SMTP TLS Reporting

The STARTTLS extension to SMTP allows SMTP clients and hosts to establish secure SMTP sessions over TLS. The protocol design is based on "Opportunistic Security" (OS) , which provides interoperability for clients that do not support STARTTLS but means that any attacker who can delete parts of the SMTP session (such as the "250 STARTTLS" response) or redirect the entire SMTP session (perhaps by overwriting the resolved MX record of the delivery domain) can perform a downgrade or interception attack. Because such "downgrade attacks" are not necessarily apparent to the receiving MTA, this document defines a mechanism for sending domains to report on failures at multiple parts of the MTA-to-MTA conversation. Recipient domains may also use the mechanisms defined by MTA-STS (TODO: Add ref) or DANE to publish additional encryption and authentication requirements; this document defines a mechanism for sending domains that are compatible with MTA-STS or DANE to share success and failure statistics with recipient domains. Specifically, this document defines a reporting schema that covers failures in routing, STARTTLS negotiation, and both DANE and MTA-STS (TODO: Add ref) policy validation errors, and standard TXT record that recipient domains can use to indicate where reports in this format should be sent. This document is intended as a companion to the specification for SMTP MTA Strict Transport Security (MTA-STS, TODO: Add ref).

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in . We also define the following terms for further use in this document: STS Policy: A definition of the expected TLS availability and behavior, as well as the desired actions for a given domain when a sending MTA encounters different results. TLSRPT Policy: A policy detailing the endpoint to which sending MTAs should deliver reports. Policy Domain: The domain against which an STS Policy is defined. Sending MTA: The MTA initiating the delivery of an email message.

This document is intended as a companion to the specification for SMTP MTA Strict Transport Security (MTA-STS, TODO: Add ref). The Public Key Pinning Extension for HTTP contains a JSON-based definition for reporting individual pin validation failures. The Domain-based Message Authentication, Reporting, and Conformance (DMARC) contains an XML-based reporting format for aggregate and detailed email delivery errors.

SMTP TLSRPT policies are distributed via DNS from the Policy Domain's zone, as TXT records (similar to DMARC policies) under the name _smtp_tlsrpt. For example, for the Policy Domain example.com, the recipient's SMTP STS policy can be retrieved from _smtp_tlsrpt.example.com. (Future implementations may move to alternate methods of policy discovery or distribution. See the section Future Work for more discussion.) Policies consist of the following directives: v: This value MUST be equal to TLSRPTv1. rua: A URI specifying the endpoint to which aggregate information about policy failures should be sent (see the section Reporting Schema for more information). Two URI schemes are supported: mailto and https.

* In the case of `https`, reports should be submitted via POST ([@!RFC2818]) to the specified URI. * In the case of `mailto`, reports should be submitted to the specified email address. When sending failure reports via SMTP, sending MTAs MUST NOT honor SMTP STS or DANE TLSA failures. ruf: Future use. (There may also be a need for enabling more detailed "forensic" reporting during initial stages of a deployment. To address this, the authors consider the possibility of an optional additional "forensic reporting mode" in which more details--such as certificate chains and MTA banners--may be reported. See the section Future Work for more details.) The formal definition of the _mta_sts TXT record, defined using , is as follows:

sts-text-record = sts-version *WSP %x3B *WSP sts-id sts-version = "v" *WSP "=" *WSP %x54 %x4C %x53 %x52 %x50 %x54 %x76 %x31 sts-id = "id" *WSP "=" *WSP 1*32(ALPHA / DIGIT)

_smtp_tlsrpt.mail.example.com. IN TXT \ "v=TLSRPTv1;rua=mailto:reports@example.com"

_smtp_tlsrpt.mail.example.com. IN TXT \ "v=TLSRPTv1; \ rua=https://reporting.example.com/v1/tlsrpt"

Aggregate reports contain the following fields: Report metadata: The organization responsible for the report Contact information for one or more responsible parties for the contents of the report A unique identifier for the report The reporting date range for the report Policy, consisting of: One of the following policy types: The SMTP MTA STS policy applied (as a string) The DANE TLSA record applied (as a string) * The literal string no-policy-found, if neither a TLSA nor

MTA-STS policy could be found. The domain for which the policy is applied The MX host An identifier for the policy (where applicable) Aggregate counts, comprising result type, sending MTA IP, receiving MTA hostname, message count, and an optional additional information field containing a URI for recipients to review further information on a failure type. Note that the failure types are non-exclusive; an aggregate report MAY contain overlapping counts of failure types where a single send attempt encountered multiple errors.

