The Terrapin Attack: How a Few Deleted Packets Weaken SSH

Short summary

Terrapin (CVE-2023-48795) is a prefix truncation attack on SSH where a man-in-the-middle attacker deletes messages during the unencrypted handshake phase, causing client and server to silently disagree on sequence numbers. This can strip extension negotiation messages, downgrading security features like stronger authentication or keystroke timing defenses. ChaCha20-Poly1305 and CBC with Encrypt-then-MAC modes are vulnerable; the attack doesn't decrypt data but weakens the negotiated security posture.

• •Terrapin manipulates SSH sequence numbers during the unencrypted handshake to silently delete messages
• •Deleting SSH_MSG_EXT_INFO strips security extensions, potentially downgrading authentication strength
• •ChaCha20-Poly1305 and CBC-ETM modes are vulnerable; the attack exploits seams between protocol phases

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