WEBSEC Working Group J. Hodges
Internet-Draft PayPal
Intended status: Standards Track C. Jackson
Expires: November 18, 2012 Carnegie Mellon University
A. Barth
Google, Inc.
May 17, 2012
HTTP Strict Transport Security (HSTS)
draft-ietf-websec-strict-transport-sec-08
Abstract
This specification defines a mechanism enabling Web sites to declare
themselves accessible only via secure connections, and/or for users
to be able to direct their user agent(s) to interact with given sites
only over secure connections. This overall policy is referred to as
HTTP Strict Transport Security (HSTS). The policy is declared by Web
sites via the Strict-Transport-Security HTTP response header field,
and/or by other means, such as user agent configuration, for example.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 18, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Organization of this specification . . . . . . . . . . . . 5
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. HTTP Strict Transport Security Policy Effects . . . . . . 5
2.3. Threat Model . . . . . . . . . . . . . . . . . . . . . . . 6
2.3.1. Threats Addressed . . . . . . . . . . . . . . . . . . 6
2.3.1.1. Passive Network Attackers . . . . . . . . . . . . 6
2.3.1.2. Active Network Attackers . . . . . . . . . . . . . 7
2.3.1.3. Web Site Development and Deployment Bugs . . . . . 7
2.3.2. Threats Not Addressed . . . . . . . . . . . . . . . . 7
2.3.2.1. Phishing . . . . . . . . . . . . . . . . . . . . . 7
2.3.2.2. Malware and Browser Vulnerabilities . . . . . . . 8
2.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.1. Overall Requirement . . . . . . . . . . . . . . . . . 8
2.4.1.1. Detailed Core Requirements . . . . . . . . . . . . 8
2.4.1.2. Detailed Ancillary Requirements . . . . . . . . . 9
3. Conformance Criteria . . . . . . . . . . . . . . . . . . . . . 9
3.1. Document Conventions . . . . . . . . . . . . . . . . . . . 10
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 10
5. HSTS Mechanism Overview . . . . . . . . . . . . . . . . . . . 12
6. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Strict-Transport-Security HTTP Response Header Field . . . 13
6.1.1. The max-age Directive . . . . . . . . . . . . . . . . 14
6.1.2. The includeSubDomains Directive . . . . . . . . . . . 14
6.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 14
7. Server Processing Model . . . . . . . . . . . . . . . . . . . 15
7.1. HTTP-over-Secure-Transport Request Type . . . . . . . . . 15
7.2. HTTP Request Type . . . . . . . . . . . . . . . . . . . . 15
8. User Agent Processing Model . . . . . . . . . . . . . . . . . 16
8.1. Strict-Transport-Security Response Header Field
Processing . . . . . . . . . . . . . . . . . . . . . . . . 16
8.1.1. Noting a HSTS Host . . . . . . . . . . . . . . . . . . 17
8.2. Known HSTS Host Domain Name Matching . . . . . . . . . . . 18
8.3. URI Loading and Port Mapping . . . . . . . . . . . . . . . 19
8.4. Errors in Secure Transport Establishment . . . . . . . . . 20
8.5. HTTP-Equiv Element Attribute . . . . . . . . . . . 20
8.6. Missing Strict-Transport-Security Response Header Field . 20
9. Domain Name IDNA-Canonicalization . . . . . . . . . . . . . . 20
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10. Server Implementation and Deployment Advice . . . . . . . . . 21
10.1. HSTS Policy expiration time considerations . . . . . . . . 21
10.2. Using HSTS in conjunction with self-signed public-key
certificates . . . . . . . . . . . . . . . . . . . . . . . 22
10.3. Implications of includeSubDomains . . . . . . . . . . . . 23
11. User Agent Implementation Advice . . . . . . . . . . . . . . . 23
11.1. No User Recourse . . . . . . . . . . . . . . . . . . . . . 24
11.2. User-declared HSTS Policy . . . . . . . . . . . . . . . . 24
11.3. HSTS Pre-Loaded List . . . . . . . . . . . . . . . . . . . 24
11.4. Disallow Mixed Security Context Loads . . . . . . . . . . 25
11.5. HSTS Policy Deletion . . . . . . . . . . . . . . . . . . . 25
12. Constructing an Effective Request URI . . . . . . . . . . . . 25
12.1. ERU Fundamental Definitions . . . . . . . . . . . . . . . 26
12.2. Determining the Effective Request URI . . . . . . . . . . 26
12.2.1. Effective Request URI Examples . . . . . . . . . . . . 27
13. Internationalized Domain Names for Applications (IDNA):
Dependency and Migration . . . . . . . . . . . . . . . . . . . 27
14. Security Considerations . . . . . . . . . . . . . . . . . . . 27
14.1. Ramifications of HSTS Policy Establishment only over
Error-free Secure Transport . . . . . . . . . . . . . . . 28
14.2. The Need for includeSubDomains . . . . . . . . . . . . . . 29
14.3. Denial of Service . . . . . . . . . . . . . . . . . . . . 29
14.4. Bootstrap MITM Vulnerability . . . . . . . . . . . . . . . 30
14.5. Network Time Attacks . . . . . . . . . . . . . . . . . . . 31
14.6. Bogus Root CA Certificate Phish plus DNS Cache
Poisoning Attack . . . . . . . . . . . . . . . . . . . . . 31
14.7. Creative Manipulation of HSTS Policy Store . . . . . . . . 31
14.8. Internationalized Domain Names . . . . . . . . . . . . . . 31
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
16.1. Normative References . . . . . . . . . . . . . . . . . . . 32
16.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. Design Decision Notes . . . . . . . . . . . . . . . . 36
Appendix B. Differences between HSTS Policy and Same-Origin
Policy . . . . . . . . . . . . . . . . . . . . . . . 37
Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . . 38
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 38
D.1. For draft-ietf-websec-strict-transport-sec . . . . . . . . 39
D.2. For draft-hodges-strict-transport-sec . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 45
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1. Introduction
The HTTP protocol [RFC2616] may be used over various transports,
typically the Transmission Control Protocol (TCP). However, TCP does
not provide channel integrity protection, confidentiality, nor secure
host identification. Thus the Secure Sockets Layer (SSL) protocol
[RFC6101] and its successor Transport Layer Security (TLS) [RFC5246],
were developed in order to provide channel-oriented security, and are
typically layered between application protocols and TCP. [RFC2818]
specifies how HTTP is layered onto TLS, and defines the Uniform
Resource Identifier (URI) scheme of "https" (in practice however,
HTTP user agents (UAs) typically offer their users choices among
SSL2, SSL3, and TLS for secure transport).
UAs employ various local security policies with respect to the
characteristics of their interactions with web resources depending on
(in part) whether they are communicating with a given web resource's
host using HTTP or HTTP-over-a-Secure-Transport. For example,
cookies ([RFC6265]) may be flagged as Secure. UAs are to send such
Secure cookies to their addressed host only over a secure transport.
This is in contrast to non-Secure cookies, which are returned to the
host regardless of transport (although subject to other rules).
UAs typically announce to their users any issues with secure
connection establishment, such as being unable to validate a TLS
server certificate trust chain, or if a TLS server certificate is
expired, or if a TLS host's domain name appears incorrectly in the
TLS server certificate (see Section 3.1 of [RFC2818]). Often, UAs
enable users to elect to continue to interact with a web resource's
host in the face of such issues. This behavior is sometimes referred
to as "click(ing) through" security [GoodDhamijaEtAl05]
[SunshineEgelmanEtAl09], and thus can be described as "click-through
insecurity".
A key vulnerability enabled by click-through insecurity is the
leaking of any cookies the web resource may be using to manage a
user's session. The threat here is that an attacker could obtain the
cookies and then interact with the legitimate web resource while
impersonating the user.
Jackson and Barth proposed an approach, in [ForceHTTPS], to enable
web resources to declare that any interactions by UAs with the web
resource must be conducted securely, and that any issues with
establishing a secure transport session are to be treated as fatal
and without direct user recourse. The aim is to prevent click-
through insecurity, and address other potential threats.
This specification embodies and refines the approach proposed in
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[ForceHTTPS]. For example, rather than using a cookie to convey
policy from a web resource's host to a UA, it defines an HTTP
response header field for this purpose. Additionally, a web
resource's host may declare its policy to apply to the entire domain
name subtree rooted at its host name. This enables HSTS to protect
so-called "domain cookies", which are applied to all subdomains of a
given web resource's host name.
This specification also incorporates notions from [JacksonBarth2008]
in that policy is applied on an "entire-host" basis: it applies to
HTTP (only) over any TCP port of the issuing host.
Note that the policy defined by this specification is distinctly
different than the "same-origin policy" defined in "The Web Origin
Concept" [RFC6454]. These differences are summarized below in
Appendix B.
1.1. Organization of this specification
This specification begins with an overview of the use cases, policy
effects, threat models, and requirements for HSTS (in Section 2).
Then, Section 3 defines conformance requirements. The HSTS mechanism
itself is formally specified in Section 4 through Section 15.
