CVE-2026-39984 Sigstore Timestamp Authority has Improper Certificate Validation in verifier
Attackers can bypass timestamp authority validation by forging certificates in the PKCS#7 bag, affecting Go applications using the verification package.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-39984?
CVE-2026-39984 is a medium-severity authentication bypass vulnerability in sigstore timestamp-authority, published on 14 Apr 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Official 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.
Weakness: CWE-295
Which versions are affected by CVE-2026-39984?
4 open source packages ship an affected version, including github.com/sigstore/timestamp-authority/v2 and github.com/sigstore/timestamp-authority. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/sigstore/timestamp-authority/v2 Go | All versions | 2.0.6 |
github.com/sigstore/timestamp-authority/v2 Go | All versions | 2.0.6 |
github.com/sigstore/timestamp-authority/v2 Go | All versions | 2.0.6 |
github.com/sigstore/timestamp-authority Go | All versions | None |
Products include Linuxfoundation Sigstore Timestamp Authority.
How do I fix CVE-2026-39984?
Upgrade github.com/sigstore/timestamp-authority/v2 to 2.0.6 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-39984 being exploited?
There is no known exploitation of CVE-2026-39984 in the wild and no public exploit code.
Other vulnerabilities in sigstore timestamp-authority
Sources
Also known as CLEANSTART-2026-DK67034, CLEANSTART-2026-KJ61978, CLEANSTART-2026-LI04504, CLEANSTART-2026-NS94125, CLEANSTART-2026-SK27128, DEBIAN-CVE-2026-39984 and 41 more.