Researchers have published details about a new attack method that exploits a vulnerability in the Secure Socket Shell or SSH networking protocol that raises concerns about the security of data transmissions across networks.

Dubbed a “Terrapin Attack” by researchers Fabian Bäumer, Marcus Brinkmann and Jörg Schwenk, the vulnerability allows attackers to manipulate sequence numbers in SSH connections. SSH sequence numbers are used to ensure the integrity and order of data packets transmitted over the network, preventing data loss or tampering during the secure communication process.

The vulnerability is said to be particularly potent against connections using ChaCha20-Poly1305 or CBC-mode ciphers with Encrypt-then-MAC. ChaCha20-Poly1305 is a modern, high-speed encryption algorithm offering strong security, while CBC-mode ciphers with Encrypt-then-MAC is an older encryption method – both are used to secure data in SSH.

To exploit the Terrapin Attack vulnerability, attackers must occupy a “man-in-the-middle” position where an attacker intercepts SSH communications between two parties. The positioning allows them to manipulate the sequence numbers in SSH packets. Specifically targeting SSH connections, the attackers can undermine the integrity and security of the data transmission, leading to potential breaches in encrypted communications.

The implications of the Terrapin Attack method are far-reaching, given SSH’s widespread use in secure data communications, with most SSH implementations, including on popular platforms, potentially at risk.

According to the researchers, many vendors have updated their SSH implementation to support an optional strict key exchange, a backward-incompatible change to the SSH handshake that introduces sequence number resets and takes away an attacker’s capability to inject packets during the initial, unencrypted handshake. However, both the client and server must support this feature to take effect.

For enterprises, the researchers recommend disabling the affected encryption modes or installing patches, where available, that support strict key exchange as immediate steps. The researchers have also released a vulnerability scanner on GitHub to help organizations assess their exposure to a Terrapin Attack.

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