CVE-2026-42013

A flaw was found in gnutls. When validating certificates, an oversized Subject Alternative Name (SAN) could cause the validation process to incorrectly fall back to checking the Common Name (CN) field. This could allow a remote attacker to bypass proper certificate validation, potentially leading to spoofing or man-in-the-middle attacks.

This vulnerability is rated as having a Moderate impact across affected Red Hat products because successful exploitation requires an active man-in-the-middle network position as well as a certificate issued by a trusted authority matching the target hostname's Common Name. Although TLS clients using GnuTLS on Red Hat Enterprise Linux and OpenShift are susceptible to improper certificate acceptance under these conditions, modern certificate authorities rarely issue certificates that omit valid Subject Alternative Names, substantially restricting practical threat vectors. Applications functioning exclusively as TLS servers or utilizing alternative cryptographic libraries like OpenSSL are not exposed to this flaw.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.