full-disclosure - Re: OpenSSH User Enumeration Time-Based Attack

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Date: Wed, 10 Jul 2013 15:17:30 -0700
From: Swair Mehta <swairmehta@...il.com>
To: Curesec Research Team <crt@...esec.com>
Cc: "full-disclosure@...ts.grok.org.uk" <full-disclosure@...ts.grok.org.uk>
Subject: Re: OpenSSH User Enumeration Time-Based Attack

I havent tried this yet but it makes sense.
To avoid linearization attacks from figuring out sensitive data, there are
specific precautions that are taken. (some UNIX login program had a similar
timing issue if I am not mistaken).
>From the looks of it, sshd is looking for the username in some file and if
it doesn't exist, it waits sometime and returns, if it does, then it
calculates the long hash.


On Wed, Jul 10, 2013 at 6:38 AM, Curesec Research Team <crt@...esec.com>wrote:

> Hi List,
>
> today, we will show a bug concerning OpenSSH. OpenSSH is the most used
> remote control software nowadays on *nix like operating systems. Legacy
> claims it replaced unencrypted daemons like rcp, rsh and telnet. Find a
> version at: https://www.openssh.com.
>
> By testing several OpenSSH installations we figured there is a delay of
> time when it comes to cracking users (not) existing on a system. A
> normal Brute-force-Attack tests for the correct user and password
> combination, usually without knowledge if the user on the system exists.
>
> For instance, the attacker is interested in the all-mighty “root” aka
> “toor” account. He might go for password combinations like:
>
> root:root
> root:toor
> root:password
> root:system
>
> and so on. Permanent attacks against the service normally running on
> Port 22/tcp implicate that Ssh-Brute-force-Attacks are still profitable.
> If you are an Auditor and want to check for interesting accounts it
> might be worthy to know which ones are available on the system to run a
> more focused attack.
>
> To assist you in this issue, there is a little trick to find out a User
> name before trying to cracking it. To do this the length of the password
> needs to be increased massively. In our case we go with 39.000
> characters(A’s). Trying those passwords at an existing and a
> non-existing account shows a quite high delay.
>
>
> Find the rest of the post + some example code at the blogpost.
>
> http://cureblog.de/openssh-user-enumeration-time-based-attack/
>
> Cheers,
> Curesec Research Team
>
> _______________________________________________
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-- 
Swair Mehta

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