[Cscwg-public] Signature authorization checks for signed code (SBOM)

Mon Jun 14 07:51:05 UTC 2021

Hello All,

Attempting to send this again, as the previous E-Mail didn't seem to display the message body. The below has been discussed on the meeting of June 3rd. There no immediate action that could be taken but a comment is warranted nevertheless.

With the current state of technology, certificate issuers have no visibility of the later use of CodeSigning certificates so that authorization checks when signing code cannot be performed by certificate issuers. However, the working group acknowledges that in addition to guaranteeing the authenticity and integrity of code with digital signatures a guarantee that the signer is an authorized party would contribute to the security of the whole ecosystem. The working group would like to encourage any certificate consumer or even third party who intends to address this problem to present their approach.

There's a special case for certificate issuer operated signing services, where certificate issuers have visibility of the code that is being signed. Such signing services will be discussed and addressed by the working group in greater detail during the coming months. The authorization of an entity to sign certain software/applications/code will not be a primary concern but definitely receive some attention during the discussion as well.

Kind regards,

Seb

Sebastian Schulz
Product Manager Client Certificates

From: Sebastian Schulz
Sent: 04 June 2021 09:53
To: cscwg-public at cabforum.org
Subject: Signature authorization checks for signed code (SBOM)

Hello All,

I wanted to bring something to your attention that a partner of ours has shared with us. In short, in securing software supply chains there's some concern about tools for the validation of code signing signatures (e.g. SignTool) not taking into account whether the signer is actually authorized to sign that software package. As a suggestion to resolving this problem, a system similar to DNS CAA is being suggested (while being well aware that it could not work the exact same way).

Full message below:

Copied from https://energycentral.com/c/ec/we-cannot-secure-software-supply-chain-without-sbom

A Software Bill of Materials (SBOM)<https://ntia.gov/sbom> has received considerable attention since the Cybersecurity Executive order was released on May 12, 2021<https://www.whitehouse.gov/briefing-room/presidential-actions/2021/05/12/executive-order-on-improving-the-nations-cybersecurity/> calling on government agencies to require SBOM's from software vendors, "(vii)   providing a purchaser a Software Bill of Materials (SBOM) for each product directly or by publishing it on a public website"

The Executive Order goes on to describe an SBOM, with an emphasis on it's component inventory capabilities, "Software Bill of Materials" or "SBOM" means a formal record containing the details and supply chain relationships of various components used in building software", "It is analogous to a list of ingredients on food packaging."

All of these claims are indeed accurate, however there is another essential and critically important element that SBOM's provide, which is essential to secure the software supply chain; The identity of the software source supplier that produced and licensed the software, which can be verified using digital signing keys issued by trustworthy Certificate Authorities.

Current practices used in the software supply chain do not require identification of the actual software supplier and licensor of a software package. Some practices require that software packages be digitally signed, however there is no verification or validation that the signer of a software package and the supplier/licensor are related, or that the supplier/licensor has authorized a given party to sign a software object on their behalf. This creates a false sense of security when parties rely on tools such as Microsoft's signtool to validate a digital signature. Signtool does not validate that a digital signature belongs to a party authorized to sign a software product on behalf of the original source supplier/licensor - it has no way of verifying that a given signer is authorized to digitally sign a software package. Signtool will report a valid signature if the cryptographic verifications and trust chain reports produce successful results, which do not verify the authorization of a signing party and supplier arrangement. This issue is similar to a past scenario where Certificate Authorities were issuing SSL certificates to domains without authorization to do so, which resulted in the implementation of DNS CAA (IETF RFC 6844<https://datatracker.ietf.org/doc/html/rfc6844>) records to identify authorized parties to issue SSL certificates. A similar authorization scheme may be appropriate to authorize the issuance of digital certificates and signing keys to parties that have been authorized by a software supplier/licensor to sign software objects on their behalf.

The ability to determine trustworthiness is the key to securing the software supply chain. SBOM is the foundation for establishing this trust by specifying the identification of the actual source supplier/licensor of a software product. This supplier information can then be used to validate a digital signature applied to a software object by verifying the relationship of the software supplier and software signer using a trusted method to verify this relationship. A solution similar to DNS CAA, ref: "DNS Certification Authority Authorization (CAA) Resource Record", IETF RFC 6844<https://datatracker.ietf.org/doc/html/rfc6844> may be worth exploring to address this need.

There is no doubt that securing the software supply chain requires a software consumer to verify the identity of a software source supplier and licensor of a software object. SBOM is a critical requirement in this process due to its standard method for identifying a software supplier/licensor of a software object. Having this SBOM supplier identifier will enable a software consumer to verify this party using digital signatures based on digital certificates issued by trusted Certificate Authorities.  What is missing today is a means to verify the authorization of a signing party by the software source supplier/licensor using trusted mechanisms, similar to RFC 6844. Perhaps, one day, we may see a DNS Digital Signature Authorization (DSA) record to meet this need.

Everyone in the Energy industry really needs to follow this wise guidance provided by NIST:

SP 800-161 R1: https://doi.org/10.6028/NIST.SP.800-161r1-draft

SOFTWARE, FIRMWARE, AND INFORMATION INTEGRITY | CODE AUTHENTICATION

Supplemental C-SCRM Guidance: The organization should ensure that code authentication mechanisms such as digital signatures are implemented to assure the integrity of software, firmware, and information.

This NIST guidance, combined with Software Supplier Identification information contained in a digitally signed SBOM will go a long way to addressing software supply chain issues.

Best,
Seb

Sebastian Schulz
Product Manager Client Certificates

<http://www.globalsign.com/>

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