CVE-2026-39395 Sigstore CVE debrief

Who should care

Users of Cosign, especially those relying on verify-blob-attestation for security, should be aware of this vulnerability and upgrade to a fixed version. This includes operators, platform administrators, vulnerability management teams, and security teams who use Cosign for code signing and transparency.

Technical summary

The vulnerability was caused by a logic flaw in the error handling of predicate type validation for old-format bundles and detached signatures, and a complete bypass of predicate type validation for new-format bundles. This could lead to incorrect verification results, potentially allowing malicious attestations to be accepted. The vulnerability is fixed in Cosign versions 3.0.6 and 2.6.3. Users should review and update verification processes to ensure correct predicate type validation and monitor for potential incorrect verification results.

Defensive priority

High

Recommended defensive actions

• Upgrade to Cosign version 3.0.6 or 2.6.3 or later
• Review and update verification processes to ensure correct predicate type validation
• Monitor for and address potential incorrect verification results
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record was published on 2026-04-07T20:16:33.140Z and last modified on 2026-07-24T21:10:00.143Z. The NVD entry is currently Analyzed. Cosign provides code signing and transparency for containers and binaries. Prior to 3.0.6 and 2.6.3, cosign verify-blob-attestation may erroneously report a 'Verified OK' result for attestations with malformed payloads or mismatched predicate types. For old-format bundles and detached signatures, this was due to a logic flaw in the error handling of the predicate type validation. For new-format bundles, the predicate type validation was bypassed completely. This vulnerability is fixed in 3.0.6 and 2.6.3. The evidence for this CVE is based on the official CVE record and NVD detail page.

Official resources