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.