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Re: Strengthen OpenSSH security?
From: Damien Miller <djm () mindrot org>
Date: Thu, 20 Apr 2006 15:13:54 +1000 (EST)

On Mon, 17 Apr 2006, 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?

It would help if you reported what the actual differences you were
experiencing are, the operating systems and versions of OpenSSH in
question and how you came to the conclusion that password guessers were
able to figure out the difference.

OpenSSH already tries to minimise any exposed difference when attempting
authentication using a valid account, a non-existent account and a valid
account barred by Allow/DenyUsers policy.

On the platforms immediately at my disposal (Linux running an older
OpenSSH and OpenBSD running CVS -current OpenSSH), there is no visible
difference in the protocol messages sent or in the rough timing between
them when making incorrect password guesses between a valid account in
AllowUsers and one that is not.

We would consider divergences from this as bugs, so if you find one then
please report it to us (see http://www.openssh.com/report.html)

-d


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