Medium [CVE-2026-66402] Server Identity Verification Bypass via TLS Certificate Validation Weaknesses

FreeRDP before 3.29.0 (affected versions <= 3.28.0) contains multiple TLS certificate identity validation weaknesses in tls_verify_certificate(), tls_match_hostname(), and x509_utils_get_dns_names().

Because FreeRDP performs custom Common Name and DNS SAN string matching instead of using OpenSSL's length-aware identity validation APIs, it (1) truncates DNS SAN values at embedded NUL bytes (accepting e.g. 'victim.example\0.attacker.example' as 'victim.example'), (2) accepts a matching Common Name even when non-matching DNS SAN entries are present, and (3) accepts IP-literal targets via DNS/CN matching without comparing iPAddress SANs.

Under a trusted or misissued certificate chain, an attacker positioned to present such a certificate can bypass server identity verification, weakening TLS server authentication. A flaw was found in FreeRDP.

This flaw involves multiple weaknesses in how FreeRDP validates TLS (Transport Layer Security) certificates. FreeRDP's custom validation logic can incorrectly process DNS Subject Alternative Name (SAN) values, accept Common Names without proper comparison to other SAN entries, and fail to validate IP address SANs for IP-literal targets.

Affected products named by the advisory: Red Hat Enterprise Linux 10; Red Hat Enterprise Linux 6; Red Hat Enterprise Linux 7; Red Hat Enterprise Linux 8; and 2 more.