sigstore-go has a multi-log threshold bypass via single compromised log

Impact

What kind of vulnerability is it? Who is impacted?

A verifier configured with WithTransparencyLog(N>1) or WithSignedCertificateTimestamps(N>1) expected defense-in-depth against the compromise of a single log instance. However, threshold counting counted verified witnesses per-entry or per-validation-path rather than per-log-authority.

As a result, a single compromised transparency log could forge multiple entries with different indices, and a single compromised CT log could verify multiple times (either across multiple certificate chains or via multiple embedded SCTs), fully satisfying the multi-log threshold requirements and defeating the multi-log policy.

Note that this does not affect Cosign, as Cosign sets a threshold of 1.

Patches

Has the problem been patched? What versions should users upgrade to?

Upgrade to v1.1.5.

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

There is no workaround, beyond relying on trusted logs.