KPMG-2002005: BitVise WinSSH Denial of Service

[Peter Gründl <pgrundl () kpmg dk>]

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        -=>BitVise WinSSH Denial of Service<=-
                      courtesy of KPMG Denmark

BUG-ID: 2002005         Released: 18th Mar 2002
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Problem:
========
Using "ill-intended connection attempts", a malicious user could
bring the server to a state where it would no longer accept incoming
SSH connections.

Vulnerable:
===========
- BitVise WinSSH prior to build 2002-03-16 on Windows 2000 Server

Details:
========
Due to differences in the SSHd and the underlying socket layer, it
was possible to abruptly end sessions and not have those freed
properly by the sshd. Each incomplete connection would use up a few
memory handles and allocate nonpaged kernel memory.

Windows can only handle a certain amount of kernel memory being
allocated, after that point most applications begin acting peculiar.

During testing we repeatedly caused the server to no longer accept
connections to port 22 (connection refused). This took about
1840x254 connections, but since it's not time or bandwidth related,
this attack could be carried out from a normal modem dialup,
and still be succesful.

The vendor has limited the amount of simultaneous unathenticated
sessions and put a timeout of 60 seconds on each connection.


Vendor URL:
===========
You can visit the vendors webpage here: http://www.bitvise.com

Vendor response:
================
The vendor was contacted on the 25th of February, 2002. On the
16th of March the vendor released the new build, that corrected
the issue. On the 18th of January, 2002, it was confirmed that the
patch corrected the issue mentioned in this advisory.

Corrective action:
==================
Upgrade to the latest build, which can be downloaded here:
http://www.bitvise.com/existing-users.html


   Author: Peter Gründl (pgrundl () kpmg dk)

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