AsyncSSH versions 2.14.1 and earlier are vulnerable to a prefix truncation attack (Terrapin attack) targeting the ChaCha20-Poly1305 and Encrypt-then-MAC SSH algorithms. This attack allows a man-in-the-middle to strip messages after the initial key exchange, breaking SSH extension negotiation and downgrading connection security. The vulnerability exploits the lack of authentication for SSH_MSG_IGNORE messages and sequence number checks only after key exchange. The attack is guaranteed to work on ChaCha20-Poly1305 and requires CBC ciphers for practical exploitation of Encrypt-then-MAC. A patch is available that implements a 'strict kex' handshake to prevent this attack, requiring support on both client and server sides. As a temporary mitigation, disabling affected algorithms and using alternatives like AES-GCM is recommended.