Sigstore Timestamp Authority has Improper Certificate Validation in verifier

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Summary

CVECVE-2026-39984
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-04-15 04:17:40 UTC
Updated2026-04-17 15:38:09 UTC
DescriptionSigstore 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.

Risk And Classification

Primary CVSS: v3.1 5.5 MEDIUM from [email protected]

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

EPSS: 0.000080000 probability, percentile 0.007410000 (date 2026-04-21)

Problem Types: CWE-295 | CWE-295 CWE-295: Improper Certificate Validation

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary5.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
3.1CNADECLARED5.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Sigstore Timestamp-authority affected < 2.0.6 Not specified

References

ReferenceSourceLinkTags
github.com/sigstore/timestamp-authority/security/advisories/GHSA-xm5m-wg... [email protected] github.com
github.com/sigstore/timestamp-authority/releases/tag/v2.0.6 [email protected] github.com
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