Terrapin Flaw, CVE-2023-48795, Leaves 11 million SSH Servers Vulnerable

Terrapin Flaw - SSH Vulnerable
Terrapin Flaw – SSH Vulnerable

SSH Server Vulnerable to Terrapin Flaw – Analysis

Security researchers from Ruhr University Bochum in Germany have identified a Terrapin flaw in the Secure Shell (SSH) cryptographic network protocol.

Recognized by the CVE-2023-48795 prefix truncation attack, can potentially be exploited by malicious actors to weaken the security measures in a normally considered secure channel.

What is a Terrapin Flaw attack?

  • Target the SSH protocol, affecting both clients and servers.
  • Manipulate sequence numbers during the handshake process to compromise the integrity of the SSH channel.
  • It can downgrade public key algorithms and turn off defenses against keystroke timing attacks.
  • Require attackers to be in an adversary-in-the-middle (AitM) position.
  • Allowing an attacker to downgrade the connection’s security by truncating the extension negotiation message (RFC8308) from the transcript.
  • The truncation can lead to using less secure client authentication algorithms and deactivating specific countermeasures against keystroke timing attacks in OpenSSH 9.5.

How To Perform Terrapin Attack?

  • To perform Terrapin attack, require MitM capabilities at the network layer (the attacker must be able to intercept and modify the connection’s traffic).
  • Additionally, the connection must be secured by either ChaCha20-Poly1305 or CBC with Encrypt-then-MAC. However, our scan indicates an extensive adoption of these encryption modes; therefore, Terrapin applies to most real-world SSH sessions.
Terrapin attack flow
Terrapin attack flow – Image by Terrapin


  • Nearly 11 million SSH servers are vulnerable (52% of scanned servers).
  • The US, China, Germany, Russia, Singapore, and Japan are the most vulnerable systems.
  • Attackers have a large pool of vulnerable servers to choose from.

Additional points:

  • Ruhr University Bochum team provides a Terrapin vulnerability scanner.
  • CISA warns of actively exploited bugs in the Chrome and Excel parsing libraries.

CVE numbers. These are:

  • CVE-2023-48795: General Protocol Flaw
  • CVE-2023-46445: Rogue Extension Negotiation Attack in AsyncSSH
  • CVE-2023-46446: Rogue Session Attack in AsyncSSH

Call to action:

  • Check your SSH client or server for Terrapin vulnerability.
  • Update to the latest version of OpenSSH.

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