Cisco Systems

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NIST

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Internet-Draft To improve cybersecurity posture, automation is necessary to locate what software is running on a device, whether that software has known vulnerabilities, and what, if any recommendations suppliers may have. This memo specifies a model to provide access this information. It may optionally be discovered through manufacturer usage descriptions.

A number of activities have been working to improve visibility to what software is running on a system, and what vulnerabilities that software may have. Put simply, we seek to answer two classes of questions at scale: Is this system vulnerable to a particular vulnerability? Which devices in a particular environment contain vulnerabilities that require some action? Software bills of material (SBOMs) are descriptions of what software, including versioning and dependencies, a device contains. There are different SBOM formats such as Software Package Data Exchange or CycloneDX. System vulnerabilities may similarly be described using several data formats, including the aforementioned CycloneDX, Common Vulnerability Reporting Framework , the Common Security Advisory Format . This information is typically used to report to customers the state of a system. These two classes of information can be used in concert. For instance, a network management tool may discover that a system makes use of a particular software component that has a known vulnerability, and a vulnerability report may be used to indicate what if any versions of software correct that vulnerability, or whether the system exercises the vulnerable code at all. Both classes of information elements are optional under the model specified in this memo. One can provide only an SBOM, only vulnerability information, or both an SBOM and vulnerability information. Note that SBOMs may also carry other information, the most common being any licensing terms. Because this specification is neutral regarding content, it is left for format developers such as the Linux Foundation, OASIS, and ISO to decide what attributes they will support. This memo specifies means by which both SBOMs and vulnerability information can be advertised and retrieved through the use of a YANG augmentation of the Manufacturer User Description (MUD) model . Note that the schema creates a grouping that can also be used independently of MUD. The mechanisms specified in this document are meant to satisfy several use cases: A network-layer management system retrieving information from an IoT device as part of its ongoing lifecycle. Such devices may or may not have query interfaces available. An application-layer management system retrieving vulnerability or SBOM information in order to evaluate the posture of an application server of some form. These application servers may themselves be containers or hypervisors. Discovery of the topology of a server is beyond the scope of this memo. To satisfy these two key use cases, objects may be found in one of three ways: on devices themselves on a web site (e.g., via URI) through some form of out-of-band contact with the supplier. In the first case, devices will have interfaces that permit direct retrieval. Examples of these interfaces might be an HTTP, COAP or endpoint for retrieval. There may also be private interfaces as well. In the second case, when a device does not have an appropriate retrieval interface, but one is directly available from the manufacturer, a URI to that information must be discovered. In the third case, a supplier may wish to make an SBOM or vulnerability information available under certain circumstances, and may need to individually evaluate requests. The result of that evaluation might be the SBOM or vulnerability itself or a restricted URL or no access. To enable application-layer discovery, this memo defines a well-known URI . Management or orchestration tools can query this well-known URI to retrieve a system’s SBOM or vulnerability information. Further queries may be necessary based on the content and structure of the response. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

[ This section to be removed prior to publication ] A separate use case may be addressed in future versions of this document: Related to the application layer, software as a service may involve multiple backend systems, depending on many factors. One example might be a large cloud-based service that offers spreadsheets, email, and document authoring and management. Depending on what service is being used, a different set of back end services may in turn be invoking different software that should be listed. The reason why this use case isn’t addressed here is that it may be better addressed inline within HTML. Further discussion is required.

For devices that can emit a URL or can establish a well-known URI, the mechanism may be highly automated. For devices that have a URL in either their documentation or within a QR code on a box, the mechanism is semi-automated (someone has to scan the QR code or enter the URL). Note that vulnerability and SBOM information is likely to change at different rates. The MUD semantics provide a way for manufacturers to control how often tooling should check for those changes through the cache-validity node.

There are multiple ways to express both SBOMs and vulnerability information. When these are retrieved either directly from the device or directly from a web server, tools will need to observe the content-type header to determine precisely which format is being transmitted. Because IoT devices in particular have limited capabilities, use of a specific Accept: header in HTTP or the Accept Option in CoAP is NOT RECOMMENDED. Instead, backend tooling is encouraged to support all known formats, and SHOULD silently discard SBOM information sent with a media type that is not understood. Some formats may support both vulnerability and software inventory information. When both vulnerability and software inventory information is available from the same location, both sbom and vuln nodes MUST indicate that. Network management systems retrieving this information MUST take note that the identical resource is being retrieved rather than retrieving it twice.

The following is discussion to be removed at time of RFC publication. Is the model structured correctly? Are there other retrieval mechanisms that need to be specified? Do we need to be more specific in how to authenticate and retrieve SBOMs? What are the implications if the MUD URL is an extension in a certificate (e.g. an IDevID cert)?

Three well known endpoints are defined: ”/.well-known/sbom” retrieves an SBOM. ”/.well-known/vuln” retrieves vulnerability information. ”/.well-known/openc2” is the HTTPS binding to OpenC2. As discussed previously, the precise format of a response is based on the Content-type provided.

