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Pablo FTP Server DoS Vulnerability



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.


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'.


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.


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.


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


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


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


10/15/2002 Issue disclosed to iDEFENSE
10/31/2002 Author notified
10/31/2002 iDEFENSE clients notified
11/01/2002 Response received from pablovandermeer@kabelfoon.nl
11/04/2002 Public disclosure


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