]> Cisco Systems

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NIST

100 Bureau Dr Gaithersburg MD 20899 USA +1 301-975-8439 scott.rose@nist.gov

Internet-Draft To improve cybersecurity posture, automation is necessary to locate the software a device is using, and whether that software has known vulnerabilities, and what, if any recommendations suppliers may have. This memo extends the MUD YANG schema to provide the locations of software bills of materials (SBOMS) and to vulnerability information by introducing a transparency schema.

Introduction 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, this memo seeks to answer two classes of questions to the scale of tens of thousands of devices and a large variety of types of devices. Those questions are as the following: Is this system vulnerable 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 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 administrators the state of any known vulnerabilities on a system. SBOM and vulnerability information can be used in concert with other sources of vulnerability information. For a network management tool could discover that a system makes use of a particular set of software components, searches a national vulnerability database to determine known vulnerabilities, and then applies information provided 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's because vulnerability information changes occur at different rates to software updates. 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 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 methods: on devices themselves on a website (e.g., via URI) 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 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 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 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.

How This Information Is Retrieved Section 4 describes a data model to extend the MUD file format to carry SBOM and vulnerability information. Section 1.5 of RFC8520 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. Section 2 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 retrieving this information MUST take note that the identical resource is being retrieved rather than retrieving it twice.

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 model extension 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:

See for a description of YANG trees.

The mud-sbom augmentation to the MUD YANG model

file "ietf-mud-transparency@2023-01-12.yang" module ietf-mud-transparency { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-mud-transparency"; prefix mudtx; import ietf-inet-types { prefix inet; reference "RFC 6991"; } import ietf-mud { prefix mud; reference "RFC 8520"; } organization "IETF OPSAWG (Ops Area) Working Group"; contact "WG Web: https://datatracker.ietf.org/wg/opsawg/ WG List: opsawg@ietf.org Editor: Eliot Lear lear@cisco.com Editor: Scott Rose scott.rose@nist.gov"; description "This YANG module augments the ietf-mud model to provide for reporting of SBOMs and vulnerability information. 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 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 2023-01-12 { description "Initial proposed standard."; reference "RFC XXXX: Discovering and Retrieving Software Transparency and Vulnerability Information"; } identity local-type { description "Base identity for local-well-known choices"; } identity http { base mudtx:local-type; description "Use http[RFC7231] (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"; } identity https { base mudtx:local-type; description "Use https (secure) to retrieve SBOM information. See RFC 9110."; } identity coap { base mudtx:local-type; description "Use COAP [RFC7252] (insecure) to retrieve SBOM. This method is NOT RECOMMENDED, although it may be unavoidable for certain classes of implementations/deployments."; } identity coaps { base mudtx:local-type; description "Use COAPS (secure) to retrieve SBOM [RFC7252]"; } 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 either a tel, http, https, or 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 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; } } ]]>

Examples 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.

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, 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.

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 the NETCONF or RESTCONF , and hence the regular security considerations for such usage are not considered here. We describe below protections relating to both discovery and some advice on protecting the underlying SBOM/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 is 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 risk, any change in a URL, and in particular to the authority section, two approaches may be used: test any cloud-based URL against a reputation service. provide the administrator an opportunity to approve further procesisng 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 well 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 The IANA is requested to add "transparency" to the MUD extensions registry as follows:

YANG Registration The following YANG module should be registered in the "YANG Module Names" registry:

The following XML registration is requested:

Well-Known Prefix The following well known URI is requested in accordance with :

Acknowledgments Thanks to Russ Housley, Dick Brooks, Tom Petch, Nicolas Comstedt, who provided review comments.

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. The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs). This document obsoletes RFC 4741. [STANDARDS-TRACK] This document introduces a collection of common data types to be used with the YANG data modeling language. This document obsoletes RFC 6021. The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation.CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in the Network Configuration Protocol (NETCONF). 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. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the "http" and "https" Uniform Resource Identifier (URI) schemes. This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230. 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. United States Of America The Linux Foundation cyclonedx.org OASIS OASIS OASIS OASIS NIST This document captures the current syntax used in YANG module tree diagrams. The purpose of this document is to provide a single location for this definition. This syntax may be updated from time to time based on the evolution of the YANG language.

Changes from Earlier Versions [[This section to be removed by RFC Editor]] Please see https://github.com/elear/mud-sbom for changes.