Terrapin Attack (CVE-2023-48795).

A 2023 prefix-truncation flaw in the SSH transport protocol that allows an active network attacker to silently downgrade or strip extensions during the handshake, weakening features like keystroke timing protection. Cette notion relève de la catégorie Attaques et menaces en cybersécurité.

A 2023 prefix-truncation flaw in the SSH transport protocol that allows an active network attacker to silently downgrade or strip extensions during the handshake, weakening features like keystroke timing protection.

The Terrapin attack (CVE-2023-48795), disclosed by Bäumer, Brinkmann, and Schwenk in December 2023, is a prefix-truncation flaw in the SSH binary packet protocol. By manipulating packet sequence numbers during the initial KEX handshake — specifically when ChaCha20-Poly1305 (`chacha20-poly1305@openssh.com`) or CBC-EtM ciphers are used — a man-in-the-middle attacker can truncate the secure channel's initial messages without the integrity check failing, causing both ends to disagree about which extensions were negotiated. The most cited practical impact is silently disabling extensions such as keystroke-timing obfuscation; the underlying primitive can also weaken implementation-specific authentication features (notably AsyncSSH's CVE-2023-46446). Terrapin spurred quick fixes: OpenSSH 9.6 introduced a strict KEX mode that resets sequence numbers, and most major SSH implementations (Dropbear, libssh, PuTTY, Bitvise, Cisco IOS) followed. Mitigation is to upgrade clients and servers so both negotiate `strict-kex`, and to prefer AES-GCM ciphers, which are unaffected.

Les défenses contre Terrapin Attack (CVE-2023-48795) combinent habituellement des contrôles techniques et des pratiques opérationnelles, comme détaillé dans la définition ci-dessus.

Noms alternatifs courants : CVE-2023-48795, SSH prefix truncation.