2. Overview
This section discusses the use cases, summarizes the HTTP Strict
Transport Security (HSTS) policy, and continues with a discussion of
the threat model, non-addressed threats, and derived requirements.
2.1. Use Cases
The high-level use case is a combination of:
o Web browser user wishes to interact with various web sites (some
arbitrary, some known) in a secure fashion.
o Web site deployer wishes to offer their site in an explicitly
secure fashion for both their own, as well as their users',
benefit.
2.2. HTTP Strict Transport Security Policy Effects
The effects of the HTTP Strict Transport Security (HSTS) Policy, as
applied by a conformant UA in interactions with a web resource host
wielding such policy (known as a HSTS Host), are summarized as
follows:
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1. UAs transform insecure URI references to a HSTS Host into secure
URI references before dereferencing them.
2. The UA terminates any secure transport connection attempts upon
any and all secure transport errors or warnings.
2.3. Threat Model
HSTS is concerned with three threat classes: passive network
attackers, active network attackers, and imperfect web developers.
However, it is explicitly not a remedy for two other classes of
threats: phishing and malware. Addressed and not addressed threats
are briefly discussed below. Readers may wish refer to [ForceHTTPS]
for details as well as relevant citations.
2.3.1. Threats Addressed
2.3.1.1. Passive Network Attackers
When a user browses the web on a local wireless network (e.g., an
802.11-based wireless local area network) a nearby attacker can
possibly eavesdrop on the user's unencrypted Internet Protocol-based
connections, such as HTTP, regardless of whether or not the local
wireless network itself is secured [BeckTews09]. Freely available
wireless sniffing toolkits (e.g., [Aircrack-ng]) enable such passive
eavesdropping attacks, even if the local wireless network is
operating in a secure fashion. A passive network attacker using such
tools can steal session identifiers/cookies and hijack the user's web
session(s), by obtaining cookies containing authentication
credentials [ForceHTTPS]. For example, there exist widely-available
tools, such as Firesheep (a Firefox extension) [Firesheep], which
enable their wielder to obtain other local users' session cookies for
various web applications.
To mitigate such threats, some Web sites support, but usually do not
force, access using end-to-end secure transport -- e.g., signaled
through URIs constructed with the "https" scheme [RFC2818]. This can
lead users to believe that accessing such services using secure
transport protects them from passive network attackers.
Unfortunately, this is often not the case in real-world deployments
as session identifiers are often stored in non-Secure cookies to
permit interoperability with versions of the service offered over
insecure transport ("Secure cookies" are those cookies containing the
"Secure" attribute [RFC6265]). For example, if the session
identifier for a web site (an email service, say) is stored in a non-
Secure cookie, it permits an attacker to hijack the user's session if
the user's UA makes a single insecure HTTP request to the site.
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2.3.1.2. Active Network Attackers
A determined attacker can mount an active attack, either by
impersonating a user's DNS server or, in a wireless network, by
spoofing network frames or offering a similarly-named evil twin
access point. If the user is behind a wireless home router, an
attacker can attempt to reconfigure the router using default
passwords and other vulnerabilities. Some sites, such as banks, rely
on end-to-end secure transport to protect themselves and their users
from such active attackers. Unfortunately, browsers allow their
users to easily opt-out of these protections in order to be usable
for sites that incorrectly deploy secure transport, for example by
generating and self-signing their own certificates (without also
distributing their CA certificate to their users' browsers).
2.3.1.3. Web Site Development and Deployment Bugs
The security of an otherwise uniformly secure site (i.e. all of its
content is materialized via "https" URIs), can be compromised
completely by an active attacker exploiting a simple mistake, such as
the loading of a cascading style sheet or a SWF movie over an
insecure connection (both cascading style sheets and SWF movies can
script the embedding page, to the surprise of many web developers,
plus some browsers do not issue so-called "mixed content warnings"
when SWF files are embedded via insecure connections). Even if the
site's developers carefully scrutinize their login page for "mixed
content", a single insecure embedding anywhere on the overall site
compromises the security of their login page because an attacker can
script (i.e., control) the login page by injecting script into
another, insecurely loaded, site page.
Note: "Mixed content" as used above (see also Section 5.3 in
[W3C.REC-wsc-ui-20100812]) refers to the notion termed "mixed
security context" in this specification, and should not be
confused with the same "mixed content" term used in the
context of markup languages such as XML and HTML.
2.3.2. Threats Not Addressed
2.3.2.1. Phishing
Phishing attacks occur when an attacker solicits authentication
credentials from the user by hosting a fake site located on a
different domain than the real site, perhaps driving traffic to the
fake site by sending a link in an email message. Phishing attacks
can be very effective because users find it difficult to distinguish
the real site from a fake site. HSTS is not a defense against
phishing per se; rather, it complements many existing phishing
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defenses by instructing the browser to protect session integrity and
long-lived authentication tokens [ForceHTTPS].
2.3.2.2. Malware and Browser Vulnerabilities
Because HSTS is implemented as a browser security mechanism, it
relies on the trustworthiness of the user's system to protect the
session. Malicious code executing on the user's system can
compromise a browser session, regardless of whether HSTS is used.
2.4. Requirements
This section identifies and enumerates various requirements derived
from the use cases and the threats discussed above, and lists the
detailed core requirements HTTP Strict Transport Security addresses,
as well as ancillary requirements that are not directly addressed.
2.4.1. Overall Requirement
o Minimize the risks to web browser users and web site deployers
that are derived from passive and active network attackers, web
site development and deployment bugs, as well as insecure user
actions.
2.4.1.1. Detailed Core Requirements
These core requirements are derived from the overall requirement, and
are addressed by this specification.
1. Web sites need to be able to declare to UAs that they should be
interacted with using a strict security policy.
2. Web sites need to be able to instruct UAs that contact them
insecurely to do so securely.
3. UAs need to persistently remember web sites that signal strict
security policy enablement, for time spans declared by the web
sites. Additionally, UAs need to cache the "freshest" strict
security policy information, in order to allow web sites to
update the information.
4. UAs need to re-write all insecure UA "http" URI loads to use the
"https" secure scheme for those web sites for which secure policy
is enabled.
5. Web site administrators need to be able to signal strict security
policy application to subdomains of higher-level domains for
which strict security policy is enabled, and UAs need to enforce
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such policy.
For example, both example.com and foo.example.com could set
policy for bar.foo.example.com.
6. UAs need to disallow security policy application to peer domains,
and/or higher-level domains, by domains for which strict security
policy is enabled.
For example, neither bar.foo.example.com nor foo.example.com can
set policy for example.com, nor can bar.foo.example.com set
policy for foo.example.com. Also, foo.example.com cannot set
policy for sibling.example.com.
7. UAs need to prevent users from clicking-through security
warnings. Halting connection attempts in the face of secure
transport exceptions is acceptable.
Note: A means for uniformly securely meeting the first core
requirement above is not specifically addressed by this
specification (see Section 14.4 "Bootstrap MITM
Vulnerability"). It may be addressed by a future revision of
this specification or some other specification. Note also
that there are means by which UA implementations may more
fully meet the first core requirement, see Section 11 "User
Agent Implementation Advice".
2.4.1.2. Detailed Ancillary Requirements
These ancillary requirements are also derived from the overall
requirement. They are not normatively addressed in this
specification, but could be met by UA implementations at their
implementor's discretion, although meeting these requirements may be
complex.
1. Disallow "mixed security context" loads (see Section 2.3.1.3).
2. Facilitate user declaration of web sites for which strict
security policy is enabled, regardless of whether the sites
signal HSTS Policy.
3. Conformance Criteria
This specification is written for hosts and user agents (UAs).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in [RFC2119].
A conformant host is one that implements all the requirements listed
in this specification that are applicable to hosts.
A conformant user agent is one that implements all the requirements
listed in this specification that are applicable to user agents.
3.1. Document Conventions
Note: This is a note to the reader. These are points that should be
expressly kept in mind and/or considered.
Warning: This is how a warning is shown. These are things that can
have negative downside risks if not heeded.
4. Terminology
Terminology is defined in this section.
ASCII case-insensitive comparison
means comparing two strings exactly, codepoint for
codepoint, except that the characters in the range
U+0041 .. U+005A (i.e. LATIN CAPITAL LETTER A to
LATIN CAPITAL LETTER Z) and the corresponding
characters in the range U+0061 .. U+007A (i.e.
LATIN SMALL LETTER A to LATIN SMALL LETTER Z) are
considered to also match. See [Unicode] for
details.
codepoint is a colloquial contraction of Code Point, which is
any value in the Unicode codespace; that is, the
range of integers from 0 to 10FFFF(hex) [Unicode].
domain name domain names, also referred to as DNS Names, are
defined in [RFC1035] to be represented outside of
the DNS protocol itself (and implementations
thereof) as a series of labels separated by dots,
e.g., "example.com" or "yet.another.example.org".
In the context of this specification, domain names
appear in that portion of a URI satisfying the reg-
name production in "Appendix A. Collected ABNF for
URI" in [RFC3986], and the host component from the
Host HTTP header field production in Section 14.23
of [RFC2616].
