Quoting Marco Ivaldi (raptorat_private): > Security Advisory @ Mediaservice.net Srl > (#01, 30/04/2003) Data Security Division > > Title: OpenSSH/PAM timing attack allows remote users identification > Application: OpenSSH-portable <= 3.6.1p1 > Platform: Linux, maybe others > Description: A remote attacker can identify valid users on vulnerable > systems, all PAM-enabled systems are potentially affected > Author: Marco Ivaldi <raptorat_private> > Contributors: Maurizio Agazzini <inodeat_private>, > Solar Designer <solarat_private>, > Andrea Ghirardini <pilaat_private> > Vendor Status: OpenSSH team notified on 12/04/2003, > vendor-sec list notified on 28/04/2003 > CVE Candidate: The Common Vulnerabilities and Exposures project has assigned > the name CAN-2003-0190 to this issue. > References: http://lab.mediaservice.net/advisory/2003-01-openssh.txt > http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2003-0190 > > 1. Abstract. > > 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. > > 2. Example Attack Session. > > root@voodoo:~# ssh [valid_user]@lab.mediaservice.net > [valid_user]@lab.mediaservice.net's password: <- arbitrary (non-null) string > [2 secs delay] > Permission denied, please try again. > > root@voodoo:~# ssh [no_such_user]@lab.mediaservice.net > [no_such_user]@lab.mediaservice.net's password: <- arbitrary (non-null) string > [no delay] > Permission denied, please try again. > > 4. Fix. The "Fix" is to encourage all users/admins of OpenSSH to _only_ work with key authentication (preferable only ssh2 protocol) on all ssh servers. Switch the default: PasswordAuthentication yes Into: PasswordAuthentication no in sshd_config In combination with the default "RSAAuthentication yes" it results in: ,-------- | kh@i4x:~$ ssh dodo@i4x <-dodo=no_such_user | [no delay] | Permission denied (publickey). `-------- The same as: ,-------- | kh@i4x:~$ ssh root@i4x | [no delay] | Permission denied (publickey). `-------- That would be my 2Cent. Karl-Heinz
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