FreeRDP before 3.29.0 improperly validates the Extended Key Usage (EKU) purpose of the peer certificate during client-side server TLS authentication. In x509_utils_verify(), when server-purpose (X509_PURPOSE_SSL_SERVER) verification fails, the code falls back to client-purpose and any-purpose verification, so a trusted, hostname-matching certificate valid only for clientAuth can be accepted as the RDP server certificate. In environments relying on EKU separation between client and server certificates, this allows a clientAuth-only certificate issued by a trusted CA to bypass server certificate purpose validation.
Weakness
The product does not validate, or incorrectly validates, a certificate.
Potential Mitigations
Related Attack Patterns
References
- https://github.com/FreeRDP/FreeRDP/commit/f3b4347105114fe7453828736bea069999af319f
- https://github.com/FreeRDP/FreeRDP/security/advisories/GHSA-89c6-jjrw-96h4
- https://www.vulncheck.com/advisories/freerdp-before-tls-certificate-eku-bypass
- https://github.com/FreeRDP/FreeRDP/security/advisories/GHSA-89c6-jjrw-96h4