Executive brief
A security flaw has been identified in GnuTLS, a widely used library that enables secure communications for applications and operating systems. When checking a website's security certificate, the library may fail to properly verify the identity of the server if the certificate contains an unusually large name field. This could allow a malicious actor to impersonate a legitimate service, potentially leading to the interception of sensitive data or unauthorized access to communications.
Technical details
A certificate validation bypass vulnerability exists in GnuTLS due to improper validation of specified quantities in input (CWE-1284). When the library encounters a certificate with an oversized Subject Alternative Name (SAN) field, the validation logic incorrectly fails over to checking the legacy Common Name (CN) field instead of rejecting the malformed or oversized SAN. A remote, unauthenticated attacker can exploit this by presenting a specially crafted certificate during a TLS handshake. Successful exploitation allows the attacker to bypass modern SAN-based identity verification, facilitating man-in-the-middle (MitM) attacks and server spoofing. Red Hat has acknowledged the issue, and fixes involve suppressing the CN fallback mechanism when SAN processing fails due to size constraints.
Affected products
- GnuTLS gnutls
Timeline
- 2026-05-06: disclosed: Initial report in Red Hat Bugzilla
- 2026-05-26: advisory: NVD publication date