Vulnerability · Published Apr 14, 2026 · Assigned by GitHub_M
CVE-2026-39984Sigstore Timestamp Authority has Improper Certificate Validation in verifier
Medium 5.5
In plain English
A medium-severity improper certificate validation in timestamp-authority from sigstore. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 1% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:13 | golang-github-sigstore-timestamp-authority | no fix listed | DEBIAN-CVE-2026-39984 |
| Debian:14 | golang-github-sigstore-timestamp-authority | 2.1.0-1 | DEBIAN-CVE-2026-39984 |
| Go | github.com/sigstore/timestamp-authority | no fix listed | GO-2026-5763 |
| Go | github.com/sigstore/timestamp-authority/v2 | 2.0.6 | GHSA-xm5m-wgh2-rrg3 |
| Go | github.com/sigstore/timestamp-authority/v2 | 2.0.6 | GO-2026-5763 |
| Ubuntu:25.10 | golang-github-sigstore-timestamp-authority | no fix listed | UBUNTU-CVE-2026-39984 |
| Ubuntu:26.04:LTS | golang-github-sigstore-timestamp-authority | no fix listed | UBUNTU-CVE-2026-39984 |
| Ubuntu:26.10 | golang-github-sigstore-timestamp-authority | no fix listed | UBUNTU-CVE-2026-39984 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| sigstore | timestamp-authority | < 2.0.6 |
Weakness: CWE-295 · Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N