“The Terrapin attack is a novel cryptographic attack targeting the integrity of the SSH protocol, the first-ever practical attack of its kind, and one of the very few attacks against SSH at all. The attack exploits weaknesses in the specification of SSH paired with widespread algorithms, namely ChaCha20-Poly1305 and CBC-EtM, to remove an arbitrary number of protected messages at the beginning of the secure channel, thus breaking integrity. In practice, the attack can be used to impede the negotiation of certain security-relevant protocol extensions. Moreover, Terrapin enables more advanced exploitation techniques when combined with particular implementation flaws, leading to a total loss of confidentiality and integrity in the worst case.”

“Although we suggest backward-compatible countermeasures to stop our attacks, we note that the security of the SSH protocol would benefit from a redesign from scratch, guided by all findings and insights from both practical and theoretical security analysis, in a similar manner as was done for TLS 1.3.”