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Re: Strengthen OpenSSH security?
From: Mike Hoskins <mhoskins () e2open com>
Date: Wed, 19 Apr 2006 20:21:25 -0700

Brett Glass wrote:
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?

This has been a known issue for some time (Google), so I guess it's about time someone started using it rather than the usual "a, aa, aaa, aba, abb, ... zzzzzzzzz" type attacks I usually see. Those always make me laugh.

While I agree with your point, I'm not versed enough in the SSH protocol (and don't feel like Googling again myself) to know if there's a technical reason (timing, etc.) that this behavior exhibits itself. This should raise an interesting discussion.

FWIW, I found that this sort of enumeration was possible myself when researching 2-factor authentication with OpenSSH.

That being said, I'd suggest configuring 2-factor authentication for any truly critical SSH gateways into your network. If you don't want to dole out the cash for key fobs (including the cash to purchase them again and again as employees break them with really creative excuses ;), you can use public keys as "something employees have" along with the usual password for "something employees know". With two-factor enabled, having either the key (stolen laptop) or the password (successful dictionary attack) won't permit access and will log the attempt.

http://bugzilla.mindrot.org/show_bug.cgi?id=983

Have been using this patch against the latest OpenSSH-stable release for awhile now along with appropriate log watchers, and getting a bit more sleep at night.

--Mike


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