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Note: The domain names appearing in actual URI
instances and matching the aforementioned
production components may or may not be a
fully qualified domain name.
domain name label is that portion of a domain name appearing "between
the dots", i.e. consider "foo.example.com": "foo",
"example", and "com" are all domain name labels.
Effective Request URI
is a URI, identifying the target resource, that can
be inferred by an HTTP host for any given HTTP
request it receives. Such inference is necessary
because HTTP requests often do not contain a
complete "absolute" URI identifying the target
resource. See Section 12 "Constructing an
Effective Request URI".
HTTP Strict Transport Security
is the overall name for the combined UA- and
server-side security policy defined by this
specification.
HTTP Strict Transport Security Host
is a HTTP host implementing the server aspects of
the HSTS policy. This means that an HSTS Host
returns the "Strict-Transport-Security" HTTP
response header field in its HTTP response messages
sent over secure transport.
HTTP Strict Transport Security Policy
is the name of the combined overall UA- and server-
side facets of the behavior specified in this
specification.
HSTS See HTTP Strict Transport Security.
HSTS Host See HTTP Strict Transport Security Host.
HSTS Policy See HTTP Strict Transport Security Policy.
Known HSTS Host is a HSTS Host for which the UA has a HSTS Policy
in effect. I.e., the UA has noted this host as a
Known HSTS Host.
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Local policy is comprised of policy rules deployers specify and
which are often manifested as "configuration
settings".
MITM is an acronym for man-in-the-middle. See "man-in-
the-middle attack" in [RFC4949].
Request URI is the URI used to cause a UA to issue an HTTP
request message.
UA is a an acronym for user agent. For the purposes
of this specification, a UA is an HTTP client
application typically actively manipulated by a
user [RFC2616] .
unknown HSTS Host is a HSTS Host that the user agent in question has
not yet noted.
5. HSTS Mechanism Overview
This section provides an overview of the mechanism by which an HSTS
Host conveys its HSTS Policy to UAs, and how UAs process the HSTS
Policies received from HSTS Hosts. The mechanism details are
specified in Section 6 through Section 15.
An HSTS Host conveys its HSTS Policy to UAs via the Strict-Transport-
Security HTTP response header field over secure transport (e.g.,
TLS). Receipt of this header field signals to UAs to enforce the
HSTS Policy for all subsequent connections made to the HSTS Host, for
a specified time duration. Application of the HSTS Policy to
subdomains of the HSTS Host name may optionally be specified.
HSTS Hosts manage their advertised HSTS Policies by sending Strict-
Transport-Security HTTP response header fields to UAs with new values
for policy time duration and application to subdomains. UAs cache
the "freshest" HSTS Policy information on behalf of an HSTS Host.
Specifying a zero time duration signals to the UA to delete the HSTS
policy for that HSTS host.
Section 6.2 presents examples of Strict-Transport-Security HTTP
response header fields.
6. Syntax
This section defines the syntax of the Strict-Transport-Security HTTP
response header field and its directives, and presents some examples.
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Section 7 "Server Processing Model" then details how hosts employ
this header field to declare their HSTS Policy, and Section 8 "User
Agent Processing Model" details how user agents process the header
field and apply the HSTS Policy.
6.1. Strict-Transport-Security HTTP Response Header Field
The Strict-Transport-Security HTTP response header field (STS header
field) indicates to a UA that it MUST enforce the HSTS Policy in
regards to the host emitting the response message containing this
header field.
The ABNF syntax for the STS header field is given below. It is based
on the Generic Grammar defined in Section 2 of [RFC2616] (which
includes a notion of "implied linear whitespace", also known as
"implied *LWS").
Strict-Transport-Security = "Strict-Transport-Security" ":"
[ directive ] *( ";" [ directive ] )
directive = token [ "=" ( token | quoted-string ) ]
where:
token =
quoted-string =
The two directives defined in this specification are described below.
The overall requirements for directives are:
1. The order of appearance of directives is not significant.
2. All directives MUST appear only once in an STS header field.
Directives are either optional or required, as stipulated in
their definitions.
3. Directive names are case-insensitive.
4. UAs MUST ignore any STS header fields containing directives, or
other header field value data, that does not conform to the
syntax defined in this specification.
Additional directives extending the semantic functionality of the STS
header field can be defined in other specifications.
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6.1.1. The max-age Directive
The REQUIRED "max-age" directive specifies the number of seconds,
after the reception of the STS header field, during which the UA
regards the host (from whom the message was received) as a Known HSTS
Host. See also Section 8.1.1 "Noting a HSTS Host". The delta-
seconds production is specified in [RFC2616].
The syntax of the max-age directive's value (after quoted-string
unescaping, if necessary) is defined as:
max-age-value = delta-seconds
delta-seconds = <1*DIGIT, defined in [RFC2616], Section 3.3.2>
Note: A max-age value of zero (i.e., "max-age=0") signals the UA to
cease regarding the host as a Known HSTS Host.
6.1.2. The includeSubDomains Directive
The OPTIONAL "includeSubDomains" directive is a valueless flag which,
if present, signals to the UA that the HSTS Policy applies to this
HSTS Host as well as any subdomains of the host's domain name.
6.2. Examples
The below HSTS header field stipulates that the HSTS policy is to
remain in effect for one year (there are approximately 31 536 000
seconds in a year), and the policy applies only to the domain of the
HSTS Host issuing it:
Strict-Transport-Security: max-age=31536000
The below HSTS header field stipulates that the HSTS policy is to
remain in effect for approximately six months and the policy applies
only to the domain of the issuing HSTS Host and all of its
subdomains:
Strict-Transport-Security: max-age=15768000 ; includeSubDomains
The max-age directive value can optionally be quoted:
Strict-Transport-Security: max-age="31536000"
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7. Server Processing Model
This section describes the processing model that HSTS Hosts
implement. The model is comprised of two facets: the first being the
processing rules for HTTP request messages received over a secure
transport (e.g., TLS [RFC5246], SSL [I-D.ietf-tls-ssl-version3], or
perhaps others), the second being the processing rules for HTTP
request messages received over non-secure transports, i.e. over
TCP/IP.
7.1. HTTP-over-Secure-Transport Request Type
When replying to an HTTP request that was conveyed over a secure
transport, a HSTS Host SHOULD include in its response message a STS
header field that MUST satisfy the grammar specified above in
Section 6.1 "Strict-Transport-Security HTTP Response Header Field".
If a STS header field is included, the HSTS Host MUST include only
one such header field.
Note: Including the STS header field is stipulated as a "SHOULD" in
order to accommodate various server- and network-side caches
and load-balancing configurations where it may be difficult to
uniformly emit STS header fields on behalf of a given HSTS
Host.
Establishing a given host as a Known HSTS Host, in the context
of a given UA, MAY be accomplished over the HTTP protocol by
correctly returning, per this specification, at least one
valid STS header field to the UA. Other mechanisms, such as a
client-side pre-loaded Known HSTS Host list MAY also be used.
E.g., see Section 11 "User Agent Implementation Advice".
7.2. HTTP Request Type
If a HSTS Host receives a HTTP request message over a non-secure
transport, it SHOULD send a HTTP response message containing a status
code indicating a permanent redirect, such as status code 301
(Section 10.3.2 of [RFC2616]), and a Location header field value
containing either the HTTP request's original Effective Request URI
(see Section 12 "Constructing an Effective Request URI") altered as
necessary to have a URI scheme of "https", or a URI generated
according to local policy (which SHOULD employ a URI scheme of
"https").
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Note: The above behavior is a "SHOULD" rather than a "MUST" because:
* There are risks in server-side non-secure-to-secure redirects
[owaspTLSGuide].
* Site deployment characteristics -- e.g., a site that
incorporates third-party components may not behave correctly
when doing server-side non-secure-to-secure redirects in the
case of being accessed over non-secure transport, but does
behave correctly when accessed uniformly over secure transport.
The latter is the case given a HSTS-capable UA that has already
noted the site as a Known HSTS Host (by whatever means, e.g.,
prior interaction or UA configuration).
A HSTS Host MUST NOT include the STS header field in HTTP responses
conveyed over non-secure transport.
8. User Agent Processing Model
This section describes the HTTP Strict Transport Security processing
model for UAs. There are several facets to the model, enumerated by
the following subsections.
This processing model assumes that the UA implements IDNA2008
[RFC5890], or possibly IDNA2003 [RFC3490], as noted in Section 13
"Internationalized Domain Names for Applications (IDNA): Dependency
and Migration". It also assumes that all domain names manipulated in
this specification's context are already IDNA-canonicalized as
outlined in Section 9 "Domain Name IDNA-Canonicalization" prior to
the processing specified in this section.
The above assumptions mean that this processing model also
specifically assumes that appropriate IDNA and Unicode validations
and character list testing have occurred on the domain names, in
conjunction with their IDNA-canonicalization, prior to the processing
specified in this section. See the IDNA-specific security
considerations in Section 14.8 "Internationalized Domain Names" for
rationale and further details.
