]> Cisco Systems

Richtistrasse 7 Wallisellen 8304 Switzerland +41 44 878 9200 lear@cisco.com

NIST

100 Bureau Dr. Gaithersburg MD 20899 United States of America +1 301-975-8439 scott.rose@nist.gov

ops opsawg sbom discovery mud vex chaff To improve cybersecurity posture, automation is necessary to locate the software a device is using, whether that software has known vulnerabilities, and what, if any, recommendations suppliers may have. This memo extends the Manufacturer User Description (MUD) YANG schema to provide the locations of software bills of materials (SBOMs) and vulnerability information by introducing a transparency schema.

Introduction A number of activities have taken place to improve the visibility of what software is running on a system and what vulnerabilities that software may have . Put simply, this memo seeks to answer two classes of questions for tens of thousands of devices and a large variety of device types. Those questions are as follows:

• Is this system susceptible to a particular vulnerability?
• Which devices in a particular environment contain vulnerabilities that require some action?

This memo doesn't specify the format of this information but rather only how to locate and retrieve these objects. That is, the model is intended to facilitate discovery and on its own provides no access to the underlying data. Software bills of materials (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 be similarly described using several data formats, including the aforementioned CycloneDX, the Common Vulnerability Reporting Framework , and the Common Security Advisory Format . This information is typically used to report the state of any known vulnerabilities on a system to administrators. SBOM and vulnerability information can be used in concert with other sources of vulnerability information. A network management tool could discover that a system uses a particular set of software components, searches a national vulnerability database to determine known vulnerabilities, and applies information provided by the manufacturer through this mechanism to produce a vulnerability report. That 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 SBOM formats 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 does not specify how vulnerability information may be retrieved directly from the endpoint. That is because vulnerability information changes occur to software updates at different rates. However, some SBOM formats may also contain vulnerability information. SBOMs and vulnerability information are 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. Moreover, other MUD features, such as access controls, needn't be present. The mechanisms specified in this document are meant to address two use cases:

• A network-layer management system retrieving information from an Internet of Things (IoT) device as part of its ongoing life cycle. 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 methods:

1. on the devices themselves
2. on a website (e.g., via a URI)
3. through some form of out-of-band contact with the supplier

Using the first method, devices will have interfaces that permit direct retrieval. Examples of these interfaces might be an HTTP or Constrained Application Protocol (CoAP) endpoint for retrieval. There may also be private interfaces as well. Using the second method, when a device does not have an appropriate retrieval interface, but one is directly available from the manufacturer, a URI to that information is discovered through interfaces such as MUD via DHCP or bootstrapping and ownership transfer mechanisms. Using the third method, 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, the vulnerability itself, 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 information. Further queries may be necessary based on the content and structure of the response.

Requirements Language 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.

How This Information Is Retrieved describes a data model to extend the MUD file format to carry SBOM and vulnerability information. describes mechanisms by which devices can emit a URL to point to this file. Additionally, devices can share this URL either through documentation or within a QR code on a box. describes a well-known URL from which an SBOM could be served from the local device. Note that vulnerability and SBOM information are likely to change at different rates. MUD's cache-validity node provides a way for manufacturers to control how often tooling should check for those changes through the cache-validity node.

Formats There are multiple ways to express both SBOMs and vulnerability information. When these are retrieved either from the device or from a remote 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. If multiple SBOMs are intended to be supported in the same file, the media type should properly reflect that. For example, one might make use of application/{someformat}+json-seq. It is left to those supporting those formats to make the appropriate registrations in this case. Some formats may support both vulnerability and software inventory information. When both vulnerability and software inventory information is available from the same URL, both sbom-url and members of the vuln-url list MUST indicate that. Network management systems MUST take note of when the SBOM and vulnerability information are accessible via the same resource and not retrieve the resource a second time.

The Well-Known Transparency Endpoint Set A well-known endpoint is defined: "/.well-known/sbom" retrieves an SBOM As discussed previously, the precise format of a response is based on the Content-Type provided.

The mud-transparency Extension We now formally define the mud-transparency extension; this is done in two parts. First, the extension name "transparency" is listed in the "extensions" array of the MUD file. Note that this schema extension is intended to be used wherever it might be appropriate (e.g., not just with MUD). Second, the "mud" container is augmented with a list of SBOM sources. This is done as follows: See for a description of YANG trees.

