Summary
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.
Workarounds
Always set --check-claims=true for attestation verification.
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.
CVE-2026-39395 has a CVSS score of 4.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.0.6, 2.6.3); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
v3.0.6, v2.6.3
Frequently Asked Questions
- What is CVE-2026-39395? CVE-2026-39395 is a medium-severity security vulnerability in github.com/sigstore/cosign (go), affecting versions >= 3.0.0, < 3.0.6. It is fixed in 3.0.6, 2.6.3.
- How severe is CVE-2026-39395? CVE-2026-39395 has a CVSS score of 4.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/sigstore/cosign are affected by CVE-2026-39395? github.com/sigstore/cosign (go) versions >= 3.0.0, < 3.0.6 is affected.
- Is there a fix for CVE-2026-39395? Yes. CVE-2026-39395 is fixed in 3.0.6, 2.6.3. Upgrade to this version or later.
- Is CVE-2026-39395 exploitable, and should I be worried? Whether CVE-2026-39395 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-39395 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-39395?
- Upgrade
github.com/sigstore/cosignto 3.0.6 or later - Upgrade
github.com/sigstore/cosignto 2.6.3 or later
- Upgrade