Sigstore Timestamp Authority has Improper Certificate Validation in verifier

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.5 · CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

EPSS

• CVE-2026-39984: 0.10% estimated exploitation activity in the next 30 days; percentile 0.80 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

14 Apr 2026, 01:01 UTC

Last modified by the source

24 Apr 2026, 20:43 UTC

Source last checked

26 Oct 2026, 15:19 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-xm5m-wgh2-rrg3

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# Patch Next advisory brief: CVE-2026-39984

Advisory: Sigstore Timestamp Authority has Improper Certificate Validation in verifier
IDs: CVE-2026-39984, GHSA-xm5m-wgh2-rrg3
Record: https://patchnext.com/advisories/cve-2026-39984 (revision 1, source checked 26 Oct 2026, 15:19 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/sigstore/timestamp-authority/v2 2.0.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/sigstore/timestamp-authority/v2: affected <= 2.0.5; fixed in 2.0.6

## Suggested inspection (read-only)
- Go github.com/sigstore/timestamp-authority/v2: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/sigstore/timestamp-authority/v2.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-xm5m-wgh2-rrg3 (GitHub-reviewed advisory GHSA-xm5m-wgh2-rrg3 lists affected ranges: Go github.com/sigstore/timestamp-authority/v2 >=0 <=2.0.5)

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