CVE-2026-39984
MEDIUMCVE-2026-39984 is a medium-severity (CVSS 5.5) vulnerability in github.com/sigstore/timestamp-authority/v2. O3 Security confirms whether CVE-2026-39984 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Sigstore Timestamp Authority has Improper Certificate Validation in verifier
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
github.com/sigstore/timestamp-authority/v2Real-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
Sigstore Timestamp Authority is a service for issuing RFC 3161 timestamps. Versions 2.0.5 and below contain an authorization bypass vulnerability in the VerifyTimestampResponse function. VerifyTimestampResponse correctly verifies the certificate chain signature, but the TSA-specific constraint checks in VerifyLeafCert uses the first non-CA certificate from the PKCS#7 certificate bag instead of the leaf certificate from the verified chain. An attacker can exploit this by prepending a forged certificate to the certificate bag while the message is signed with an authorized key, causing the library to validate the signature against one certificate but perform authorization checks against another. This vulnerability only affects users of the timestamp-authority/v2/pkg/verification package and does not affect the timestamp-authority service itself or sigstore-go. The issue has been fixed in version 2.0.6.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/sigstore/timestamp-authority/v2 | all versions | 2.0.6 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/sigstore/timestamp-authority/v2. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/sigstore/timestamp-authority/v2 to 2.0.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39984 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.
How O3 protects you
O3 pinpoints whether CVE-2026-39984 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-39984. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-39984 in your dependencies?
O3 detects CVE-2026-39984 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.