Cosign's verify-blob-attestation reports false positive when payload parsing fails

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

EPSS

• CVE-2026-39395: 0.24% estimated exploitation activity in the next 30 days; percentile 15.27 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Description 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. Impact When cosign verify-blob-attestation is used without --check-claims set to true, an attestation that has a valid signature but a malformed or unparsable payload would be incorrectly validated. Additionally, systems relying on --type <predicate type to reject attestations with mismatched types would be lead to trust the unexpected attestation type. Patches v3.0.6, v2.6.3 Workarounds Always set --check-claims=true for attestation verification.

Published by the source

8 Apr 2026, 00:15 UTC

Last modified by the source

8 Apr 2026, 00:15 UTC

Source last checked

26 Oct 2026, 15:19 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-w6c6-c85g-mmv6

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# Patch Next advisory brief: CVE-2026-39395

Advisory: Cosign's verify-blob-attestation reports false positive when payload parsing fails
IDs: CVE-2026-39395, GHSA-w6c6-c85g-mmv6
Record: https://patchnext.com/advisories/cve-2026-39395 (revision 1, source checked 26 Oct 2026, 15:19 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/sigstore/cosign 3.0.6, 2.6.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/sigstore/cosign: affected >= 3.0.0, < 3.0.6; < 2.6.3; fixed in 3.0.6, 2.6.3

## Suggested inspection (read-only)
- Go github.com/sigstore/cosign: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/sigstore/cosign.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-w6c6-c85g-mmv6 (GitHub-reviewed advisory GHSA-w6c6-c85g-mmv6 lists affected ranges: Go github.com/sigstore/cosign >=3.0.0 =0 <2.6.3)

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