Terrapin SSH Connection Weakening

In this paper, the authors show that as new encryption algorithms and mitigations were added to SSH, the SSH Binary Packet Protocol is no longer a secure channel: SSH channel integrity (INT-PST) is broken for three widely used encryption modes. This allows prefix truncation attacks where some encrypted packets at the beginning of the SSH channel can be deleted without the client or server noticing it. They demonstrate several real-world applications of this attack. They show that they can fully break SSH extension negotiation (RFC 8308), such that an attacker can downgrade the public key algorithms for user authentication or turn off a new countermeasure against keystroke timing attacks introduced in OpenSSH 9.5. They also identified an implementation flaw in AsyncSSH that, together with prefix truncation, allows an attacker to redirect the victim's login into a shell controlled by the attacker. Related proof of concept code from their github has been added to this archive.