Dreambox DM500 webserver long URL request denial of service

scip AG Vulnerability ID 3807 (08/29/2008)
http://www.scip.ch/cgi-bin/smss/showadvf.pl?id=3807

I. INTRODUCTION

Dreambox is a Linux-based DVB satellite, terrestrial and cable digital 
television decoder (set-top box), produced by German multimedia vendor 
Dream Multimedia.

More information is available at the official product web site at the 
following URL:

     http://www.dream-multimedia-tv.de/

II. DESCRIPTION

Marc Ruef at scip AG found an input validation error within the web 
interface of the model DM500C. Other models, for example DM500S, might 
be affected too.

An attacker is able to send a very long http request string (approx. 512 
bytes) to the web server which will cause a denial of service. The web 
interface and some parts of the operating system might not be 
responsible anymore.

III. EXPLOITATION

It is possible to exploit the vulnerability with a common web browser by 
using a long url.

The http web server fingerprinting suite httprecon[1] is able to 
reproduce the problem too. Just use the test plugin get_long (activated 
by default).

The Attack Tool Kit (ATK)[2] is able to exploit this vulnerability with 
the following generic ASL code (expand the long URL request):

    open|send GET http://192.168.0.1/aaa(...) 
HTTP/1.0\n\n|sleep|close|pattern_not_exists HTTP/1.# ### *

IV. IMPACT

Because the attack is possible without further authentication, users 
with access possibilities to the web server might affect the behavior of 
the device.

The web interface and some parts of the operating system are not 
responsible anymore. A restart of the device is required to provide full 
functionality again.

V. DETECTION

Detection of web based attacks requires a specialized web proxy and/or 
intrusion detection system. Patterns for such a detection are available 
and easy to implement.

VI. SOLUTION

We have informed Dream Multimedia on an early stage. Our technical 
requests were not answered nor confirmed. Therefore, not official 
statement, patch or upgrade is available.

We suggest to close the web interface in insecure environments. 
Especially the access via Internet should be prevented with restrictive 
configuation settings or an additional firewall device.

VII. VENDOR RESPONSE

Dream Multimedia has been informed first at 04/30/2008 via email to 
support-at-Dream-Multimedia-Tv.de. This message has been added to the 
ticketing system at http://tickets.dream-multimedia-tv.de.

The same day it was assured to verify the mentioned condition. No 
further response came back. Other requests remained unanswered too.

VIII. SOURCES

scip AG - Security Consulting Information Process (german)
http://www.scip.ch/

scip AG Vulnerability Database (german)
http://www.scip.ch/cgi-bin/smss/showadvf.pl?id=3807

computec.ch document data base (german)
http://www.computec.ch/download.php

IX. DISCLOSURE TIMELINE

2008/04/28 Identification of the vulnerability by Marc Ruef
2008/04/30 First information to support-at-Dream-Multimedia-Tv.de
2008/04/30 Automated reply by the ticketing system (ticket id 
2008043010000466)
2008/04/30 Manual response by Nils Weiberg announcing further investigation
2008/05/22 Request for current state of investigation
2008/05/22 Another response by Nils Weiberg that the research is ongoing
2008/07/15 Last request for current state of investigation
2008/07/15 Another response by Nils Weiberg without further details
2008/08/29 Public disclosure of the advisory

X. CREDITS

The vulnerability was discovered by Marc Ruef.

     Marc Ruef, scip AG, Zuerich, Switzerland
     maru-at-scip.ch
     http://www.scip.ch/

A1. BIBLIOGRAPHY

[1] http://www.computec.ch/projekte/httprecon/
[2] http://www.computec.ch/projekte/atk/

A2. LEGAL NOTICES

Copyright (c) 2007-2008 scip AG, Switzerland.

Permission is granted for the re-distribution of this alert. It may not 
be edited in any way without permission of scip AG.

The information in the advisory is believed to be accurate at the time 
of publishing based on currently available information. There are no 
warranties with regard to this information. Neither the author nor the 
publisher accepts any liability for any direct, indirect or 
consequential loss or damage from use of or reliance on this advisory.