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Strengthen OpenSSH security?
From: Brett Glass <brett () lariat org>
Date: Mon, 17 Apr 2006 22:31:52 -0600

I'm sure that most folks with hosts that expose an OpenSSH daemon to the Internet have been getting the usual probes and password guessing attempts and have been taking appropriate actions (e.g. setting AllowUsers and using strong passwords) to protect yourselves. But today, on one of my servers, I noticed a new trend: the attackers are getting smarter. Apparently, they can tell the difference between a user ID which is not named in an AllowUsers directive -- or which does not exist at all -- from one for which they just haven't guessed the correct password. I've now watched as some attackers (but not all... yet) tried various user IDs, noted which ones existed and were in AllowUsers, and focused password guessing attacks on just those user IDs.

It seems to me that sshd should not tip its hand by returning different responses when a user ID can be used for logins than when it can't -- allowing an attacker to focus password guessing attacks on user IDs with which it would have a chance of gaining access. For those folks out there who are more familiar with OpenSSH than I am: How hard would it be to make the responses indistinguishable?

--Brett Glass

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