Attack Discovered Against SSH

jd writes: Ars Technica is reporting a newly-discovered man-in-the-middle attack against SSH. This only works if you are using 'ChaCha20-Poly1305' or 'CBC with Encrypt-then-MAC', so it isn't a universal flaw. The CVE numbers for this vulnerability are CVE-2023-48795, CVE-2023-46445, and CVE-2023-46446. From TFA: At its core, Terrapin works by altering or corrupting information transmitted in the SSH data stream during the handshake -- the earliest stage of a connection, when the two parties negotiate the encryption parameters they will use to establish a secure connection. The attack targets the BPP, short for Binary Packet Protocol, which is designed to ensure that adversaries with an active position can't add or drop messages exchanged during the handshake. Terrapin relies on prefix truncation, a class of attack that removes specific messages at the very beginning of a data stream.

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.

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