CVE-2026-39395: Cosign provides code signing and transparency for containers and binaries

Routine

MEDIUM 4.3UPGRADE

CVE-2026-39395 is a medium-severity flaw (CVSS 4.3) affecting cosign. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.2% (top 85%)
CISA exploited list	No
Weakness	CWE-754
Published	2026-04-07
Last modified	2026-07-24

Publisher (CNA): GitHub, Inc. · Vendor: sigstore

Last updated 2026-07-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-07 20:16 UTCPublished CVE record published
2026-07-24 21:10 UTCUpdated Record last modified

sigstore cosign UPGRADE

Affected: < 2.6.3 · >= 3.0.0 and < 3.0.6

Fixed in: 2.6.3, 3.0.6

Safe to downgrade to: a release before 3.0.0

1. Check installed version

apt-cache policy cosign || rpm -q cosign || cosign --version

2. UPGRADE to 2.6.3 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade cosign

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade cosign

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.0.0 (Debian / Ubuntu)

apt-cache madison cosign sudo apt install cosign=<VERSION_BEFORE_3.0.0> sudo apt-mark hold cosign

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list cosign sudo dnf downgrade cosign-<VERSION_BEFORE_3.0.0> sudo dnf versionlock add cosign

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-39395?

cosign: upgrade to 2.6.3, 3.0.6. Exact commands for each OS are on this page.

Is CVE-2026-39395 being exploited?

CVE-2026-39395 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-39395 affect?

CVE-2026-39395 affects cosign. Severity is medium (CVSS 4.3). 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 predica

How urgent is CVE-2026-39395?

Priority 19/100. No elevated risk signals yet.

References

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