Analysis of SSH crc32 compensation attack detector exploit

Dave Dittrich dittrich at
Wed Nov 14 18:21:49 GMT 2001

On Thu, 8 Nov 2001, Dave Dittrich wrote:

>     ==========================================================
>     Analysis of SSH crc32 compensation attack detector exploit
>     ==========================================================

I received comments from a number of people about identification
of "affected" SSH servers.  One such message is here:

>From markus at Tue Nov 13 18:43:41 2001
Date: Sun, 11 Nov 2001 14:35:31 +0100
Subject: Re: Analysis of SSH crc32 compensation attack detector exploit
From: Markus Friedl <markus at>
To: Dave Dittrich <dittrich at>

 . . .

> are quite visible (remember, this is stock 1.2.31 on
> Red Hat Linux 6.0 -- syslog signatures for OpenSSH were not obtained
> in this analysis):

with OpenSSH you would only see the 'fatal:' messages,
the 'Connection from' will not be displayed in the default

> One final point.  Note the last syslog entry.  The successful exploit
> causes an authentication attempt to pause while the shell code back door
> becomes active.  You can connect to the shell and do whatever you
> want.  Only problem is, the original SSH daemon (at least with
> 1.2.31) will timeout when the authentication doesn't complete, and the
> shell will be terminated.

same applies to openssh and probably all ssh-1.x versions.

the rules are simpler:

1) protocol 2 only

are not affected, since no protocol v1 is iisnvolved.

2) protocol 1 und 2 support

supports both protocol versions, it gets more difficult.
for the commercial server, you never know the version
of the server that will be called for the fallback,
you have to assume that all
are affected, and
are affected for versions x.y < 2.3

3) protocol 1 only
is affected versions x.y < 2.3

and the commercial versions.


 [updates to table removed]


The analysis has been updated to reflect this, and the script
modified somewhat.  The most recent version can be found at:

Dave Dittrich                           Computing & Communications
dittrich at             University Computing Services    University of Washington

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