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Re: OpenSSH/PAM timing attack allows remote users identification
From: Thilo Schulz <arny () ats s bawue de>
Date: Thu, 1 May 2003 17:20:44 +0200

On Wednesday, 30th April 2003 16:34 Marco Ivaldi wrote:
During a pen-test we stumbled across a nasty bug in OpenSSH-portable with
PAM support enabled (via the --with-pam configure script switch). This bug
allows a remote attacker to identify valid users on vulnerable systems,
through a simple timing attack. The vulnerability is easy to exploit and may
have high severity, if combined with poor password policies and other
security problems that allow local privilege escalation.

This is !!NOT!! a problem specific to openssh.
When I saw this topic come up I tried the same with proftpd, which also can 
use pam to establish the user's authentication.

here is an example with the simple ftp tool:
thilo () Thilo thilo $ ftp www.someftphost.net
Connected to www.someftphost.net.
220 ProFTPD 1.2.5rc1 Server (Debian) [www.someftphost.net]
Name (www.someftphost.net:thilo): thilo
331 Password required for thilo.
Password:
[valid user account, but wrong password: 2seconds wait]
530 Login incorrect.
Login failed.
ftp>

same here, if this is an invalid user, there is no delay between the entering 
of the password and the 530 reply.
I tested the postfix smtp daemon, apache and ipopd pop3 daemon which have pam 
support, there this weakness is obviously not present, yet don't consider all 
daemons secure, there may still be many others out there that suffer from the 
same weakness.

 - Thilo Schulz


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