The list of result types will start with the minimal set below, and is expected to grow over time based on real-world experience. The initial set is:

success: This indicates that the sending MTA was able to successfully negotiate a policy-compliant TLS connection, and serves to provide a "heartbeat" to receiving domains that reporting is functional and tabulating correctly.

mx-mismatch: This indicates that the MX resolved for the recipient domain did not match the MX constraint specified in the policy. certificate-host-mismatch: This indicates that the certificate presented by the receiving MX did not match the MX hostname.

starttls-not-supported: This indicates that the recipient MX did not support STARTTLS. invalid-certificate: This indicates that the certificate presented by the receiving MX did not validate. certificate-host-mismatch: This indicates that the certificate presented did not adhere to the constraints specified in the STS or DANE policy, e.g. if the CN field did not match the hostname of the MX. certificate-name-constraints-not-permitted: The certificate request contains a name that is not listed as permitted in the name constraints extension of the cert issuer. certificate-name-constraints-excluded: The certificate request contains a name that is listed as excluded in the name constraints extension of the issuer. expired-certificate: This indicates that the certificate has expired.

tlsa-invalid: This indicates a validation error in the TLSA record associated with a DANE policy. dnssec-invalid: This indicates a failure to authenticate DNS records for a Policy Domain with a published TLSA record.

sts-invalid: This indicates a validation error for the overall MTA-STS policy. webpki-invalid: This indicates that the MTA-STS policy could not be authenticated using PKIX validation.

Note that, when sending failure reports via SMTP, sending MTAs MUST NOT honor SMTP STS or DANE TLSA failures.

There are no IANA considerations at this time.

SMTP TLS Reporting provides transparency into misconfigurations or attempts to intercept or tamper with mail between hosts who support STARTTLS. There are several security risks presented by the existence of this reporting channel: Flooding of the Aggregate report URI (rua) endpoint: An attacker could flood the endpoint and prevent the receiving domain from accepting additional reports. This type of Denial-of-Service attack would limit visibility into STARTTLS failures, leaving the receiving domain blind to an ongoing attack. Untrusted content: An attacker could inject malicious code into the report, opening a vulnerability in the receiving domain. Implementers are advised to take precautions against evaluating the contents of the report. Report snooping: An attacker could create a bogus TLSRPT record to receive statistics about a domain the attacker does not own. Since an attacker able to poison DNS is already able to receive counts of SMTP connections (and, absent DANE or MTA-STS policies, actual SMTP message payloads) today, this does not present a significant new vulnerability.

The JSON schema is derived from the HPKP JSON schema (cf. Section 3)

{ "organization-name": organization-name, "date-range": { "start-datetime": date-time, "end-datetime": date-time }, "contact-info": email-address, "report-id": report-id, "policy": { "policy-type": policy-type, "policy-string": policy-string, "policy-domain": domain, "mx-host": mx-host-pattern }, "report-items": [ { "result-type": result-type, "sending-mta-ip": ip-address, "receiving-mx-hostname": receiving-mx-hostname, "message-count": message-count, "additional-information": additional-info-uri } ] } organization-name: The name of the organization responsible for the report. It is provided as a string. date-time: The date-time indicates the start- and end-times for the report range. It is provided as a string formatted according to Section 5.6, "Internet Date/Time Format", of . email-address: The contact information for a responsible party of the report. It is provided as a string formatted according to Section 3.4.1, "Addr-Spec", of . report-id: A unique identifier for the report. Report authors may use whatever scheme they prefer to generate a unique identifier. It is provided as a string. policy-type: The type of policy that was applied by the sending domain. Presently, the only two valid choices are tlsa and sts. It is provided as a string. policy-string: The string serialization of the policy, whether TLSA record or STS policy. Any linefeeds from the original policy MUST be replaced with [SP]. TODO: Help with specifics. domain: The Policy Domain upon which the policy was applied. For messages sent to user@example.com this field would contain example.com. It is provided as a string. mx-host-pattern: The pattern of MX hostnames from the applied policy. It is provided as a string, and is interpreted in the same manner as the "Checking of Wildcard Certificates" rules in Section 6.4.3 of . result-type: A value from the Result Types section above. ip-address: The IP address of the sending MTA that attempted the STARTTLS connection. It is provided as a string representation of an IPv4 or IPv6 address in dot-decimal or colon-hexadecimal notation. receiving-mx-hostname: The hostname of the receiving MTA MX record with which the sending MTA attempted to negotiate a STARTTLS connection. message-count: The number of (attempted) messages that match the relevant result-type for this section. additional-info-uri: An optional URI pointing to additional information around the relevant result-type. For example, this URI might host the complete certificate chain presented during an attempted STARTTLS session.

{ "organization-name": "Company-X", "date-range": { "start-datetime": "2016-04-01T00:00:00Z", "end-datetime": "2016-04-01T23:59:59Z" }, "contact-info": "sts-reporting@company-x.com", "report-id": "5065427c-23d3-47ca-b6e0-946ea0e8c4be", "policy": { "policy-type": "sts", "policy-string": "TODO: Add me", "policy-domain": "company-y.com", "mx-host": "*.mail.company-y.com" }, "report-items": [{ "result-type": "ExpiredCertificate", "sending-mta-ip": "98.136.216.25", "receiving-mx-hostname": "mx1.mail.company-y.com", "message-count": 100 }, { "result-type": "StarttlsNotSupported", "sending-mta-ip": "98.22.33.99", "receiving-mx-hostname": "mx2.mail.company-y.com", "message-count": 200, "additional-information": "hxxps://reports.company-x.com/ report_info?id=5065427c-23d3#StarttlsNotSupported" }] }