How severe is CVE-2026-39984?

CVE-2026-39984 has a CVSS score of 5.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2026-39984?

CVE-2026-39984 affects the following packages: github.com/sigstore/timestamp-authority/v2 (Go). Ecosystems affected: Go.

How do I fix CVE-2026-39984?

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.

How do I detect CVE-2026-39984 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/sigstore/timestamp-authority/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-39984 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-2026-39984 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-39984 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-2026-39984?

CVE-2026-39984 has an EPSS (Exploit Prediction Scoring System) score of 0.1%, placing it in the 1th 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-2026-39984?

CVE-2026-39984 is classified as CWE-295 (CWE-295). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-39984 published, and has it been updated?

CVE-2026-39984 was published on April 14, 2026 and was last updated on October 30, 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-2026-39984 — timestamp-authority

MEDIUM

CVE-2026-39984 is a medium-severity (CVSS 5.5) CWE-295 vulnerability in github.com/sigstore/timestamp-authority/v2. A fix is available for github.com/sigstore/timestamp-authority/v2 — see the affected versions and patch details below.

Sigstore Timestamp Authority has Improper Certificate Validation in verifier

Also known asGHSA-xm5m-wgh2-rrg3GO-2026-5763

Published

Apr 14, 2026

Updated

Oct 30, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 9, 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-2026-39984.

EPSS Exploitation Probability

via FIRST.org ↗

0.1%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs1th percentile — riskier than 1% 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

1 pkg affected

🐹github.com/sigstore/timestamp-authority/v2

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

Authorization bypass via certificate bag manipulation in sigstore/timestamp-authority verifier

An authorization bypass vulnerability exists in sigstore/timestamp-authority verifier (timestamp-authority/v2/pkg/verification): VerifyTimestampResponse function correctly verifies the certificate chain but when the TSA specific constraints are verified in VerifyLeafCert, the first non-CA certificate from the PKCS#7 certificate bag is used instead of the leaf certificate from the certificate chain. An attacker can exploit this by prepending a forged certificate to the certificate bag while the message is signed with an authorized key. The library validates the signature using the one certificate but performs authorization checks on the another, allowing an attacker to bypass some authorization controls.

This vulnerability does not apply to timestamp-authority service, only to users of timestamp-authority/v2/pkg/verification package.

This vulnerability does not apply to sigstore-go even though it is a user of timestamp-authority/v2/pkg/verification: Providing TSACertificate option to VerifyTimestampResponse fully mitigates the issue.

Patches

The issue will be fixed in timestamp-authority 2.0.6

Workarounds

Users of VerifyTimestampResponse can use the TSACertificate option to specify the exact certificate they expect to be used: this fully mitigates the issue.

References

This issue was found after reading CVE-2026-33753 / GHSA-3xxc-pwj6-jgrj (originally reported by @Jaynornj and @Pr00fOf3xpl0it)

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/sigstore/timestamp-authority/v2`	all versions	2.0.6`go get github.com/sigstore/timestamp-authority/v2@v2.0.6`

Affected Products

1 product · 1 configurations

Application

sigstore timestamp authoritylinuxfoundation

< 2.0.6

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/timestamp-authority/v2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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.

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

This vulnerability allows an attacker to bypass authorization checks by prepending a forged certificate to the certificate bag. The library validates the signature against one certificate while performing authorization checks against another. Additionally, a user or an automated system must actively run the…

Workaround published by Red Hat

To mitigate this vulnerability, users of VerifyTimestampResponse can use the TSACertificate option to explicitly specify the exact certificate they expect to be used.

Source: Red Hat security advisory for CVE-2026-39984 (CC BY 4.0)

Frequently Asked Questions

### Authorization bypass via certificate bag manipulation in sigstore/timestamp-authority verifier An authorization bypass vulnerability exists in sigstore/timestamp-authority verifier (timestamp-authority/v2/pkg/verification): `VerifyTimestampResponse` function correctly verifies the certificate chain but when the TSA specific constraints are verified in `VerifyLeafCert`, the first non-CA certificate from the PKCS#7 certificate bag is used instead of the leaf certificate from the certificate chain. An attacker can exploit this by prepending a forged certificate to the certificate bag while t