8.1. Strict-Transport-Security Response Header Field Processing
If an HTTP response, received over a secure transport, includes a STS
header field, conforming to the grammar specified in Section 6.1
"Strict-Transport-Security HTTP Response Header Field", and there are
no underlying secure transport errors or warnings (see Section 8.4),
the UA MUST either:
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o Note the host as a Known HSTS Host if it is not already so noted
(see Section 8.1.1 "Noting a HSTS Host"),
or,
o Update the UA's cached information for the Known HSTS Host if
either or both of the max-age and includeSubDomains header field
value tokens are conveying information different than that already
maintained by the UA.
Note: The max-age value is essentially a "time to live" value
relative to the reception time of the STS header field.
If the max-age header field value token has a value of zero,
the UA MUST remove its cached HSTS Policy information if the
HSTS Host is known, or, MUST NOT note this HSTS Host if it is
not yet known.
If a UA receives more than one STS header field in a HTTP
response message over secure transport, then the UA MUST
process only the first such header field.
Otherwise:
o If an HTTP response is received over insecure transport, the UA
MUST ignore any present STS header field(s).
o The UA MUST ignore any STS header fields not conforming to the
grammar specified in Section 6.1 "Strict-Transport-Security HTTP
Response Header Field".
8.1.1. Noting a HSTS Host
If the substring matching the host production from the Request-URI
(of the message that the host responded to) syntactically matches the
IP-literal or IPv4address productions from Section 3.2.2 of
[RFC3986], then the UA MUST NOT note this host as a Known HSTS Host.
Otherwise, if the substring does not congruently match a Known HSTS
Host's domain name, per the matching procedure specified in
Section 8.2 "Known HSTS Host Domain Name Matching", then the UA MUST
note this host as a Known HSTS Host, caching the HSTS Host's domain
name and noting along with it the expiry time of this information, as
effectively stipulated per the given max-age value, as well as
whether the includeSubDomains flag is asserted or not. See also
Section 10.1 "HSTS Policy expiration time considerations".
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Note: The UA MUST NOT modify the expiry time nor the
includeSubDomains flag of any superdomain matched Known HSTS
Host.
8.2. Known HSTS Host Domain Name Matching
A given domain name may match a Known HSTS Host's domain name in one
or both of two fashions: a congruent match, or a superdomain match.
Or, there may be no match.
The below steps determine whether there are any matches, and if so,
of which fashion:
Compare the given domain name with the domain name of each of the
UA's Known HSTS Hosts. For each Known HSTS Host's domain name,
the comparison is done with the given domain name label-by-label
(comparing only labels) using an ASCII case-insensitive comparison
beginning with the rightmost label, and continuing right-to-left.
See also Section 2.3.2.4. of [RFC5890].
* Superdomain Match
If a label-for-label match between an entire Known HSTS Host's
domain name and a right-hand portion of the given domain name
is found, then this Known HSTS Host's domain name is a
superdomain match for the given domain name. There could be
multiple superdomain matches for a given domain name.
For example:
Given Domain Name: qaz.bar.foo.example.com
Superdomain matched
Known HSTS Host DN: bar.foo.example.com
Superdomain matched
Known HSTS Host DN: foo.example.com
* Congruent Match
If a label-for-label match between a Known HSTS Host's domain
name and the given domain name is found -- i.e., there are no
further labels to compare -- then the given domain name
congruently matches this Known HSTS Host.
For example:
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Given Domain Name: foo.example.com
Congruently matched
Known HSTS Host DN: foo.example.com
* Otherwise, if no matches are found, the given domain name does
not represent a Known HSTS Host.
8.3. URI Loading and Port Mapping
Whenever the UA prepares to "load", also known as "dereference", any
"http" URI [RFC3986], the UA MUST first determine whether a domain
name is given in the URI and whether it matches a Known HSTS Host,
using these steps:
1. Extract from the URI any substring described by the host
component of the authority component of the URI.
2. If the substring is null, then there is no match with any Known
HSTS Host.
3. Else, if the substring is non-null and syntactically matches the
IP-literal or IPv4address productions from Section 3.2.2 of
[RFC3986], then there is no match with any Known HSTS Host.
4. Otherwise, the substring is a given domain name, which MUST be
matched against the UA's Known HSTS Hosts using the procedure in
Section 8.2 "Known HSTS Host Domain Name Matching".
If a superdomain match with an asserted includeSubDomains flag is
found, or if a congruent match is found -- without any found
superdomain matches having asserted includeSubDomains flags -- then
before proceeding with the load:
The UA MUST replace the URI scheme with "https" [RFC2818], and,
if the URI contains an explicit port component of "80", then the
UA MUST convert the port component to be "443", or,
if the URI contains an explicit port component that is not equal
to "80", the port component value MUST be preserved, otherwise,
if the URI does not contain an explicit port component, the UA
MUST NOT add one.
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Note: The the above steps ensure that the HSTS Policy applies to
HTTP over any TCP port of a HSTS Host.
8.4. Errors in Secure Transport Establishment
When connecting to a Known HSTS Host, the UA MUST terminate the
connection (see also Section 11 "User Agent Implementation Advice")
if there are any errors (e.g., certificate errors), whether "warning"
or "fatal" or any other error level, with the underlying secure
transport. This includes any issues with certificate revocation
checking whether via the Certificate Revocation List (CRL) [RFC5280],
or via the Online Certificate Status Protocol (OCSP) [RFC2560].
8.5. HTTP-Equiv Element Attribute
UAs MUST NOT heed http-equiv="Strict-Transport-Security" attribute
settings on elements [W3C.REC-html401-19991224] in received
content.
8.6. Missing Strict-Transport-Security Response Header Field
If a UA receives HTTP responses from a Known HSTS Host over a secure
channel, but they are missing the STS header field, the UA MUST
continue to treat the host as a Known HSTS Host until the max-age
value for the knowledge of that Known HSTS Host is reached. Note
that the max-age could be infinite for a given Known HSTS Host. For
example, if the Known HSTS Host is part of a pre-configured list that
is implemented such that the list entries never "age out".
9. Domain Name IDNA-Canonicalization
An IDNA-canonicalized domain name is the output string generated by
the following steps. The input is a putative domain name string
ostensibly composed of any combination of "A-labels", "U-labels", and
"NR-LDH labels" (see Section 2 of [RFC5890]) concatenated using some
separator character (typically ".").
1. Convert the input putative domain name string to a order-
preserving sequence of individual label strings.
2. When implementing IDNA2008, convert, validate, and test each
A-label and U-label found among the sequence of individual label
strings, using the procedures defined in Sections 5.3 through 5.5
of [RFC5891].
Otherwise, when implementing IDNA2003, convert each label using
the "ToASCII" conversion in Section 4 of [RFC3490] (see also the
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definition of "equivalence of labels" in Section 2 of the latter
specification).
3. If no errors occurred during the foregoing step, concatenate all
the labels in the sequence, in order, into a string, separating
each label from the next with (".") a %x2E character. The
resulting string, known as a IDNA-canonicalized domain name, is
appropriate for use in the context of Section 8 "User Agent
Processing Model".
Otherwise, errors occurred. The input putative domain name
string was not successfully IDNA-canonicalized. Invokers of this
procedure should attempt appropriate error recovery.
See also Section 13 "Internationalized Domain Names for Applications
(IDNA): Dependency and Migration" and Section 14.8 "Internationalized
Domain Names" of this specification for further details and
considerations.
10. Server Implementation and Deployment Advice
This section is non-normative.
10.1. HSTS Policy expiration time considerations
Server implementations and deploying web sites need to consider
whether they are setting an expiry time that is a constant value into
the future, or whether they are setting an expiry time that is a
fixed point in time.
The constant value into the future approach can be accomplished by
constantly sending the same max-age value to UAs.
For example, a max-age value of 778000 is 90 days:
Strict-Transport-Security: max-age=778000
Note that each receipt of this header by a UA will require the UA to
update its notion of when it must delete its knowledge of this Known
HSTS Host.
The fixed point in time approach can be accomplished by sending max-
age values that represent the remaining time until the desired expiry
time. This would require the HSTS Host to send a newly-calculated
max-age value on each HTTP response.
A consideration here is whether a deployer wishes to have the
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signaled HSTS Policy expiry time match that for the web site's domain
certificate.
Additionally, server implementers should consider employing a default
max-age value of zero in their deployment configuration systems.
This will require deployers to wilfully set max-age in order to have
UAs enforce the HSTS Policy for their host, and protects them from
inadvertently enabling HSTS with some arbitrary non-zero duration.
10.2. Using HSTS in conjunction with self-signed public-key
certificates
If a web site/organization/enterprise is generating their own secure
transport public-key certificates for web sites, and that
organization's root certification authority (CA) certificate is not
typically embedded by default in browser and/or operating system CA
certificate stores, and if HSTS Policy is enabled on a host
identifying itself using a certificate signed by the organization's
CA (i.e., a "self-signed certificate"), and this certificate does not
match a usable TLS certificate association (as defined by Section 4
of the TLSA protocol specification [I-D.ietf-dane-protocol]), then
secure connections to that site will fail, per the HSTS design. This
is to protect against various active attacks, as discussed above.
