go-witness: AWS attestor accepts unsigned or forged EC2 instance identity documents

Impact

The AWS attestor in go-witness treats verification as successful when the instance identity document carries no signature or an empty one, and when RSA verification outright fails. Anything downstream that trusts a witness attestation to prove "this build ran on this EC2 instance in this account" can be fed a forged document - for example by impersonating the Instance Metadata Service - and will record the forged provenance as verified. For an operator whose image or model-artifact promotion gates on witness attestations, that is a supply-chain admission check that can be talked into saying yes, and the artifact it blesses then lands on GPU nodes. The attestor also pins a single legacy global AWS public certificate and does not know about the region-specific certificates issued in 2024, so even a correctly implemented check has no trust material for many regions.

Who can reach it

An attacker who can supply or intercept the instance identity document seen by the attestor - IMDS impersonation on the build host, or control of the data path feeding verification. No authentication to the witness tooling itself is needed; network position or code execution in the attesting environment is.

What to do

Upgrade go-witness to 0.9.1 or later and the witness CLI to 0.10.1 or later, then rebuild and redeploy whatever embeds them (CI runner images, policy verifiers, admission webhooks) - this is a dependency bump and a redeploy of the attestation tooling, not a node-level action. Until then, the advisory's workaround is to disable the AWS attestor, or verify the identity document, signature and AWS region certificate out of band with standard tooling per AWS's published guidance. Attestations already produced by a vulnerable version should be treated as unproven rather than trusted retroactively.

References

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