GHSA-wvc4-j7g5-4f79: NATS TLS certificate common name validation bypass

The NATS official Rust clients are vulnerable to MitM when using TLS.

A fix for the nats crate hasn’t been released yet. Since the nats crate is going to be deprecated anyway, consider switching to async-nats >= 0.29 which already fixed this vulnerability.

The common name of the server’s TLS certificate is validated against the hostname provided by the server’s plaintext INFO message during the initial connection setup phase. A MitM proxy can tamper with the host field’s value by substituting it with the common name of a valid certificate it controls, fooling the client into accepting it.

Reproduction steps

1. The NATS Rust client tries to establish a new connection
2. The connection is intercepted by a MitM proxy
3. The proxy makes a separate connection to the NATS server
4. The NATS server replies with an INFO message
5. The proxy reads the INFO, alters the host JSON field and passes the tampered INFO back to the client
6. The proxy upgrades the client connection to TLS, presenting a certificate issued by a certificate authority present in the client’s keychain. In the previous step the host was set to the common name of said certificate
7. rustls accepts the certificate, having verified that the common name matches the attacker-controlled value it was given
8. The client has been fooled by the MitM proxy into accepting the attacker-controlled certificate

References

• nats-io/nats.rs#881
• nats-io/nats.rs#887
• https://rustsec.org/advisories/RUSTSEC-2023-0029.html