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CVE-2026-39984

Sigstore Timestamp Authority has Improper Certificate Validation in verifier

Published: Apr 14, 2026Updated: Apr 23, 2026 Sources: CVE List NVDCWE-295
NVD MITRE
5.5CVSSMEDIUM
EPSS Score
0.1%
Top 99.0%
CISA KEV
Not in KEV
Exploits
No Known Exploits
Remediation
Workaround Only

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.

CVSS Vector Breakdown

AV:LAC:LPR:NUI:RS:UC:NI:HA:N
Exploitability
AV:LAttack Vector
Local
AC:LAttack Complexity
Low
PR:NPrivileges Required
None
UI:RUser Interaction
Required
Scope
S:UScope
Unchanged
Impact
C:NConfidentiality
None
I:HIntegrity
High
A:NAvailability
None
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Weaknesses

CWE-295

Affected Products

timestamp-authoritysigstore
On-prem
Security Products / pki-crypto
sigstore timestamp authoritylinuxfoundation
Mixed
AI / ML / ml-framework
sigstoreoss-projectSecurity Productsaka cosign, gitsign, rekor
linuxfoundationoss-projectUSAI / MLaka yocto, pytorch, everest

Attack Graph

Products CVE Techniques Tactics

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Exploitability

Workaround Available

MITRE ATT&CK

1 technique
Collection
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References

https://github.com/sigstore/timestamp-authority/security/advisories/GHSA-xm5m-wgh2-rrg3
github.com
https://github.com/sigstore/timestamp-authority/releases/tag/v2.0.6
github.com

Timeline

Published
Apr 14, 2026
Last Updated
Apr 23, 2026

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