CVE-2026-42013: GnuTLS Auth Bypass Vulnerability

CVE-2026-42013 Overview

CVE-2026-42013 is a certificate validation flaw in GnuTLS, a widely deployed open-source TLS library. When the library processes a certificate containing an oversized Subject Alternative Name (SAN) extension, it incorrectly falls back to validating the Common Name (CN) field. This fallback contradicts modern TLS validation practice, which requires SAN to be authoritative when present. A remote attacker can exploit this behavior to present a certificate that passes hostname validation against a target the attacker does not legitimately control. The flaw enables spoofing and machine-in-the-middle attacks against TLS-protected connections that rely on GnuTLS. The vulnerability is tracked under [CWE-1284] (Improper Validation of Specified Quantity in Input).

Critical Impact
Remote attackers can bypass TLS certificate validation, enabling spoofing of trusted endpoints and interception of encrypted traffic without user interaction.

Affected Products

GnuTLS (affected versions per upstream advisory)
Red Hat Enterprise Linux distributions shipping GnuTLS (see RHSA-2026:20611)
Downstream applications and services that link against vulnerable GnuTLS builds for TLS validation

Discovery Timeline

2026-05-26 - CVE-2026-42013 published to the National Vulnerability Database
2026-05-27 - Last updated in NVD database

Technical Details for CVE-2026-42013

Vulnerability Analysis

The vulnerability resides in the certificate validation routines of GnuTLS. RFC 6125 requires that when a Subject Alternative Name extension is present, validators must use SAN entries to match the expected hostname and must not fall back to the Common Name. GnuTLS violates this requirement when a SAN extension exceeds an internal size constraint. Instead of treating the oversized SAN as a validation failure, the library silently ignores the SAN and reverts to CN-based matching. An attacker who can obtain or craft a certificate with a manipulated SAN structure and a CN matching the targeted hostname can therefore defeat hostname verification.

Root Cause

The root cause is improper handling of a bounded input quantity inside the SAN parsing path. The validator treats the size constraint as a soft failure and continues validation rather than rejecting the certificate or restricting evaluation to SAN entries. This logic flaw causes a deprecated CN fallback to execute against attacker-influenced input.

Attack Vector

Exploitation requires network position and a certificate crafted to trigger the SAN size condition while carrying a CN value matching the victim's expected hostname. A network-adjacent attacker performing a machine-in-the-middle attack against TLS traffic, or an operator of a malicious server contacted by a vulnerable client, can present such a certificate. No privileges or user interaction are required. The vulnerability does not affect availability but degrades the integrity of authenticated TLS sessions and can expose limited confidential data exchanged during handshake-bound flows.

The vulnerability manifests in the SAN extension parsing and hostname matching logic of GnuTLS. See the Red Hat Bug Report for upstream technical analysis.

Detection Methods for CVE-2026-42013

Indicators of Compromise

TLS handshakes accepting certificates whose CN matches the target host while the SAN extension is malformed or unusually large
Unexpected certificate chains from non-standard issuers being accepted by GnuTLS-linked clients
Repeated TLS sessions to sensitive endpoints originating from unexpected upstream network paths

Detection Strategies

Inventory installed GnuTLS packages across Linux hosts and compare against fixed versions listed in vendor errata
Inspect captured TLS certificates at network egress and flag certificates with SAN extensions exceeding typical size thresholds
Correlate certificate fingerprints observed in production with expected issuer and SAN values from internal PKI baselines

Monitoring Recommendations

Enable TLS inspection or passive certificate logging on network sensors to record SAN and CN fields for outbound and inbound sessions
Monitor package management logs for gnutls updates and verify patch deployment across all hosts
Alert on TLS clients negotiating connections without matching SAN entries when communicating with internal services

How to Mitigate CVE-2026-42013

Immediate Actions Required

Apply the GnuTLS update distributed in RHSA-2026:20611 or the equivalent advisory from your Linux distribution
Restart all services that load GnuTLS, including web servers, mail servers, and VPN daemons, to ensure the patched library is in use
Audit certificate trust stores and remove unnecessary or untrusted certificate authorities to reduce attacker options

Patch Information

Red Hat has shipped fixed GnuTLS packages through RHSA-2026:20611. Additional vendor details are available in the Red Hat CVE Details advisory. Administrators of non-Red Hat distributions should track upstream GnuTLS releases and apply distribution-specific updates as they become available.

Workarounds

Restrict outbound TLS connections from sensitive workloads to known certificate pinning sets where feasible
Enforce certificate transparency log verification for externally facing services to detect rogue certificates
Where supported, configure applications to use an alternative validated TLS stack until GnuTLS is patched

bash

# Update GnuTLS on Red Hat Enterprise Linux
sudo dnf update gnutls

# Verify the installed version after patching
rpm -q gnutls

# Identify processes still using the pre-patch library
sudo lsof | grep -i libgnutls