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Roughly 11 million Internet-exposed servers remain susceptible to a recently discovered vulnerability that allows attackers with a foothold inside affected networks. Once they’re in, attackers compromise the integrity of SSH sessions that form the lynchpin for admins to securely connect to computers inside the cloud and other sensitive environments.

Terrapin, as the vulnerability has been named, came to light two weeks ago in a research paper published by academic researchers. Tracked as CVE-2023-48795, the attack the researchers devised works when attackers have an adversary-in-the-middle attack (also abbreviated as AitM and known as man-in-the-middle or MitM), such as when they’re positioned on the same local network and can secretly intercept communications and assume the identity of both the recipient and the sender.

In those instances, Terrapin allows attackers to alter or corrupt information transmitted in the SSH data stream during the handshake—the earliest connection stage, when the two parties negotiate the encryption parameters they will use to establish a secure connection. As such, Terrapin represents the first practical cryptographic attack targeting the integrity of the SSH protocol itself. It works by targeting BPP (Binary Packet Protocol), which is designed to ensure AitMs can’t add or drop messages exchanged during the handshake. This prefix truncation attack works when implementations support either the “ChaCha20-Poly1305” or “CBC with Encrypt-then-MAC,” cipher modes, which, at the time the paper was published, was found in 77 percent of SSH servers.