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Software Assurance Tips
A product of the Software Assurance Tips Team[2]
Generated Saturday 3rd July, 2021

Jon Hood

Monday 5th July, 2021

1 A Pedigree of Dropping S-BOMs

Recently, a large software project encountered a serious vulnerability. The project has been in existence since the early 1990s and consists of millions of lines of code. On May 20th, CISA issued an advisory that resulted in several CVEs being created for some of the older RTOSes.[1] When we marked this as a critical finding against the legacy system, the response we received was, “I didn’t even know that was in there!”

1.1 Establishing a Pedigree

While the DoD is still proceeding with its RMF implementation of NIST 800-53 Revision 4, Revision 5 of the RMF controls creates a new family: SR - Supply Chain Risk Management. One of the new controls is SR-4, related to the provenance of the supply chain. To date, no categorization baseline requires the implementation of SR-4; however, it should be anticipated that this control will be tailored in for high-importance and tactical systems in the future.

Part of SR-4 is enhancement SR-4(4) which requires the establishment of provenance and pedigree by keeping up with the internal composition of software and hardware components. “For software this includes the composition of open-source and proprietary code, including the version of the component at a given point in time.”[3, p. 66] This is a step above the hardware and software lists currently implemented in RMF; the program must manage the composition at a more granular level. Claiming not to know that a component includes a dependency would be a failure against this control.

1.2 Compliance

For software, I recommend implementing Software Package Data Exchange (SPDX) and a Software Bill of Materials (S-BOM). A compliant policy would include a statement like the following:

Our organization requires that each main software delivery must define a Software Package Data Exchange (SPDX) file and Software Bill of Materials (S-BOM) detailing all first-order dependencies.[4] The SPDX file, at a minimum, must include the PackageName and PackageLicenseDeclared. If they are available, PackageOriginator and PackageHomePage must also be provided. Each main delivery product will also provide an S-BOM consisting of an SPDX file for each first-order dependency when that first order dependency properly manages its own dependencies. For example, if Product A is the deliverable and it depends on Product B, Product B is a first-order dependency. If Product B depends on Product C, Product C is a second-order dependency and only requires documentation in the S-BOM if it is also a first-order dependency or if Product B requires Product A to manage its dependencies.

For every major release of the deliverable or every three years (whichever occurs first), a Software Composition Analysis is performed, and the S-BOM’s SPDX files are compared to the results. The software composition analysis may be conducted automatically as part of the CI/CD pipeline or in our Software Assurance assessments. Undocumented dependencies are triaged as security concerns in our issue tracking system.

Such a policy requires listings of dependencies with their version numbers, encourages developers to automate composition analysis, documents the POCs for each dependency, and records the license restrictions of each component.

1.3 Recommendations

The Application Sescurity and Development Security Technical Implementation Guide (STIG) should be updated to check that dependencies are documented appropriately. There is not currently a STIG requirement to document and manage dependencies.

When issuing Control Correlation Identifiers (CCI) against SR-4(4), the DoD should divide this into at least two checks: one for documenting the composition correctly, and the other for verifying the integrity and correctness of that composition.

References

[1]

US-CERT. ICS Advisory (ICSA-21-119-04): Multiple RTOS (Update B). CISA. May 20, 2021. url: https://us-cert.cisa.gov/ics/advisories/icsa-21-119-04 (visited on 06/23/2021).

[2]

Jon Hood, ed. SwATips. https://www.SwATips.com/.

[3]

National Institute of Standards and Technology. Security and Privacy Controls for Information Systems and Organizations. Tech. rep. Special Publication (SP) 800-53 Revision 5. Washington, D.C.: U.S. Department of Commerce, 2001. doi: 10 . 6028 / NIST . SP . 800 - 53r5. url: https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-53r5.pdf.

[4]

David A. Wheeler. SPDX Tutorial. url: https://github.com/david-a-wheeler/spdx-tutorial/blob/cee3cbe7ae5f83ec478e2acf2c9282eafd42ff0f/README.md (visited on 06/23/2021).