Attack Chain

1. Attacker crafts a malicious certificate with an oversized Subject Alternative Name (SAN) field.
2. The attacker sets the Common Name (CN) field in the certificate to a value they wish to impersonate (e.g., a legitimate domain).
3. The attacker initiates a TLS connection to a target server or client using the crafted certificate.
4. The gnutls library on the target attempts to validate the presented certificate.
5. Due to the oversized SAN, the gnutls library fails to properly process the SAN field.
6. The gnutls library incorrectly falls back to validating the CN field.
7. The CN field matches the expected value, and the gnutls library incorrectly considers the certificate valid.
8. The attacker successfully bypasses certificate validation, enabling potential spoofing or man-in-the-middle attacks.

Impact

Successful exploitation of CVE-2026-42013 allows a remote attacker to bypass certificate validation, potentially leading to spoofing or man-in-the-middle attacks. This could allow the attacker to intercept sensitive data, inject malicious content, or compromise the confidentiality and integrity of communications. The CVSS v3.1 base score is 8.2, indicating a high severity.

Recommendation

• Apply the necessary patches or updates provided by Red Hat to address CVE-2026-42013 on systems using the affected gnutls library.
• Monitor network traffic for TLS connections using certificates with unusually large SAN fields, as these could indicate exploitation attempts. Consider implementing a network connection rule targeting connections utilizing certificates with large SAN sizes.
• Deploy the Sigma rule Detect GnuTLS Certificate Validation Bypass - Large SAN to identify potential exploitation attempts based on process execution patterns and network connections.