SPDX 2.x has the concept of an External Reference for a Package to "reference an external source of additional information, metadata, enumerations, asset identifiers, or downloadable content believed to be relevant to the Package."

The specification for External Reference identifiers (Annex F) has four defined categories:

• Security: CPE, SWID tag identifier, or reference to security information
• Package-Manager: package identifier and locator
• Persistent-id: identifier which is guaranteed to remain stable (persistent) over time
• Other: Use if none of the above match your use case

This section provides usage scenarios of how to leverage the Security and Persistent-id category external references specified above to refer to external security information. A complete SPDX document using these can be found in the examples directory within the SPDX code repository. Multiple instances and types of external security information may be included within a SPDX document.

Note that identifiers (e.g. CPE, GitBOM, SWID) are spread throughout Annex F and sometimes locators refer to identifiers.

Including a reference to a Common Vulnerabilities and Exposures (CVE) advisory applicable to a package is shown in the example below. A SPDX creator should include current publicly known vulnerabilities at the time of document creation. SPDX consumers should always assume vulnerabilities enumerated by a SPDX creator to be out-of-date.

"externalRefs" : [ {
 "referenceCategory" : "SECURITY",
 "referenceLocator" : "https://nvd.nist.gov/vuln/detail/CVE-2020-29573",
 "referenceType" : "advisory"
}, {
 "referenceCategory" : "SECURITY",
 "referenceLocator" : "https://nvd.nist.gov/vuln/detail/CVE-2020-6096",
 "referenceType" : "advisory"
}, {
}, {
 "referenceCategory" : "SECURITY",
 "referenceLocator" : "https://nvd.nist.gov/vuln/detail/CVE-2020-3326",
 "referenceType" : "advisory"
} ]

To learn how to reference to CSAF formatted security information applicable to a package see the example below, and additional examples here and here.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://github.com/oasis-tcs/csaf/blob/master/csaf_2.0/examples/csaf/csaf_vex/2022-evd-uc-01-a-001.json",
  "referenceType" : "advisory"
} ]

To reference to CycloneDX formatted security information applicable to a package see the example below.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://raw.githubusercontent.com/CycloneDX/bom-examples/ed522d1f051c364e045b87c20665003a0c4ea777/SBOM/laravel-7.12.0/bom.json",
  "referenceType" : "advisory"
} ]

To learn how to include a reference to Open Source Vulnerability (OSV) formatted security information applicable to a package see the example below.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://github.com/github/advisory-database/tree/6b9d5bc96a62bb845ee71e4551a214eb1457e2c6/advisories/github-reviewed/2022/04/GHSA-2gwj-7jmv-h26r/GHSA-2gwj-7jmv-h26r.json",
  "referenceType" : "advisory"
} ]

To reference to GitBOM formatted security information applicable to a package see the example below.

"externalRefs" : [ {
  "referenceCategory" : "PERSISTENT-ID",
  "referenceLocator" : "gitoid:blob:sha1:d8bcd58df2b14818b8237bb70c979d62c7df5747",
  "referenceType" : "gitbom"
  "referenceComment" : "GitBOM Object Id for the HeartBleed fix in ssl/d1_both.c"
} ,
{
  "referenceCategory" : "PERSISTENT-ID",
  "referenceLocator" : "gitoid:blob:sha1:bcb99b819dadaebdf2c8f88d92ee9024c45f9df3",
  "referenceType" : "gitbom"
  "referenceComment" : "GitBOM Object Id for the HeartBleed fix in ssl/t1_lib.c"
} ]

To express a reference to a vulnerability disclosure document for a package such Cisco’s response to Apache log4j vulnerability.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-apache-log4j-qRuKNEbd",
  "referenceType" : "advisory"
} ]

To communicate that a package is not vulnerable to a specific vulnerability it is recommended to reference a web page indicating why given vulnerabilities are not applicable.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://example.com/product-x/security-info.html",
  "referenceType" : "advisory"
} ]

To refer to a security disclosure feed, such as the security bulletins from CERT-EU.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://cert.europa.eu/cert/Data/newsletter/reviewlatest-SecurityBulletins.xml",
  "referenceType" : "advisory"
} ]

To reference a code fix for a security issue applicable to a package see the example below. In this example, the link points to a specific code revision containing the fix for CVE-2020-28498.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://github.com/indutny/elliptic/commit/441b7428b0e8f6636c42118ad2aaa186d3c34c3f",
  "referenceType" : "fix"
} ]

A fix reference may point to a configuration change for example the patch file as one of the fixes for CVE-2022-26499.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://downloads.digium.com/pub/security/AST-2022-002-16.diff",
  "referenceType" : "fix"
} ]

