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RhinoSoft Serv-U FTP Anonymous Remote DoS Vulnerability

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secondmotion-SM-SA-02-03                            Security Advisory
Topic: RhinoSoft Serv-U FTP Anonymous Remote DoS Vulnerability
Announced: 2002-06-11
Updated: 2002-06-11
Tested on: Serv-U FTP and earler
Not affected: Serv-U FTP 4.1
Obsoletes: /

This advisory is based on trial and error results both locally over
a standard LAN FTP, and remote Internet FTP configurations.  This
vulnerability was reproduced remotely at Cat-Soft with the permission
of Rob Beckers.  This document is subject to change without prior

The software developers and software vendors were informed of this
vulnerability on 17 September 2002.

If anyone reading this is aware of any further information relating
to this vulnerability, please contact the authors below or report
via BugTraq.

I. Background

	While working on a new security product to detect bugs in
	software, we considered that some FTP servers may work as 
	fast as possible to clear the buffer in Windows sockets. 
	Looking into this further in conjunction with our application
	we realised it may be possible to cause a Denial of Service
	(DoS) against certain FTP server products.

II. Problem Description

	By connecting to the Serv-U FTP server as a anonymous user or
	a local user then its possible to issue MKD commands.
	Looping a MKD command to Serv-U it will cause the application
	to stop accepting connections. Although this may be likened
	to a normal DoS attack by sending mass amounts of data to the server
	this vulnerability can be launched over a 56k connection, and
	therefore should not be categorised as a straight DoS weakness.
	The fault is caused due to Serv-U having no flood protection 
	against commands itself, only hammer attacks. MKD is used as it 
	forces Serv-u to check the user has access to the folder,
	which causes it to stop processing requests.

III. Impact:

	Version 4.04 and earler are affected by this vulnerability.
	Many home users/businesses use Serv-u FTP since it has a simple 
	GUI and also has many easy-to-use features. Using this
	it is possible to remotely shutdown FTP servers operating this 
	server application.

IV. Solution

	As of November 01, 2002 Rhinosoft/Cat-Soft have release version 4.1
	which is patched against this vulnerability. We recommend all
	users upgrade to Version 4.1 of Serv-U immediately.

V. Credits

	matt@xxxxxxxxxxxxxxxx - Matt Thompson [Proof of Concept]
	paul@xxxxxxxxxxxxxxxx - Paul Smurthwaite
	Rob Beckers - Cat-Soft [for working with us on this]

VI. Source code

	A Proof of Concept tool can be provided at short notice on request.

- -ends-

Matt Thompson

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