We now formally define this extension. This is done in two parts. First, the extension name “transparency” is listed in the “extensions” array of the MUD file. N.B., this schema extension is intended to be used wherever it might be appropriate (e.g., not just MUD). Second, the “mud” container is augmented with a list of SBOM sources. This is done as follows:

file "ietf-mud-transparency@2021-07-06.yang" module ietf-mud-transparency { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-mud-transparency"; prefix mud-transparency; import ietf-inet-types { prefix inet; } import ietf-mud { prefix mud; } organization "IETF OPSAWG (Ops Area) Working Group"; contact "WG Web: http://tools.ietf.org/wg/opsawg/ WG List: opsawg@ietf.org Author: Eliot Lear lear@cisco.com Author: Scott Rose scott.rose@nist.gov"; description "This YANG module augments the ietf-mud model to provide for reporting of SBOMs. Copyright (c) 2020 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC XXXX (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself for full legal notices. The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document are to be interpreted as described in BCP 14 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here. "; revision 2021-07-06 { description "Initial proposed standard."; reference "RFC XXXX: Extension for software transparency"; } grouping transparency-extension { description "Transparency extension grouping"; container transparency { description "container of methods to get an SBOM."; choice sbom-retrieval-method { description "How to find SBOM information"; case cloud { list sboms { key "version-info"; description "A list of SBOMs tied to different s/w or h/w versions."; leaf version-info { type string; description "The version to which this SBOM refers."; } leaf sbom-url { type inet:uri; description "A statically located URI."; } } } case local-well-known { leaf sbom-local-well-known { type enumeration { enum http { description "Use http (insecure) to retrieve SBOM information."; } enum https { description "Use https (secure) to retrieve SBOM information."; } enum coap { description "Use COAP (insecure) to retrieve SBOM"; } enum coaps { description "Use COAPS (secure) to retrieve SBOM"; } enum openc2 { description "Use OpenC2 endpoint. This is https://{host}/.well-known/openc2"; } } description "Which communication protocol to choose."; } } case sbom-contact-info { leaf sbom-contact-uri { type inet:uri; mandatory true; description "This MUST be either a tel, http, https, or mailto uri schema that customers can use to contact someone for SBOM information."; } } } choice vuln-retrieval-method { description "How to find vulnerability information"; case cloud { leaf vuln-url { type inet:uri; description "A statically located URL."; } } case vuln-local-well-known { leaf vuln-local-well-known { type enumeration { enum http { description "Use http (insecure) to retrieve vulnerability information."; } enum https { description "Use https to retrieve vulnerability information."; } enum coap { description "Use COAP (insecure) to retrieve vulnerability information"; } enum coaps { description "Use COAPS to retrieve vulnerability information"; } enum openc2 { description "Use OpenC2 endpoint. This is https://{host}/.well-known/openc2"; } } description "What communication protocol to use."; } } case vuln-contact-info { leaf contact-uri { type inet:uri; mandatory true; description "This MUST be either a tel, http, https, or mailto uri schema that customers can use to contact someone for vulnerability information."; } } } } } augment "/mud:mud" { description "Add extension for software transparency."; uses transparency-extension; } } ]]>

In this example MUD file that uses a cloud service, the modelX presents a location of the SBOM in a URL. Note, the ACLs in a MUD file are NOT required, although they are a very good idea for IP-based devices.

This first MUD file demonstrates how to get SBOM and vulnerability information without ACLs.

The second example demonstrates that just SBOM information is included.

In this example, the SBOM is retrieved from the device, while vulnerability information is available from the cloud. This is likely a common case, because vendors may learn of vulnerability information more frequently than they update software.

In this example, the network manager must take further steps to retrieve SBOM information. Vulnerability information is still available.

Finally, here is a complete example where the device provides SBOM and vulnerability information, as well as access-control information.

At this point, the management system can attempt to retrieve the SBOM, and determine which format is in use through the content-type header on the response to a GET request, independently repeat the process for vulnerability information, and apply ACLs, as appropriate.

SBOMs provide an inventory of software. If firmware is available to an attacker, the attacker may well already be able to derive this very same software inventory. Manufacturers MAY restrict access to SBOM information using appropriate authorization semantics within HTTP. In particular, if a system attempts to retrieve an SBOM via HTTP and the client is not authorized, the server MUST produce an appropriate error, with instructions on how to register a particular client. One example may be to issue a certificate to the client for this purpose after a registration process has taken place. Another example would involve the use of OAUTH in combination with a federations of SBOM servers. Another risk is a skew in the SBOM listing and the actual software inventory of a device/container. For example, a manufacturer may update the SBOM on its server, but an individual device has not be upgraded yet. This may result in an incorrect policy being applied to a device. A unique mapping of a device’s firmware version and its SBOM can minimize this risk. To further mitigate attacks against a device, manufacturers SHOULD recommend access controls through the normal MUD mechanism. Vulnerability information is generally made available to such databases as NIST’s National Vulnerability Database. It is possible that vendor may wish to release information early to some customers. We do not discuss here whether that is a good idea, but if it is employed, then appropriate access controls and authoration would be applied to the vulnerability resource.

The IANA is requested to add “transparency” to the MUD extensions registry as follows:

The following well known URIs are requested in accordance with :

In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This memo specifies a component-based architecture for Manufacturer Usage Descriptions (MUDs). The goal of MUD is to provide a means for end devices to signal to the network what sort of access and network functionality they require to properly function. The initial focus is on access control. Later work can delve into other aspects.This memo specifies two YANG modules, IPv4 and IPv6 DHCP options, a Link Layer Discovery Protocol (LLDP) TLV, a URL, an X.509 certificate extension, and a means to sign and verify the descriptions. This memo defines a path prefix for "well-known locations", "/.well-known/", in selected Uniform Resource Identifier (URI) schemes.In doing so, it obsoletes RFC 5785 and updates the URI schemes defined in RFC 7230 to reserve that space. It also updates RFC 7595 to track URI schemes that support well-known URIs in their registry. The Linux Foundation cylonedx.org OASIS OASIS OASIS

Draft -02: include vulnerability information Draft -01: some modest changes Draft -00: Initial revision