Re: Remote DoS attack against SSH Secure Shell for Windows Servers Vulnerability

[Kirsi Niskanen <Kirsi.Niskanen () SSH COM>]

If there are more than 64 active connections in the SSH Secure Shell
Windows server, the sshd mother process dies. We (SSH Communications
Security) are currently fixing this problem. In the meantime, a
temporary solution is available. As every active connection creates at
least one event, setting the MaxConnections keyword in the sshd2_config
file to a value under 64 fixes this problem.

## General settings
        MaxConnections                  50

Setting the maximum number of connections to 50 leaves a safe margin for
additional events which are generated depending on the environment.

--
Kirsi Niskanen
Technical Service Engineer
SSH Communications Security Corp

http://www.ssh.com/
http://www.ssh.com/support/ssh/
http://www.ssh.com/tech/ssh_query.html/

--Security for your convenience--


> Hash: SHA1
>
>                                    U.S.S.R labs
>                              Buenos Aires, Argentina
>                              http://www.ussrback.com
>
> Topic:              Remote DoS attack against SSH Secure Shell for
> Windows Servers Vulnerability
> USSR Advisory Code: USSR-2001001
> Announced:          2001-03-16
> Credits:            Luciano Martins <luck () ussrback com>
> Affects:            SSH Secure Shell for Windows Server 2.4
> Corrected:          SSH Secure Shell for Windows Server 2.5 available
> soon
> Vendors Affected:   SSH Communications Security Corp
>                     http://www.ssh.com/
>
> 1. Discussion
>
> UssrLabs has recently discovered a problem with Windows versions of
> sshd.
> The problem lies with adjacent connection handling where the sshd is
> unable to handle 64 simulataneous connections. As a result the sshd
> will
> crash, and no services to the sshd will be accepted.
>
> The problem lies in ssheloop.c where the assertion test fails.
>
> The Event Log displays the following code after 64 connections are
> spawned:
>
>   FATAL ERROR: E:\src\lib\sshutil\ssheloop\win32\ssheloop.c:1597
> SshEventLoop
>   (function name unavailable) Assertion failed:
> ssh_adt_num_objects(ssh_eloop_events) < 64
>
> No doubt, proper error handling techniques have not been implemented
> for
> the SshEventLoop which ultimately causes the crash.
>
> This results in a Denial of Service against the service in question.
>
> 2. Vendor Status:
> Informed, Contacted.
>
> 3. Fix:
> official fix will be available soon.
>
> Related Links:
>
> Underground Security Systems Research:
> http://www.ussrback.com
>
> CrunchSp Product:
> http://www.crunchsp.com
>
> About:
>
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> Copyright (c) 1999-2000 Underground Security Systems Research.
> Permission is hereby granted for the redistribution of this alert
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>
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>
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