Bug exposes OpenSSH servers to brute-force password guessing attacks

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A bug in OpenSSH, the most popular software for secure remote access to UNIX-based systems, could allow attackers to bypass authentication retry restrictions and execute many password guesses.

A security researcher who uses the online alias Kingcope disclosed the issue on his blog last week, but he waited until Tuesday to request a public vulnerability ID to be assigned.

By default, OpenSSH servers allow six authentication retries before closing a connection and the OpenSSH client allows three incorrect password entries, Kingcope said.

However, OpenSSH servers with keyboard-interactive authentication enabled, which is the default setting on many systems including FreeBSD ones, can be tricked to allow many authentication retries over a single connection, according to the researcher.

"With this vulnerability an attacker is able to request as many password prompts limited by the 'login grace time' setting, that is set to two minutes by default," Kincope said.

Depending on the server and the connection, two minutes could allow for thousands of retries, which could be enough to guess common or weak passwords using dictionary-based attacks.

According to a discussion on Reddit, setting PasswordAuthentication to 'no' in the OpenSSH configuration and using public-key authentication does not prevent this attack, because keyboard-interactive authentication is a different subsystem that also relies on passwords.

Therefore, users should set ChallengeResponseAuthentication and KbdInteractiveAuthentication to 'no' in their configurations.

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