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CVE-2026-39984

Sigstore Timestamp Authority has Improper Certificate Validation in verifier

MEDIUM
5.5
CVSS Score
Published: April 14, 2026 at 11:41 PM UTC
Updated: October 26, 2026 at 09:10 PM UTC
2 products affected across 2 vendorsSigstore Timestamp AuthoritySigstore Timestamp Authority

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Severity
MEDIUM5.5
Exploit prediction
EPSS 0.1%
SSVC
No known exploit

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

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Sigstore Timestamp Authority· Linuxfoundation
95%
v < 2.0.6
Affected range stops below 2.0.6, so it looks safe — our inference, not a vendor statement
Sigstore Timestamp Authority· Sigstore
90%
v < 2.0.6
Affected range stops below 2.0.6, so it looks safe — our inference, not a vendor statement

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (2)

x_refsource_CONFIRMMitigationVendor Advisory
Source: cverecord
https://github.com/sigstore/timestamp-authority/security/advisories/GHSA-xm5m-wgh2-rrg3
x_refsource_MISCProductRelease Notes
Source: cverecord
https://github.com/sigstore/timestamp-authority/releases/tag/v2.0.6

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

5.5MEDIUMGitHub_M
Attack Vector
LOCAL
Complexity
LOW
Privileges
NONE
User Interaction
REQUIRED
Scope
UNCHANGED
Confidentiality
NONE
Integrity
HIGH
Availability
NONE
Vector?

Quick Info

StatusANALYZED
CWE
CWE-295

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