TI-2026-28838 Aliases CVE-2026-39984 GHSA-xm5m-wgh2-rrg3

Sigstore Timestamp Authority has Improper Certificate Validation in verifier

LOW 24CWE-295Published 2026-04-14 · Updated 2026-04-16

Key facts

Attack vector: Local
Privileges required: None
User interaction: Required
Attack complexity: Low
Impact type: Spoofing
Confidentiality · integrity · availability: None · High · None
Fix status: Partial Red Hat: 3 without a fix

Attack profile from CVSS 3.1 · CNA. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-295.

Summary

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00099 0th percentileScored 2026-11-07

Fix Partial timestamp-authority (sigstore): 2.0.6github.com/sigstore/timestamp-authority/v2 (Go): 2.0.6Red Hat: 3 without a fix

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: No
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
sigstore	timestamp-authority	before 2.0.6	2.0.6	Patch available	CNA
—	github.com/sigstore/timestamp-authority/v2 (Go)	before 2.0.6	2.0.6	Patch available	GHSA-xm5m-wgh2-rrg3

timestamp-authority Vendorsigstore Affected before 2.0.6 Fixed in 2.0.6 Patch available CNA
github.com/sigstore/timestamp-authority/v2 (Go) Vendor— Affected before 2.0.6 Fixed in 2.0.6 Patch available GHSA-xm5m-wgh2-rrg3

CPE configurations NVD · 1

CPE	Versions	Role	Vendor
cpe:2.3:a:linuxfoundation:sigstore_timestamp_authority::::::::	before 2.0.6	Vulnerable	backstage

cpe:2.3:a:linuxfoundation:sigstore_timestamp_authority:*:*:*:*:*:*:*:* Versions before 2.0.6 Vulnerable ·backstage

Remediation

Fix · CNA timestamp-authority (sigstore) Fixed in 2.0.6
Fix · GHSA-xm5m-wgh2-rrg3 github.com/sigstore/timestamp-authority/v2 (Go) Fixed in 2.0.6 Advisory →

Vendor assessments

Red Hat

Red Hat rating: Moderate. Red Hat's page for this CVE →

Product	Status	Plan or fix
Red Hat Advanced Cluster Security 4	Affected	No fix yet: Fix deferred
Red Hat Trusted Artifact Signer	Affected	No fix yet: Fix deferred
Security Profiles Operator	Affected	No fix yet: Fix deferred

Timeline

2026-10-02 Threatist record created from CVE-2026-39984
2026-10-03 Alias added: GHSA-xm5m-wgh2-rrg3

References

Advisory 2

Other 2

TI-2022-16356 False positive signature verification in cosign CVSS 3.1CRITICAL 9.8
TI-2026-36084 sigstore-js: `certificateOIDs` verification constraints are silently dropped and never enforced CVSS 3.1HIGH 7.5
TI-2024-19670 Cosign vulnerable to machine-wide denial of service via malicious artifacts CVSS 3.1HIGH 7.5
TI-2026-18290 Fulcio vulnerable to Server-Side Request Forgery (SSRF) via MetaIssuer Regex Bypass CVSS 3.1MEDIUM 5.8
TI-2026-36085 sigstore-js: Insufficient Verification of Data Authenticity CVSS 3.1MEDIUM 6.5
TI-2024-19669 Cosign vulnerable to system-wide denial of service via malicious attachments CVSS 3.1MEDIUM 5.9

See all 39 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences