iDEFENSE Security Advisory 11.04.02a: Pablo FTP Server DoS Vulnerability

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iDEFENSE Security Advisory 11.04.02a:
http://www.idefense.com/advisory/11.04.02a.txt
Pablo FTP Server DoS Vulnerability
November 4, 2002

I. BACKGROUND

Pablo Software Solutions' FTP Server is a multi-threaded FTP server
for Windows 98, NT 4.0, 2000 and XP. More information about it is
available at http://www.pablovandermeer.nl/ftp_server.html.

II. DESCRIPTION

Because of its incorrect handling of format string markers in
user-provided input, the FTP Server can be remotely crashed if it
attempts to process such malformed input; code execution is also a
possibility. The denial of service condition is exploited by
attempting to login to the target FTP server as '%n'.

III. ANALYSIS

Successful exploitation should crash the FTP server. What is most
damaging about this is that the files and resources readily made
available by the server's proper functionality are inaccessible for
the duration that the server is attacked. While no exploit currently
exists, it is possible to execute arbitrary code.

IV. DETECTION

Pablo FTP Server 1.3 and 1.5, running on Windows 2000; version 1.2 is
reportedly vulnerable as well. Connecting to an arbitrary Pablo FTP
Server and providing a username of "%x%x%x%x" can determine
susceptibility. The server is vulnerable if an entry such as the
following is found in the produced log files:

[1064] 530 Please login with USER and PASS
[1064] USER f7db018409be31
[1064] 331 Password required for 247db018409be32

The username values that show up in the log files are pulled from
memory (the stack) and should differ from system to system.

V. WORKAROUND

Use a filtering proxy server to help mitigate the attack by blocking
requests that contain format string markers.

VI. VENDOR FIX

Version 1.51, which fixes the problem, is available at
http://www.pablovandermeer.nl/ftpserver.zip.

VII. CVE INFORMATION

The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project
assigned the identification number CAN-2002-1244 to this issue.

VIII. DISCLOSURE TIMELINE

10/15/2002 Issue disclosed to iDEFENSE
10/31/2002 Author notified
10/31/2002 iDEFENSE clients notified
11/01/2002 Response received from [email protected]
11/04/2002 Coordinated public disclosure

IX. CREDIT

Texonet (http://www.texonet.com) discovered this vulnerability.

Get paid for security research
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• -dave

David Endler, CISSP
Director, Technical Intelligence
iDEFENSE, Inc.
14151 Newbrook Drive
Suite 100
Chantilly, VA 20151
voice: 703-344-2632
fax: 703-961-1071

[email protected]
www.idefense.com

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