CVE-2026-39984

Name of the Vulnerable Software and Affected Versions Sigstore Timestamp Authority versions prior to 2.0.6

Description An authorization bypass exists in the timestamp-authority/v2/pkg/verification package. The VerifyTimestampResponse() function correctly verifies the certificate chain signature, but the VerifyLeafCert() function 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.

Recommendations Update to version 2.0.6. As a temporary workaround, users of VerifyTimestampResponse() can use the TSACertificate option to specify the exact certificate they expect to be used.