Alternatively, it may also link to a landing page with patches for a variety of products such as Oracle patch information for CVE-2021-44228.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://www.oracle.com/security-alerts/cpujan2022.html",
  "referenceType" : "fix"
} ]

If you want to reference any security information related to a package but cannot or do not wish to specify its kind, use the url referenceType.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://github.com/christianlundkvist/blog/blob/aa3a69b5e4c06e4435070610c0c4a2b1e8731783/2020_05_26_secp256k1_twist_attacks/secp256k1_twist_attacks.md",
  "referenceType" : "url"
} ]

One can also use it to refer to guidance related to a vulnerability such as CISA guidance for Apache Log4j.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://www.cisa.gov/uscert/apache-log4j-vulnerability-guidance",
  "referenceType" : "url"
} ]

The National Institute of Standards and Technology (NIST) describes the concept of correlating vulnerability and SBOM information for a software product at the component level in “Software Security in Supply Chains: Software Bill of Materials (SBOM)”. Use the ExternalRefs SECURITY category and advisory referenceType to report on vulnerabilities related to the components contained in a software product’s SBOM.

This enables a software producer to articulate to software consumers the status of vulnerabilities contained in the software product, by means of reporting vulnerability information at either the SBOM document or component level.

Providing a link to such data at the time the SBOM is published provides a pointer for where to find this relevant vulnerability information without promulgating vulnerability information inside the SBOM. This is advantageous because the vulnerability information has a short shelf-life (it will change frequently) while the SBOM component data isn’t likely to change if the software has not changed.

"externalRefs" : [ {
  "referenceCategory" : "SECURITY",
  "referenceLocator" : "https://github.com/rjb4standards/REA-Products/blob/master/SBOM_and_VDRbaseline/sag-pm-118_VDR.json",
  "referenceType" : "advisory"
} ]

US Executive Order 14028 in conjunction with the National Telecommunications and Information Administration (NTIA) outlined minimum elements for an SBOM. The minimum elements are detailed in NTIA's Framing Software Component Transparency: Establishing a Common Software Bill of Maternials and The Minimum Elements for a SBOM documents and summarized below:

The SPDX Specification contains fields able to address each of the NTIA minimum required data fields.

Several use cases for SPDX depend on the consumer being able to verify the provenance and integrity of their software. SPDX can support several different scenarios depending on what information is available to the producer, what information is available to the consumer, and how the SPDX document is delivered. These scenarios are described below along with recommended approaches to verifying the SPDX packages.

If a Package can be represented as a single blob of bytes, such as a tar archive:

• PackageChecksum must be computed by applying one of the supported hashing algorithms to the package blob.
• PackageDownloadLocation should be a download locator that retrieves the package blob.
• A supplier can define a PackageChecksum in a Package without providing a PackageDownloadLocation. This lets consumers perform an offline verification of private blobs.

If a Package represents an artifact that logically binds a number of single files together (such as a zip file or a directory):

• If the files bound by the Package are described in the document, PackageVerificationCode should be computed by using the files' SHA1 checksums. Additionally, the FilesAnalyzed field in the Package MUST be set to true.
• If the SHA1 checksum of any files bound by the Package is not available or the File needs to be excluded from the computation, it MUST be marked so by appending (excludes: FileName) at the end of the package verification code value.

Examples include: tarball binding one or more files to a SPDX package, installation file which installs the package and extracts SPDX document in the same directory

SPDX Field To Use: 7.9 Package verification code

Guidance:

• With the SPDX document included in the archive, it is not possible for the SPDX document to include a checksum for the archive itself. Generate a Package verification code and include the SPDX Document file name in the Excluded Files field.

Examples include: tarball, installation file

SPDX Field to Use: 7.10 Package checksum

Guidance:

• Generate a checksum for the archive file and include it in the SPDX Package checksum field. The archive file name should also be included in the Package file name field.
• If source files for the Package are included in the Package distribution archive, the Package verification code for that Package should also be included in the SPDX document and the Files analyzed field should be set to true.

Examples include: tarball, binary image, single executable

SPDX Field To Use: 7.10 Package checksum

Guidance:

• If a Package download location exists, the Package checksum should be the cryptographic hash of the Package blob at the Package download location specified.
• If the Package download location is not known, not available, or not accessible to the software consumer, the producer should include a Package checksum for the included Package file.

Examples include: source code, containers

SPDX Field To Use: 7.9 Package verification code

Guidance:

• Include File name field in the SPDX document for every file in the directory, include each file’s cryptographic hash as a File checksum, create a CONTAINS relationship between the Package and the files, and set Files analyzed to true on the Package. Note: if Files analyzed is set to false you cannot provide a Package verification code.