However, if said organization wishes to employ their own CA, and
self-signed certificates, in concert with HSTS, they can do so by
deploying their root CA certificate to their users' browsers or
operating system CA root certificate stores. They can also, in
addition or instead, distribute to their users' browsers the end-
entity certificate(s) for specific hosts. There are various ways in
which this can be accomplished (details are out of scope for this
specification). Once their root CA certificate is installed in the
browsers, they may employ HSTS Policy on their site(s).
Alternately, that organization can deploy the TLSA protocol; all
browsers that also use TLSA will then be able to trust the
certificates identified by usable TLS certificate associations as
denoted via TLSA.
Note: Interactively distributing root CA certificates to users,
e.g., via email, and having the users install them, is
arguably training the users to be susceptible to a possible
form of phishing attack, see Section 14.6 "Bogus Root CA
Certificate Phish plus DNS Cache Poisoning Attack". Thus care
should be taken in the manner in which such certificates are
distributed and installed on users' systems and browsers.
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10.3. Implications of includeSubDomains
The includeSubDomains directive has some practical implications --
for example, if a HSTS Host offers HTTP-based services on various
ports or at various subdomains of its host domain name, then they
will all have to be available over secure transport in order to work
properly.
For example, certification authorities often offer their CRL
distribution and OCSP services [RFC2560] over plain HTTP, and
sometimes at a subdomain of a publicly-available web application
which may be secured by TLS/SSL. For example,
is a publicly-available web application for
"Example CA", a certification authority. Customers use this web
application to register their public keys and obtain certificates.
Example CA generates certificates for customers containing
as the value for the "CRL
Distribution Points" and "Authority Information Access:OCSP"
certificate fields.
If example-ca.com were to issue an HSTS Policy with the
includeSubDomains directive, then HTTP-based user agents implementing
HSTS, and that have interacted with the example-ca.com web
application, would fail to retrieve CRLs and fail to check OCSP for
certificates because these services are offered over plain HTTP.
In this case, Example CA can either:
o not use the includeSubDomains directive, or,
o ensure HTTP-based services offered at subdomains of example-ca.com
are uniformly offered over TLS/SSL, or,
o offer plain HTTP-based services at a different domain name, e.g.,
example-ca-services.net.
11. User Agent Implementation Advice
This section is non-normative.
In order to provide users and web sites more effective protection, as
well as controls for managing their UA's caching of HSTS Policy, UA
implementors should consider including features such as:
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11.1. No User Recourse
Failing secure connection establishment on any warnings or errors
(per Section 8.4 "Errors in Secure Transport Establishment"), should
be done with "no user recourse". This means that the user should not
be presented with a dialog giving her the option to proceed. Rather,
it should be treated similarly to a server error where there is
nothing further the user can do with respect to interacting with the
target web application, other than wait and re-try.
Essentially, "any warnings or errors" means anything that would cause
the UA implementation to announce to the user that something is not
entirely correct with the connection establishment.
Not doing this, i.e., allowing user recourse such as "clicking-
through warning/error dialogs", is a recipe for a Man-in-the-Middle
attack. If a web application advertises HSTS, then it is opting into
this scheme, whereby all certificate errors or warnings cause a
connection termination, with no chance to "fool" the user into making
the wrong decision and compromising themselves.
11.2. User-declared HSTS Policy
A User-declared HSTS Policy is the ability for users to explicitly
declare a given Domain Name as representing a HSTS Host, thus seeding
it as a Known HSTS Host before any actual interaction with it. This
would help protect against the Section 14.4 "Bootstrap MITM
Vulnerability".
Note: Such a feature is difficult to get right on a per-site basis
-- see the discussion of "rewrite rules" in Section 5.5 of
[ForceHTTPS]. For example, arbitrary web sites may not
materialize all their URIs using the "https" scheme, and thus
could "break" if a UA were to attempt to access the site
exclusively using such URIs. Also note that this feature
would complement, but is independent of the following
described facility.
11.3. HSTS Pre-Loaded List
A HSTS Pre-Loaded List is a facility whereby web site administrators
can have UAs pre-configured with HSTS Policy for their site(s) by the
UA vendor(s) -- a so-called "pre-loaded list" -- in a manner similar
to how root CA certificates are embedded in browsers "at the
factory". This would help protect against the Section 14.4
"Bootstrap MITM Vulnerability".
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Note: Such a facility would complement a "User-declared HSTS Policy"
feature.
11.4. Disallow Mixed Security Context Loads
"Mixed security context" loads are when an web application resource,
fetched by the UA over a secure transport, subsequently fetches and
loads one or more other resources without using secure transport.
This is also generally referred to as "mixed content" loads (see
Section 5.3 "Mixed Content" in [W3C.REC-wsc-ui-20100812]), but should
not be confused with the same "mixed content" term that is also used
in the context of markup languages such as XML and HTML.
Note: In order to provide behavioral uniformity across UA
implementations, the notion of mixed security context will
require further standardization work, e.g., to more clearly
define the term(s) and to define specific behaviors with
respect to it.
11.5. HSTS Policy Deletion
HSTS Policy Deletion is the ability to delete a UA's cached HSTS
Policy on a per HSTS Host basis.
Note: Adding such a feature should be done very carefully in both
the user interface and security senses. Deleting a cache
entry for a Known HSTS Host should be a very deliberate and
well-considered act -- it shouldn't be something users get
used to just "clicking through" in order to get work done.
Also, implementations need to guard against allowing an
attacker to inject code, e.g. ECMAscript, into the UA that
silently and programmatically removes entries from the UA's
cache of Known HSTS Hosts.
12. Constructing an Effective Request URI
This section specifies how an HSTS Host must construct the Effective
Request URI for a received HTTP request.
HTTP requests often do not carry an absoluteURI for the target
resource; instead, the URI needs to be inferred from the Request-URI,
Host header field, and connection context ([RFC2616], Sections 3.2.1
and 5.1.2). The result of this process is called the "effective
request URI (ERU)". The "target resource" is the resource identified
by the effective request URI.
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12.1. ERU Fundamental Definitions
The first line of an HTTP request message, Request-Line, is specified
by the following ABNF from [RFC2616], Section 5.1:
Request-Line = Method SP Request-URI SP HTTP-Version CRLF
The Request-URI, within the Request-Line, is specified by the
following ABNF from [RFC2616], Section 5.1.2:
Request-URI = "*" | absoluteURI | abs_path | authority
The Host request header field is specified by the following ABNF from
[RFC2616], Section 14.23:
Host = "Host" ":" host [ ":" port ]
12.2. Determining the Effective Request URI
If the Request-URI is an absoluteURI, then the effective request URI
is the Request-URI.
If the Request-URI uses the abs_path form or the asterisk form, and
the Host header field is present, then the effective request URI is
constructed by concatenating:
o the scheme name: "http" if the request was received over an
insecure TCP connection, or "https" when received over a TLS/
SSL-secured TCP connection, and,
o the octet sequence "://", and,
o the host, and the port (if present), from the Host header field,
and
o the Request-URI obtained from the Request-Line, unless the
Request-URI is just the asterisk "*".
If the Request-URI uses the abs_path form or the asterisk form, and
the Host header field is not present, then the effective request URI
is undefined.
Otherwise, when Request-URI uses the authority form, the effective
request URI is undefined.
Effective request URIs are compared using the rules described in
[RFC2616] Section 3.2.3, except that empty path components MUST NOT
be treated as equivalent to an absolute path of "/".
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12.2.1. Effective Request URI Examples
Example 1: the effective request URI for the message
GET /pub/WWW/TheProject.html HTTP/1.1
Host: www.example.org:8080
(received over an insecure TCP connection) is "http", plus "://",
plus the authority component "www.example.org:8080", plus the
request-target "/pub/WWW/TheProject.html". Thus it is:
"http://www.example.org:8080/pub/WWW/TheProject.html".
Example 2: the effective request URI for the message
OPTIONS * HTTP/1.1
Host: www.example.org
(received over an SSL/TLS secured TCP connection) is "https", plus
"://", plus the authority component "www.example.org". Thus it is:
"https://www.example.org".
13. Internationalized Domain Names for Applications (IDNA): Dependency
and Migration
Textual domain names on the modern Internet may contain one or more
"internationalized" domain name labels. Such domain names are
referred to as "internationalized domain names" (IDNs). The
specification suites defining IDNs and the protocols for their use
are named "Internationalized Domain Names for Applications (IDNA)".
At this time, there are two such specification suites: IDNA2008
[RFC5890] and its predecessor IDNA2003 [RFC3490].
