CVE-2026-42010: GnuTLS RSA-PSK NUL Byte Authentication Bypass

The National Vulnerability Database has detailed CVE-2026-42010, a critical flaw in GnuTLS. This vulnerability specifically impacts servers configured with RSA-PSK (Rivest–Shamir–Adleman – Pre-Shared Key) authentication. The core issue lies in how these servers handle usernames containing a NUL character, causing them to incorrectly match them with truncated usernames. This isn’t a theoretical issue; it’s a direct authentication bypass.

An attacker can exploit this by crafting a specific username, tricking the server into granting unauthorized access. The CVSSv3.1 score of 7.1 (High) with an AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N vector indicates a network-exploitable flaw requiring low privileges to achieve high confidentiality impact and low integrity impact, with no availability impact. This is a severe problem for any organization relying on GnuTLS with RSA-PSK.

For defenders, this means a remote attacker can circumvent authentication entirely. The attacker’s calculus is simple: find a target running GnuTLS with RSA-PSK, craft the malicious username, and gain entry. This bypass opens the door to further internal reconnaissance, data exfiltration, or lateral movement, making it a prime initial access vector.