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Category:   Application (Security)  >   Bitvise SSH Server Vendors:   Bitvise
Bitvise WinSSHD Protocol State Error Allows Remote Users to Cause the Secure Shell Server to Stop Accepting Incoming Connections
SecurityTracker Alert ID:  1003840
SecurityTracker URL:  http://securitytracker.com/id/1003840
CVE Reference:   GENERIC-MAP-NOMATCH   (Links to External Site)
Date:  Mar 18 2002
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  Exploit Included:  Yes  
Version(s): Prior to build 2002-03-16
Description:   KPMG reported a denial of service vulnerability in Bitvise's WinSSHD secure shell server. A remote user can cause the server to stop accepting incoming SSH connections.

It is reported that a remote user can exploit a flaw based on a difference in the session processing in the daemon and the underlying socket layer. A remote user can abruptly end a session in such a manner that the WinSSHD will not properly free the connection. It is reported that each incomplete connection uses up a few memory handles and allocates nonpaged kernel memory. After a certain amount of kernel memory has been allocated, the server will apparently stop behaving properly.

It is reported that a remote user can initiate 1840x254 connections (which could be over a long period of time from, for example, a dial-up connection) to cause the server to stop accepting SSH connections.

Impact:   A remote user can cause the server to stop accepting secure shell connections.
Solution:   The vendor has released a fixed version. In the fixed version, the vendor has reportedly limited the amount of simultaneous unathenticated sessions that are permitted and has placed a timeout of 60 seconds on each connection. The fix is available at:

http://www.bitvise.com/existing-users.html


Vendor URL:  www.bitvise.com/winsshd.html (Links to External Site)
Cause:   Resource error, State error
Underlying OS:  Windows (NT), Windows (2000), Windows (XP)

Message History:   None.


 Source Message Contents

Subject:  KPMG-2002005: BitVise WinSSH Denial of Service



--------------------------------------------------------------------

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

BUG-ID: 2002005         Released: 18th Mar 2002
--------------------------------------------------------------------
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



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through our security advisories. These advisories are a service to
the professional security community. In no event shall KPMG be lia-
ble for any consequences whatsoever arising out of or in connection
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