The mud-sbom Augmentation to the MUD YANG Data Model This YANG module references , , , , and . WG List: Editor: Eliot Lear Editor: Scott Rose "; description "This YANG module augments the ietf-mud model to provide for reporting of SBOMs and vulnerability information. 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. Copyright (c) 2023 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 Revised 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 9472 (https://www.rfc-editor.org/info/rfc9472); see the RFC itself for full legal notices."; revision 2023-10-10 { description "Initial proposed standard."; reference "RFC 9472: A YANG Data Model for Reporting Software Bills of Materials (SBOMs) and Vulnerability Information"; } identity local-type { description "Base identity for local well-known choices."; } identity http { base mudtx:local-type; description "Use http (RFC 7231) (insecure) to retrieve SBOM information. This method is NOT RECOMMENDED but may be unavoidable for certain classes of deployment where TLS has not or cannot be implemented."; reference "RFC 7231: Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content"; } identity https { base mudtx:local-type; description "Use https (secure) to retrieve SBOM information. See RFC 9110."; reference "RFC 9110: HTTP Semantics"; } identity coap { base mudtx:local-type; description "Use COAP (RFC 7252) (insecure) to retrieve SBOM. This method is NOT RECOMMENDED, although it may be unavoidable for certain classes of implementations/deployments."; reference "RFC 7252: The Constrained Application Protocol (CoAP)"; } identity coaps { base mudtx:local-type; description "Use COAPS (secure) to retrieve SBOM (RFC 7252)."; } grouping transparency-extension { description "This grouping provides a means to describe the location of software bills of material and vulnerability descriptions."; container transparency { description "Container of methods to get SBOMs and vulnerability information."; 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 software or hardware versions."; leaf version-info { type string; description "The version to which this SBOM refers."; } leaf sbom-url { type inet:uri { pattern '((coaps?)|(https?)):.*'; } description "A statically located URL."; } } } case local-well-known { leaf sbom-local-well-known { type identityref { base mudtx:local-type; } description "Which communication protocol to choose."; } } case sbom-contact-info { leaf sbom-contact-uri { type inet:uri { pattern '((mailto)|(https?)|(tel)):.*'; } description "This MUST be a tel, an http, an https, or a mailto uri schema that customers can use to contact someone for SBOM information."; } } } leaf sbom-archive-list { type inet:uri; description "This URI returns a JSON list of URLs that consist of SBOMs that were previously published for this device. Publication dates can be found inside the SBOMs."; } choice vuln-retrieval-method { description "How to find vulnerability information."; case cloud { leaf-list vuln-url { type inet:uri; description "List of statically located URLs that reference vulnerability information."; } } case vuln-contact-info { leaf vuln-contact-uri { type inet:uri { pattern '((mailto)|(https?)|(tel)):.*'; } description "This MUST be a tel, an http, an https, or a 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; } } ]]>

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

Without ACLS This first MUD file demonstrates how to get SBOM and vulnerability information without ACLs. The second example demonstrates that just SBOM information is included from the cloud.

SBOM Located on the Device In the next example, the SBOM is located on the device, and there is no vulnerability information provided. 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.

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

With ACLS 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, 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.

Security Considerations This document describes a schema for discovering the location of information relating to software transparency and does not specify the access model for the information itself. In particular, the YANG module specified in this document is not necessarily intended to be accessed via regular network management protocols, such as NETCONF or RESTCONF , and hence the regular security considerations for such usage are not considered here. Below, we describe protections relating to both discovery and some advice on protecting the underlying SBOM and vulnerability information. The model specifies both encrypted and unencrypted means to retrieve information. This is a matter of pragmatism. Unencrypted communications allow for manipulation of information being retrieved. Therefore, it is RECOMMENDED that implementations offer a means to configure endpoints so that they may make use of TLS or DTLS. The ietf-mud-transparency module has no operational impact on the element itself and is used to discover state information that may be available on or off the element. In as much as the module itself is made writeable, this only indicates a change in how to retrieve read-only elements. There are no means, for instance, to upload an SBOM. Additional risks are discussed below and are applicable to all nodes within the transparency container. If an attacker modifies the elements, they may misdirect automation to retrieve a different set of URLs than was intended by the designer. This in turn leads to two specific sets of risks:

• the information retrieved would be false
• the URLs themselves point to malware

To address either of these risks or any tampering of a URL:

• test any cloud-based URL against a reputation service
• provide the administrator an opportunity to approve further processing when the authority changes to one not known to be reputable

SBOMs provide an inventory of software. Knowledge of which specific software is loaded on a system can aid an attacker in identifying an appropriate exploit for a known vulnerability or guide the development of novel exploit against this system. However, if software is available to an attacker, the attacker may already be able to derive this very same software inventory. When this information resides on the endpoint itself, the endpoint SHOULD NOT provide unrestricted access to the well-known URL by default. Other servers that offer the data MAY restrict access to SBOM information using appropriate authorization semantics within HTTP. One way to do this would be to issue a certificate to the client for this purpose after a registration process has taken place. Another approach would involve the use of OAuth in combination. In particular, if a system attempts to retrieve an SBOM via HTTP or CoAP and the client is not authorized, the server MUST produce an appropriate error with instructions on how to register a particular client. 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 been upgraded yet. This may result in an incorrect policy being applied to a device. A unique mapping of a device's software version and its SBOM can minimize this risk. To further mitigate attacks against a device, manufacturers SHOULD recommend network access controls. Vulnerability information is generally made available to such databases as NIST's National Vulnerability Database . It is possible that vendors 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 authorization SHOULD be applied to that information.

IANA Considerations

MUD Extension IANA has added "transparency" to the "MUD Extensions" registry as follows:

Value:

transparency

Reference:

RFC 9472

YANG Registration IANA has registered the following YANG module in the "YANG Module Names" registry :

Name:

ietf-mud-transparency

Namespace:

urn:ietf:params:xml:ns:yang:ietf-mud-transparency

Maintained by IANA:

N

Prefix:

mudtx

Reference:

RFC 9472

The following URI has been registered in the "IETF XML Registry" :

URI:

urn:ietf:params:xml:ns:yang:ietf-mud-transparency

Registrant Contact:

IESG

XML:

None. Namespace URIs do not represent an XML specification.

Well-Known Prefix IANA has added the following URI suffix to the "Well-Known URIs" registry in accordance with :

URI Suffix:

sbom

Change Controller:

IETF

Reference:

RFC 9472

Status:

permanent

Related Information:

See ISO/IEC 5962:2021 and SPDX.org

References Normative References Informative References The White House EO 14028 The Linux Foundation Version 2.3 CycloneDX Version 1.5.0 OASIS OASIS OASIS OASIS Standard OASIS NIST

Acknowledgments Thanks to , , , and , who provided review comments.