“Terrapin is not a simple software bug that can be fixed with an update to a single library or component.”
Ruhr University Bochum found that an attacker in the middle can delete SSH handshake messages by shifting sequence numbers, and neither side notices. That can downgrade user authentication or switch off OpenSSH 9.5’s keystroke timing defense. It works against ChaCha20-Poly1305 and CBC with Encrypt-then-MAC, modes that scans show on 77% of servers. The fix only works when client and server both get it, so the long tail of unpatched embedded SSH stays exposed for years.