# CVE-2026-39395: Sigstore Cosign validation bypass in verify-blob-attestation

Source: https://junglewise.ai/threats/cve-2026-39395-sigstore-cosign-validation-bypass-in-verify-blob-attestation  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-07. Updated: 2026-07-24  
License: CC BY 4.0 with a link back

CVE-2026-39395 · Severity: medium · CVSS 4.3 · Published 2026-04-07

Sigstore Cosign, a tool used to sign and verify the integrity of software containers and binaries, contains a flaw in how it validates certain security attestations. An attacker could provide a malformed or mismatched attestation that the tool incorrectly marks as 'Verified OK,' potentially leading users to trust unverified or incorrect software metadata. This issue affects the 'verify-blob-attestation' command and can be mitigated by updating to the latest version or ensuring specific claim-checking flags are enabled.

Technologies: [github.com/sigstore/cosign (Go)](https://junglewise.ai/threats/technologies/github-com-sigstore-cosign), [github.com/sigstore/cosign/v2 (Go)](https://junglewise.ai/threats/technologies/github-com-sigstore-cosign-v2), [github.com/sigstore/cosign/v3 (Go)](https://junglewise.ai/threats/technologies/github-com-sigstore-cosign-v3). Vendors: [Go](https://junglewise.ai/threats/vendors/go), [Sigstore](https://junglewise.ai/threats/vendors/sigstore).

## Quick answers

**Is CVE-2026-39395 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-39395 being exploited in the wild.

**How severe is CVE-2026-39395?** Medium severity, with a CVSS score of 4.3, published 7 April 2026.

**Which products and versions are affected?** github.com/sigstore/cosign (Go) <= 1.13.6, SEMVER:introduced=0, SEMVER:introduced=3.0.0,fixed=3.0.6; github.com/sigstore/cosign/v2 (Go) < 2.6.3, SEMVER:introduced=0,fixed=2.6.3; github.com/sigstore/cosign/v3 (Go) >= 3.0.0, < 3.0.6, SEMVER:introduced=3.0.0,fixed=3.0.6; Sigstore Cosign < 2.6.3, >= 3.0.0, < 3.0.6.

**Is there a fix for CVE-2026-39395?** Yes. github.com/sigstore/cosign (Go): upgrade to 3.0.6 or later; github.com/sigstore/cosign/v2 (Go): upgrade to 2.6.3 or later; github.com/sigstore/cosign/v3 (Go): upgrade to 3.0.6 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-39395 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-39395?** Check whether you run github.com/sigstore/cosign (Go), github.com/sigstore/cosign/v2 (Go), github.com/sigstore/cosign/v3 (Go) or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- github.com/sigstore/cosign (Go): upgrade to 3.0.6 or later

  ```sh
  go get github.com/sigstore/cosign@v3.0.6
  ```

- github.com/sigstore/cosign/v2 (Go): upgrade to 2.6.3 or later

  ```sh
  go get github.com/sigstore/cosign/v2@v2.6.3
  ```

- github.com/sigstore/cosign/v3 (Go): upgrade to 3.0.6 or later

  ```sh
  go get github.com/sigstore/cosign/v3@v3.0.6
  ```


## Technical details

A logic flaw in Sigstore Cosign's 'verify-blob-attestation' command leads to improper validation of attestation payloads. In older bundle formats and detached signatures, error handling during predicate type validation was flawed; in newer formats, the validation was bypassed entirely. This allows an attestation with a valid signature but a malformed payload or incorrect type to return a successful verification result if '--check-claims' is not set to true. Attackers can exploit this to bypass type-based rejection policies. The vulnerability is addressed in versions 3.0.6 and 2.6.3.

## Affected products

- Sigstore Cosign < 2.6.3, >= 3.0.0, < 3.0.6

## Identifiers

- CVE-2026-39395
- GHSA-w6c6-c85g-mmv6
- GO-2026-5694

## Timeline

- 2026-04-06: advisory: GitHub Security Advisory published
- 2026-04-07: disclosed: CVE-2026-39395 published

## References

- [https://github.com/sigstore/cosign/security/advisories/GHSA-w6c6-c85g-mmv6](https://github.com/sigstore/cosign/security/advisories/GHSA-w6c6-c85g-mmv6)

## Related threats

- [Cosign considered signatures valid with expired intermediate certificates when transparency log verification is skipped](https://junglewise.ai/threats/cve-2026-24122-cosign-considered-signatures-valid-with-expired-intermediate)
- [GO-2026-4309 - Cosign verification accepts any valid Rekor entry under certain conditions in github.com/sigstore/cosign](https://junglewise.ai/threats/cve-2026-22703-cosign-verification-accepts-any-valid-rekor-entry-under-certain)
- [GO-2024-2719 - Cosign malicious artifacts can cause machine-wide DoS in github.com/sigstore/cosign](https://junglewise.ai/threats/cve-2024-29903-cosign-malicious-artifacts-can-cause-machine-wide-dos)
- [GO-2024-2718 - Cosign malicious attachments can cause system-wide denial of service in github.com/sigstore/cosign](https://junglewise.ai/threats/cve-2024-29902-cosign-malicious-attachments-can-cause-system-wide-denial-of)
- [GO-2022-0998 - Improper blob verification in github.com/sigstore/cosign](https://junglewise.ai/threats/cve-2022-36056-cosign-bundle-can-be-crafted-to-successfully-verify-a-blob-even)