IDNA2008 obsoletes IDNA2003, but there are differences between the
two specifications, and thus there can be differences in processing
(e.g., converting) domain name labels that have been registered under
one from those registered under the other. There will be a
transition period of some time during which IDNA2003-based domain
name labels will exist in the wild. In order to facilitate their
IDNA transition, user agents SHOULD implement IDNA2008 [RFC5890] and
MAY implement [RFC5895] (see also Section 7 of [RFC5894]) or [UTS46].
If a user agent does not implement IDNA2008, the user agent MUST
implement IDNA2003.
14. Security Considerations
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14.1. Ramifications of HSTS Policy Establishment only over Error-free
Secure Transport
The User Agent Processing Model defined in Section 8, stipulates that
a host is initially noted as a Known HSTS Host, or that updates are
made to a Known HSTS Host's cached information, only if the UA
receives the STS header field over a secure transport connection
having no underlying secure transport errors or warnings.
The rationale behind this is that if there is a man-in-the-middle
(MITM) -- whether a legitimately deployed proxy or an illegitimate
entity -- it could cause various mischief (see also Appendix A
"Design Decision Notes", item 3, as well as Section 14.4 "Bootstrap
MITM Vulnerability" ), for example:
o Unauthorized notation of the host as a Known HSTS Host,
potentially leading to a denial of service situation if the host
does not uniformly offer its services over secure transport (see
also Section 14.3 "Denial of Service".
o Resetting the time-to-live for the host's designation as a Known
HSTS Host by manipulating the max-age header field parameter value
that is returned to the UA. If max-age is returned as zero, this
will cause the host to no longer be regarded as an Known HSTS Host
by the UA, leading to either insecure connections to the host or
possibly denial-of-service if the host delivers its services only
over secure transport.
However, this means that if a UA is "behind" a proxy -- within a
corporate intranet, for example -- and interacts with an unknown HSTS
Host beyond the proxy, the user will possibly be presented with the
legacy secure connection error dialogs. And even if the risk is
accepted and the user clicks-through, the host will not be noted as a
HSTS Host. Thus as long as the UA is behind such a proxy the user
will be vulnerable, and possibly be presented with the legacy secure
connection error dialogs for as yet unknown HSTS Hosts.
But once the UA successfully connects to an unknown HSTS Host over
error-free secure transport, the host will be noted as a Known HSTS
Host. This will result in the failure of subsequent connection
attempts from behind interfering proxies.
The above discussion relates to the recommendation in Section 11
"User Agent Implementation Advice" that the secure connection be
terminated with "no user recourse" whenever there are warnings and
errors and the host is a Known HSTS Host. Such a posture protects
users from "clicking through" security warnings and putting
themselves at risk.
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14.2. The Need for includeSubDomains
Without the includeSubDomains directive, a web application would not
be able to adequately protect so-called "domain cookies" (even if
these cookies have their "Secure" flag set and thus are conveyed only
on secure channels). These are cookies the web application expects
UAs to return to any and all subdomains of the web application.
For example, suppose example.com represents the top-level DNS name
for a web application. Further suppose that this cookie is set for
the entire example.com domain, i.e. it is a "domain cookie", and it
has its Secure flag set. Suppose example.com is a Known HSTS Host
for this UA, but the includeSubDomains flag is not set.
Now, if an attacker causes the UA to request a subdomain name that is
unlikely to already exist in the web application, such as
"https://uxdhbpahpdsf.example.com/", but the attacker has established
somewhere and registered in the DNS, then:
1. The UA is unlikely to already have an HSTS policy established for
"uxdhbpahpdsf.example.com", and,
2. The HTTP request sent to uxdhbpahpdsf.example.com will include
the Secure-flagged domain cookie.
3. If "uxdhbpahpdsf.example.com" returns a certificate during TLS
establishment, and the user clicks through any warning that might
be annunciated (it is possible, but not certain, that one may
obtain a requisite certificate for such a domain name such that a
warning may or may not appear), then the attacker can obtain the
Secure-flagged domain cookie that's ostensibly being protected.
Without the "includeSubDomains" directive, HSTS is unable to protect
such Secure-flagged domain cookies.
14.3. Denial of Service
HSTS could be used to mount certain forms of Denial-of- Service (DoS)
attacks against web sites. A DoS attack is an attack in which one or
more network entities target a victim entity and attempt to prevent
the victim from doing useful work. This section discusses such
scenarios in terms of HSTS, though this list is not exhaustive. See
also [RFC4732] for a discussion of overall Internet DoS
considerations.
o Web applications available over HTTP
There is an opportunity for perpetrating DoS attacks with web
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applications that are -- or critical portions of them are --
available only over HTTP without secure transport, if attackers
can cause UAs to set HSTS Policy for such web applications'
host(s).
This is because once the HSTS Policy is set for a web
application's host in a UA, the UA will only use secure transport
to communicate with the host. If the host is not using secure
transport, or is not for critical portions of its web application,
then the web application will be rendered unusable for the UA's
user.
An HSTS Policy can be set for a victim host in various ways:
* If the web application has a HTTP response splitting
vulnerability [CWE-113] (which can be abused in order to
facilitate "HTTP Header Injection").
* If an attacker can spoof a redirect from an insecure victim
site, e.g., to ,
where the latter is attacker-controlled and has an apparently
valid certificate, then the attacker can set an HSTS Policy for
example.com, and also for all subdomains of example.com.
* If an attacker can convince users to manually configure HSTS
Policy for a victim host, assuming their UAs offer this
capability (see Section 11 "User Agent Implementation Advice").
Or if such UA configuration is scriptable, and the attacker can
cause UAs to execute his script.
o Inadvertent use of includeSubDomains
The includeSubDomains directive instructs UAs to automatically
regard all subdomains of the given HSTS Host as Known HSTS Hosts.
If any such subdomains do not support properly configured secure
transport, then they will be rendered unreachable from such UAs.
14.4. Bootstrap MITM Vulnerability
The bootstrap MITM (Man-In-The-Middle) vulnerability is a
vulnerability users and HSTS Hosts encounter in the situation where
the user manually enters, or follows a link, to an unknown HSTS Host
using a "http" URI rather than a "https" URI. Because the UA uses an
insecure channel in the initial attempt to interact with the
specified server, such an initial interaction is vulnerable to
various attacks [ForceHTTPS] .
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Note: There are various features/facilities that UA implementations
may employ in order to mitigate this vulnerability. Please
see Section 11 "User Agent Implementation Advice".
14.5. Network Time Attacks
Active network attacks can subvert network time protocols (such as
NTP [RFC5905]) - making HSTS less effective against clients that
trust NTP or lack a real time clock. Network time attacks are beyond
the scope of this specification. Note that modern operating systems
use NTP by default. See also Section 2.10 of [RFC4732].
14.6. Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack
If an attacker can convince users of, say, https://bank.example.com
(which is protected by HSTS Policy), to install their own version of
a root CA certificate purporting to be bank.example.com's CA, e.g.,
via a phishing email message with a link to such a certificate.
Then, if they can perform an attack on the users' DNS, (e.g., via
cache poisoning) and turn on HSTS Policy for their fake
bank.example.com site, then they have themselves some new users.
14.7. Creative Manipulation of HSTS Policy Store
Since an HSTS Host may select its own host name and subdomains
thereof, and this information is cached in the HSTS Policy store of
conforming UAs, it is possible for those who control a HSTS Host(s)
to encode information into domain names they control and cause such
UAs to cache this information as a matter of course in the process of
noting the HSTS Host. This information can be retrieved by other
hosts through clever loaded page construction causing the UA to send
queries to (variations of) the encoded domain names. Such queries
can reveal whether the UA had prior visited the original HSTS Host
(and subdomains).
Such a technique could potentially be abused as yet another form of
"web tracking" [WebTracking].
14.8. Internationalized Domain Names
Internet security relies in part on the DNS and the domain names it
hosts. Domain names are used by users to identify and connect to
Internet hosts and other network resources. For example, Internet
security is compromised if a user entering an internationalized
domain name (IDN) is connected to different hosts based on different
interpretations of the IDN.
The processing models specified in this specification assume that the
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domain names they manipulate are IDNA-canonicalized, and that the
canonicalization process correctly performed all appropriate IDNA and
Unicode validations and character list testing per the requisite
specifications (e.g., as noted in Section 9 "Domain Name IDNA-
Canonicalization"). These steps are necessary in order to avoid
various potentially compromising situations.
In brief, some examples of issues that could stem from lack of
careful and consistent Unicode and IDNA validations are things such
as unexpected processing exceptions, truncation errors, and buffer
overflows, as well as false-positive and/or false-negative domain
name matching results. Any of the foregoing issues could possibly be
leveraged by attackers in various ways.
Additionally, IDNA2008 [RFC5890] differs from IDNA2003 [RFC3490] in
terms of disallowed characters and character mapping conventions.
This situation can also lead to false-positive and/or false-negative
domain name matching results, resulting in, for example, users
possibly communicating with unintended hosts, or not being able to
reach intended hosts.
For details, refer to the Security Considerations sections of
[RFC5890], [RFC5891], and [RFC3490], as well as the specifications
they normatively reference. Additionally, [RFC5894] provides
detailed background and rationale for IDNA2008 in particular, as well
as IDNA and its issues in general, and should be consulted in
conjunction with the former specifications.
