GHSA-473q-m89c-ghf8: 
C# vulnerability analysis and mitigation

Summary

With SslMode set to VerifyCA or VerifyFull with no configured CA certificate, an unvalidated TLS connection is treated as secure and a clear-text password can be sent before the certificate is checked. This can occur when relying on the password to validate a self-signed cert, e.g., MariaDB's zero-configuration TLS.

Impact

An attacker who can deploy a man-in-the-middle MySQL proxy presenting any self-signed certificate can capture the plaintext database password of a client that:

• uses SslMode = VerifyCA or VerifyFull, and
• does not use the SslCa connection string option, and
• uses a non-empty password. The attacker completes the TLS handshake with its own cert, requests a clear-text password authentication scheme (mysql_clear_password, or forces the caching_sha2_password/sha256_password clear-text fallback), and receives the password before the fingerprint check rejects the connection, defeating the man-in-the-middle protection VerifyFull is expected to provide. Not affected: a configured CA (SslCa), CA-validated TLS, local connections (Unix socket / loopback), or empty passwords. Using SslMode set to Preferred or Required is out of scope (this is documented as not authenticating the server).

Workarounds

Provide SslCa with VerifyCA/VerifyFull, use a local/Unix-socket connection, or avoid clear-text authentication.

References

• https://mariadb.org/mission-impossible-zero-configuration-ssl/