How severe is CVE-2026-39395?

CVE-2026-39395 has a CVSS score of 4.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-39395?

CVE-2026-39395 affects the following packages: github.com/sigstore/cosign (Go), github.com/sigstore/cosign (Go). Ecosystems affected: Go.

How do I fix CVE-2026-39395?

Update github.com/sigstore/cosign to 3.0.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39395 is resolved across your whole dependency graph.

How do I detect CVE-2026-39395 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/sigstore/cosign, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-39395 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-39395 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-39395 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-39395?

CVE-2026-39395 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 16th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-39395?

CVE-2026-39395 is classified as CWE-754 (CWE-754). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-39395 published, and has it been updated?

CVE-2026-39395 was published on April 7, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

MEDIUM severity

CVE-2026-39395 — cosign

MEDIUM

CVE-2026-39395 is a medium-severity (CVSS 4.3) CWE-754 vulnerability in github.com/sigstore/cosign. A fix is available for github.com/sigstore/cosign — see the affected versions and patch details below.

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

Also known asBIT-cosign-2026-39395GHSA-w6c6-c85g-mmv6GO-2026-5694

Published

Apr 7, 2026

Updated

Oct 28, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-39395.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk+0.01%

Lower risk than most CVEs16th percentile — riskier than 16% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected

🐹github.com/sigstore/cosign🐹github.com/sigstore/cosign

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

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.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/sigstore/cosign`	≥ 3.0.0&&< 3.0.6	3.0.6`go get github.com/sigstore/cosign@v3.0.6`
🐹Go	`github.com/sigstore/cosign`	all versions	2.6.3`go get github.com/sigstore/cosign@v2.6.3`

Affected Products

1 product · 2 configurations

Application

cosignsigstore

≥ 3.0.0 && < 3.0.6

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/sigstore/cosign, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/sigstore/cosign to 3.0.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39395 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.

Source: Red Hat security advisory for CVE-2026-39395 (CC BY 4.0)

Frequently Asked Questions

## 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. Additionall