15. IANA Considerations
Below is the Internet Assigned Numbers Authority (IANA) Permanent
Message Header Field registration information per [RFC3864].
Header field name: Strict-Transport-Security
Applicable protocol: HTTP
Status: standard
Author/Change controller: IETF
Specification document(s): this one
16. References
16.1. Normative References
[I-D.ietf-dane-protocol]
Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Protocol for Transport Layer
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Security (TLS)", draft-ietf-dane-protocol-19 (work in
progress), April 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June 1999.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
This specification is referenced due to its ongoing
relevance to actual deployments for the foreseeable
future.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
This specification is referenced due to its ongoing
relevance to actual deployments for the foreseeable
future.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
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[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[RFC5894] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Background, Explanation, and
Rationale", RFC 5894, August 2010.
[RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for
Internationalized Domain Names in Applications (IDNA)
2008", RFC 5895, September 2010.
[UTS46] Davis, M. and M. Suignard, "Unicode IDNA Compatibility
Processing", Unicode Technical Standards # 46, 2010,
.
[Unicode] The Unicode Consortium, "The Unicode Standard",
.
[W3C.REC-html401-19991224]
Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
.
16.2. Informative References
[Aircrack-ng]
d'Otreppe, T., "Aircrack-ng", Accessed: 11-Jul-2010,
.
[BeckTews09]
Beck, M. and E. Tews, "Practical Attacks Against WEP and
WPA", Second ACM Conference on Wireless Network
Security Zurich, Switzerland, 2009, .
[CWE-113] "CWE-113: Improper Neutralization of CRLF Sequences in
HTTP Headers ('HTTP Response Splitting')", Common Weakness
Enumeration , The Mitre
Corporation ,
.
[Firesheep]
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Various, "Firesheep", Wikipedia Online, on-going,
.
[ForceHTTPS]
Jackson, C. and A. Barth, "ForceHTTPS: Protecting High-
Security Web Sites from Network Attacks", In Proceedings
of the 17th International World Wide Web Conference
(WWW2008) , 2008,
.
[GoodDhamijaEtAl05]
Good, N., Dhamija, R., Grossklags, J., Thaw, D.,
Aronowitz, S., Mulligan, D., and J. Konstan, "Stopping
Spyware at the Gate: A User Study of Privacy, Notice and
Spyware", In Proceedings of Symposium On Usable Privacy
and Security (SOUPS) Pittsburgh, PA, USA, July 2005, .
[I-D.ietf-tls-ssl-version3]
Freier, A., Karlton, P., and P. Kocher, "The SSL Protocol
Version 3.0", draft-ietf-tls-ssl-version3-00 (work in
progress), November 1996, .
This is the canonical reference for SSLv3.0.
[JacksonBarth2008]
Jackson, C. and A. Barth, "Beware of Finer-Grained
Origins", Web 2.0 Security and Privacy Oakland, CA, USA,
2008,
.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
RFC 4949, August 2007.
[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
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[RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure
Sockets Layer (SSL) Protocol Version 3.0", RFC 6101,
August 2011.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
April 2011.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
December 2011.
[SunshineEgelmanEtAl09]
Sunshine, J., Egelman, S., Almuhimedi, H., Atri, N., and
L. Cranor, "Crying Wolf: An Empirical Study of SSL Warning
Effectiveness", In Proceedings of 18th USENIX Security
Symposium Montreal, Canada, Augus 2009, .
[W3C.REC-wsc-ui-20100812]
Saldhana, A. and T. Roessler, "Web Security Context: User
Interface Guidelines", World Wide Web Consortium
Recommendation REC-wsc-ui-20100812, August 2010,
.
[WEBSEC] "WebSec -- HTTP Application Security Minus Authentication
and Transport",
.
Mailing list for IETF WebSec Working Group. [RFCEditor:
please remove this reference upon publication as an RFC.]
[WebTracking]
Schmucker, N., "Web Tracking", SNET2 Seminar Paper Summer
Term, 2011, .
[owaspTLSGuide]
Coates, M., Wichers, d., Boberski, M., and T. Reguly,
"Transport Layer Protection Cheat Sheet", Accessed: 11-
Jul-2010, .
Appendix A. Design Decision Notes
This appendix documents various design decisions.
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1. Cookies aren't appropriate for HSTS Policy expression as they are
potentially mutable (while stored in the UA), therefore an HTTP
header field is employed.
2. We chose to not attempt to specify how "mixed security context
loads" (aka "mixed content loads") are handled due to UA
implementation considerations as well as classification
difficulties.
3. A HSTS Host may update UA notions of HSTS Policy via new HSTS
header field parameter values. We chose to have UAs honor the
"freshest" information received from a server because there is
the chance of a web site sending out an errornous HSTS Policy,
such as a multi-year max-age value, and/or an incorrect
includeSubDomains flag. If the HSTS Host couldn't correct such
errors over protocol, it would require some form of annunciation
to users and manual intervention on the users' part, which could
be a non-trivial problem for both web application providers and
their users.
4. HSTS Hosts are identified only via domain names -- explicit IP
address identification of all forms is excluded. This is for
simplification and also is in recognition of various issues with
using direct IP address identification in concert with PKI-based
security.
Appendix B. Differences between HSTS Policy and Same-Origin Policy
HSTS Policy has the following primary characteristics:
HSTS Policy stipulates requirements for the security
characteristics of UA-to-host connection establishment, on a per-
host basis.
Hosts explicitly declare HSTS Policy to UAs. Conformant UAs are
obliged to implement hosts' declared HSTS Policies.
HSTS Policy is conveyed over protocol from the host to the UA.
The UA maintains a cache of Known HSTS Hosts.
UAs apply HSTS Policy whenever making a HTTP connection to a Known
HSTS Host, regardless of host port number. I.e., it applies to
all ports on a Known HSTS Host. Hosts are unable to affect this
aspect of HSTS Policy.
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Hosts may optionally declare that their HSTS Policy applies to all
subdomains of their host domain name.
In contrast, the Same-Origin Policy (SOP) [RFC6454] has the following
primary characteristics:
An origin is the scheme, host, and port of a URI identifying a
resource.
A UA may dereference a URI, thus loading a representation of the
resource the URI identifies. UAs label resource representations
with their origins, which are derived from their URIs.
The SOP refers to a collection of principles, implemented within
UAs, governing the isolation of and communication between resource
representations within the UA, as well as resource
representations' access to network resources.
In summary, although both HSTS Policy and SOP are enforced by UAs,
HSTS Policy is optionally declared by hosts and is not origin-based,
while the SOP applies to all resource representations loaded from all
hosts by conformant UAs.
Appendix C. Acknowledgments
The authors thank Devdatta Akhawe, Michael Barrett, Tobias Gondrom,
Paul Hoffman, Murray Kucherawy, James Manger, Alexey Melnikov, Yoav
Nir, Laksh Raghavan, Marsh Ray, Julian Reschke, Tom Ritter, Peter
Saint-Andre, Sid Stamm, Maciej Stachowiak, Andy Steingrubl, Brandon
Sterne, Martin Thomson, Daniel Veditz, as well as all the websec
working group participants and others for their review and
contributions.
Thanks to Julian Reschke for his elegant re-writing of the effective
request URI text, which he did when incorporating the ERU notion into
the HTTPbis work. Subsequently, the ERU text in this spec was lifted
from Julian's work in [I-D.draft-ietf-httpbis-p1-messaging-17] and
adapted to the [RFC2616] ABNF.
Appendix D. Change Log
[RFCEditor: please remove this section upon publication as an RFC.]
Changes are grouped by spec revision listed in reverse issuance
order.
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D.1. For draft-ietf-websec-strict-transport-sec
Changes from -07 to -08:
1. Clarified requirement #4 for STS header field directives in
Section 6.1, and removed "(which "update" this
specification)". Also added explicit "max-age=0" to Section
6.1.1. Reworked final sentence in 2nd para of Section 13.
This addresses issue ticket #45.
Changes from -06 to -07:
1. Various minor/modest editorial tweaks throughout as I went
through it pursuing the below issue tickets. Viewing a visual
diff against -06 revision recommended.
2. fixed some minor editorial issues noted in review by Alexey,
fixes noted in here:
3. Addressed ABNF exposition issues, specifically inclusion of
quoted-string syntax for directive values. Fix STS header
ABNF such that a leading ";" isn't required. Add example of
quoted-string-encoded max-age-value. This addresses (re-
opened) issue ticket #33.
4. Reworked sections 8.1 through 8.3 to ensure matching algorithm
and resultant HSTS Policy application is more clear, and that
it is explicitly stipulated to not muck with attributes of
superdomain matching Known HSTS Hosts. This addresses issue
ticket #37.
5. Added reference to [I-D.ietf-dane-protocol], pared back
extraneous discussion in section 2.2, and updated discussion
in 10.2 to accomodate TLSA (nee DANE). This addresses issue
ticket #39.
