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PatchSiren cyber security CVE debrief

CVE-2026-42012 Red Hat CVE debrief

A certificate validation bypass vulnerability exists in GnuTLS where specially crafted certificates containing URI or SRV Subject Alternative Names (SANs) can cause the validation process to incorrectly fall back to checking DNS hostnames against the Common Name (CN) field. This fallback behavior may allow attackers to spoof legitimate services or intercept sensitive information by presenting certificates that exploit this validation logic flaw. The vulnerability stems from improper handling of non-DNS SAN types during certificate chain validation, violating the expectation that SANs should strictly supersede CN checking when present.

Vendor
Red Hat
Product
Red Hat Enterprise Linux 8
CVSS
HIGH 7.1
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-26
Original CVE updated
2026-08-25
Advisory published
2026-05-26
Advisory updated
2026-08-25

Who should care

Organizations operating TLS-secured services using GnuTLS for certificate validation, including mail servers, VPN endpoints, and API gateways. Certificate authority operators and developers implementing custom TLS validation logic. Security teams responsible for TLS configuration auditing and compliance with certificate validation standards.

Technical summary

The vulnerability affects GnuTLS certificate chain validation. When a certificate contains URI or SRV SAN entries, the validation logic incorrectly falls back to comparing the expected hostname against the CN field rather than enforcing strict SAN-based validation. This violates RFC 6125 guidance that SANs must be checked exclusively when present. Attackers can craft certificates with URI/SRV SANs that pass initial parsing but trigger the fallback path, potentially matching attacker-controlled CN values against legitimate service hostnames. The flaw represents a state machine error in certificate name verification where SAN presence does not properly suppress CN evaluation for non-DNS name types.

Defensive priority

HIGH

Recommended defensive actions

  • Audit systems for GnuTLS library usage and identify all TLS/SSL-terminating applications
  • Monitor vendor security advisories for GnuTLS patch availability
  • Review certificate validation implementations for proper SAN handling
  • Implement certificate pinning for critical services where feasible
  • Enable detailed TLS handshake logging to detect anomalous certificate presentations
  • Validate that applications reject certificates with unexpected SAN types when strict hostname verification is required

Evidence notes

Official CVE record published 2026-05-26. Red Hat Bugzilla entry 2467441 confirms vendor awareness. CVSS 3.1 vector AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:N indicates network attack vector with low attack complexity, requiring user interaction but no privileges, with high impact to integrity. CWE-295 (Improper Certificate Validation) assigned as primary weakness.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-42012 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-42012

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-42012 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-42012

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.