Services using internationalized (non-ASCII) domain names are potentially vulnerable to TLS connections made from AnyIO's connect tcp() or directly via TLSStream.wrap() where the connection has (through other means) been hijacked and redirected to a malicious server. The attacker would obtain a legitimate certificate using the IDNA 2003 encoded version of the domain name and offer it to the connecting client, making the certificate validate properly on the client's end.

The vulnerability will be patched in v4.14.2.

Encode host names via the idna package prior to connecting.