6. Addressed various editorial items from issue ticket #40.
7. Loosened up the language regarding redirecting "http" requests
to "https" in section 7.2 such that future flavors of
permanent redirects are accommodated. This addresses issue
ticket #43.
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8. Reworked the terminology and language in Section 9, in
particular defining the term "putative domain name string" to
replace "valid Unicode-encoded string-serialized domain name".
This addresses issue ticket #44.
Changes from -05 to -06:
1. Addressed various editorial comments provided by Tobias G.
This addresses issue ticket #38.
Changes from -04 to -05:
1. Fixed up references to move certain ones back to the normative
section -- as requested by Alexey M. Added explanation for
referencing obsoleted [RFC3490] and [RFC3492]. This addresses
issue ticket #36.
2. Made minor change to Strict-Transport-Security header field
ABNF in order to address further feedback as appended to
ticket #33. This addresses issue ticket #33.
Changes from -03 to -04:
1. Clarified that max-age=0 will cause UA to forget a known HSTS
host, and more generally clarified that the "freshest" info
from the HSTS host is cached, and thus HSTS hosts are able to
alter the cached max-age in UAs. This addresses issue ticket
#13.
2. Updated section on "Constructing an Effective Request URI" to
remove remaining reference to RFC3986 and reference RFC2616
instead. Further addresses issue ticket #14.
3. Addresses further ABNF issues noted in comment:1 of issue
ticket #27.
4. Reworked the introduction to clarify the denotation of "HSTS
policy" and added the new Appendix B summarizing the primary
characteristics of HSTS Policy and Same-Origin Policy, and
identifying their differences. Added ref to [RFC4732]. This
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addresses issue ticket #28.
5. Reworked language in Section 2.3.1.3. wrt "mixed content",
more clearly explain such vulnerability, disambiguate "mixed
content" in web security context from its usage in markup
language context. This addresses issue ticket #29.
6. Expanded Denial of Service discussion in Security
Considerations. Added refs to [RFC4732] and [CWE-113]. This
addresses issue ticket #30.
7. Mentioned in prose the case-insensitivity of directive names.
This addresses issue ticket #31.
8. Added Section 10.3 "Implications of includeSubDomains". This
addresses issue ticket #32.
9. Further refines text and ABNF definitions of STS header field
directives. Retains use of quoted-string in directive
grammar. This addresses issue ticket #33.
10. Added Section 14.7 "Creative Manipulation of HSTS Policy
Store", including reference to [WebTracking]. This addresses
issue ticket #34.
11. Added Section 14.1 "Ramifications of HSTS Policy
Establishment only over Error-free Secure Transport" and made
some accompanying editorial fixes in some other sections.
This addresses issue ticket #35.
12. Refined references. Cleaned out un-used ones, updated to
latest RFCs for others, consigned many to Informational.
This addresses issue ticket #36.
13. Fixed-up some inaccuracies in the "Changes from -02 to -03"
section.
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Changes from -02 to -03:
1. Updated section on "Constructing an Effective Request URI" to
remove references to RFC3986. Addresses issue ticket #14.
2. Reference RFC5890 for IDNA, retaining subordinate refs to
RFC3490. Updated IDNA-specific language, e.g. domain name
canonicalization and IDNA dependencies. Addresses issue
ticket #26
.
3. Completely re-wrote the STS header ABNF to be fully based on
RFC2616, rather than a hybrid of RFC2616 and httpbis.
Addresses issue ticket #27
.
Changes from -01 to -02:
1. Updated Section 8.3 "URI Loading and Port Mapping" fairly
thoroughly in terms of refining the presentation of the
steps, and to ensure the various aspects of port mapping are
clear. Nominally fixes issue ticket #1
2. Removed dependencies on
[I-D.draft-ietf-httpbis-p1-messaging-15]. Thus updated STS
ABNF in Section 6.1 "Strict-Transport-Security HTTP Response
Header Field" by lifting some productions entirely from
[I-D.draft-ietf-httpbis-p1-messaging-15] and leveraging
[RFC2616]. Addresses issue ticket #2
.
3. Updated Effective Request URI section and definition to use
language from [I-D.draft-ietf-httpbis-p1-messaging-15] and
ABNF from [RFC2616]. Fixes issue ticket #3
.
4. Added explicit mention that the HSTS policy applies to all
TCP ports of a host advertising the HSTS policy. Nominally
fixes issue ticket #4
5. Clarified the need for the "includeSubDomains" directive,
e.g. to protect Secure-flagged domain cookies. In
Section 14.2 "The Need for includeSubDomains". Nominally
fixes issue ticket #5
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6. Cited Firesheep as real-live threat in Section 2.3.1.1
"Passive Network Attackers". Nominally fixes issue ticket #6
.
7. Added text to Section 11 "User Agent Implementation Advice"
justifying connection termination due to tls warnings/errors.
Nominally fixes issue ticket #7
.
8. Added new subsection Section 8.6 "Missing Strict-Transport-
Security Response Header Field". Nominally fixes issue
ticket #8
.
9. Added text to Section 8.4 "Errors in Secure Transport
Establishment" explicitly note revocation check failures as
errors causing connection termination. Added references to
[RFC5280] and [RFC2560]. Nominally fixes issue ticket #9
.
10. Added a sentence, noting that distributing specific end-
entity certificates to browsers will also work for self-
signed/private-CA cases, to Section 10 "Server Implementation
and Deployment Advice" Nominally fixes issue ticket #10
.
11. Moved "with no user recourse" language from Section 8.4
"Errors in Secure Transport Establishment" to Section 11
"User Agent Implementation Advice". This nominally fixes
issue ticket #11
.
12. Removed any and all dependencies on
[I-D.draft-ietf-httpbis-p1-messaging-15], instead depending
on [RFC2616] only. Fixes issue ticket #12
.
13. Removed the inline "XXX1" issue because no one had commented
on it and it seems reasonable to suggest as a SHOULD that web
apps should redirect incoming insecure connections to secure
connections.
14. Removed the inline "XXX2" issue because it was simply for
raising consciousness about having some means for
distributing secure web application metadata.
15. Removed "TODO1" because description prose for "max-age" in
the Note following the ABNF in Section 6 seems to be fine.
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16. Decided for "TODO2" that "the first STS header field wins".
TODO2 had read: "Decide UA behavior in face of encountering
multiple HSTS headers in a message. Use first header?
Last?". Removed TODO2.
17. Added Section 1.1 "Organization of this specification" for
readers' convenience.
18. Moved design decision notes to be a proper appendix
Appendix A.
Changes from -00 to -01:
1. Changed the "URI Loading" section to be "URI Loading and Port
Mapping".
2. [HASMAT] reference changed to [WEBSEC].
3. Changed "server" -> "host" where applicable, notably when
discussing "HSTS Hosts". Left as "server" when discussing
e.g. "http server"s.
4. Fixed minor editorial nits.
Changes from draft-hodges-strict-transport-sec-02 to
draft-ietf-websec-strict-transport-sec-00:
1. Altered spec metadata (e.g. filename, date) in order to submit
as a WebSec working group Internet-Draft.
D.2. For draft-hodges-strict-transport-sec
Changes from -01 to -02:
1. updated abstract such that means for expressing HSTS Policy
other than via HSTS header field is noted.
2. Changed spec title to "HTTP Strict Transport Security (HSTS)"
from "Strict Transport Security". Updated use of "STS"
acronym throughout spec to HSTS (except for when specifically
discussing syntax of Strict-Transport-Security HTTP Response
Header field), updated "Terminology" appropriately.
3. Updated the discussion of "Passive Network Attackers" to be
more precise and offered references.
4. Removed para on nomative/non-normative from "Conformance
Criteria" pending polishing said section to IETF RFC norms.
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5. Added examples subsection to "Syntax" section.
6. Added OWS to maxAge production in Strict-Transport-Security
ABNF.
7. Cleaned up explanation in the "Note:" in the "HTTP-over-
Secure-Transport Request Type" section, folded 3d para into
"Note:", added conformance clauses to the latter.
8. Added exaplanatory "Note:" and reference to "HTTP Request
Type" section. Added "XXX1" issue.
9. Added conformance clause to "URI Loading".
10. Moved "Notes for STS Server implementors:" from "UA
Implementation dvice " to "HSTS Policy expiration time
considerations:" in "Server Implementation Advice", and also
noted another option.
11. Added cautionary "Note:" to "Ability to delete UA's cached
HSTS Policy on a per HSTS Server basis".
12. Added some informative references.
13. Various minor editorial fixes.
Changes from -00 to -01:
1. Added reference to HASMAT mailing list and request that this
spec be discussed there.
Authors' Addresses
Jeff Hodges
PayPal
2211 North First Street
San Jose, California 95131
US
Email: Jeff.Hodges@PayPal.com
Collin Jackson
Carnegie Mellon University
Email: collin.jackson@sv.cmu.edu
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Adam Barth
Google, Inc.
Email: ietf@adambarth.com
URI: http://www.adambarth.com/
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