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New Terrapin Flaw Might Let Attackers Downgrade SSH Protocol Safety

Jan 01, 2024NewsroomEncryption / Community Safety

SSH Protocol Security

Safety researchers from Ruhr College Bochum have found a vulnerability within the Safe Shell (SSH) cryptographic community protocol that might permit an attacker to downgrade the connection’s safety by breaking the integrity of the safe channel.

Known as Terrapin (CVE-2023-48795, CVSS rating: 5.9), the exploit has been described because the “first ever virtually exploitable prefix truncation assault.”

“By rigorously adjusting the sequence numbers through the handshake, an attacker can take away an arbitrary quantity of messages despatched by the shopper or server firstly of the safe channel with out the shopper or server noticing it,” researchers Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk mentioned.


SSH is a methodology for securely sending instructions to a pc over an unsecured community. It depends on cryptography to authenticate and encrypt connections between gadgets.

That is completed via a handshake during which a shopper and server agree upon cryptographic primitives and trade keys required for establishing a safe channel that may present confidentiality and integrity ensures.

Nonetheless, a foul actor in an energetic adversary-in-the-middle (AitM) place with the flexibility to intercept and modify the connection’s visitors on the TCP/IP layer can downgrade the safety of an SSH connection when utilizing SSH extension negotiation.

“The assault might be carried out in follow, permitting an attacker to downgrade the connection’s safety by truncating the extension negotiation message (RFC8308) from the transcript,” the researchers defined.

“The truncation can result in utilizing much less safe shopper authentication algorithms and deactivating particular countermeasures in opposition to keystroke timing assaults in OpenSSH 9.5.”

One other essential prerequisite essential to pulling off the assault is the usage of a weak encryption mode similar to ChaCha20-Poly1305 or CBC with Encrypt-then-MAC to safe the connection.

“In a real-world situation, an attacker might exploit this vulnerability to intercept delicate information or achieve management over crucial programs utilizing administrator privileged entry,” Qualys mentioned. “This danger is especially acute for organizations with giant, interconnected networks that present entry to privileged information.”


The flaw impacts many SSH shopper and server implementations, similar to OpenSSH, Paramiko, PuTTY, KiTTY, WinSCP, libssh, libssh2, AsyncSSH, FileZilla, and Dropbear, prompting the maintainers to launch patches to mitigate potential dangers.

“As a result of SSH servers and OpenSSH specifically are so generally used all through cloud-based enterprise software environments, it is crucial for corporations to make sure they’ve taken acceptable measures to patch their servers,” Yair Mizrahi, senior safety researcher of safety analysis at JFrog, informed The Hacker Information.

“Nonetheless, a weak shopper connecting to a patched server will nonetheless end in an weak connection. Thus, corporations should additionally take steps to establish each weak prevalence throughout their whole infrastructure and apply a mitigation instantly.”

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