Terrapin

cryptographysecurity

A 2023 attack that can weaken or truncate the SSH handshake.

Terrapin manipulated sequence numbers during the SSH handshake to strip messages and downgrade security in specific configurations. It prompted updates to OpenSSH and other SSH implementations.

Terrapin attacked SSH by manipulating sequence numbers during the handshake, allowing an attacker positioned in the network to delete messages from the beginning of the encrypted connection without detection. The effect is that security features negotiated at the start can be silently stripped.

Its interest is in what it says about integrity at boundaries. The SSH transport protects data after the handshake, and the handshake itself had a window where message deletion was not covered by the integrity checks that protect everything afterwards. That is a seam, and seams between protocol phases are where this whole family of attacks lives, alongside downgrade attacks and renegotiation flaws in TLS.

The practical remediation was a strict key exchange mode that both ends must support, which means the fix arrives only when both client and server are updated. That is the recurring difficulty with protocol vulnerabilities as opposed to implementation ones: patching your own side is insufficient, deployment is gated by the slowest participant, and the vulnerable configuration remains negotiable until support is removed rather than merely deprecated.

Also known as: Terrapin, CVE-2023-48795

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