D-Link DIR-100 long url filter evasion scip AG Vulnerability ID 3808 (09/08/2008) http://www.scip.ch/cgi-bin/smss/showadvf.pl?id=3808 I. INTRODUCTION D-Link DIR-100 is a small and cost-effective router and firewall device for small offices and home users. More details are available at the official product web site (German link): http://www.dlink.de/?go=gNTyP9CgrdFOIC4AStFCF834mptYKO9ZTdvhLPG3yV3oV492 gqltbNlwaaFp6DQoHDrpxC5H+40AAdvl II. DESCRIPTION Marc Ruef at scip AG found a possibility to evade url filters of the web proxy to prevent access to web sites. An attacker might add a very long string to the url to access web resources althought their access is forbidden. This problem could be verified in all firmware versions up to v1.12. A similar vulnerability was already detected years ago in a similar device Netgear RP114. [1, 2] III. EXPLOITATION It is possible to exploit the vulnerability with a common web browser by using a long url (approx. 1'300 chars). You can expand the length of the url by adding a non-used http get request parameter. Example url: http://www.scip.ch/?foo=aaa(...) A video illustrating this issue is available at the following url: http://de.youtube.com/watch?v=WTzPn37XNl4 The Attack Tool Kit (ATK)[3] is able to exploit this vulnerability with the following generic ASL code (expand the long URL request): open|send GET http://www.scip.ch/?foo=aaa(...) HTTP/1.0\n\n|sleep|close|pattern_not_exists *This URL is <font color=red>blocked</font> by administrator !* IV. IMPACT With this vulnerability users are able to access forbidden web resources without being filtered by the integrated web proxy service. 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 D-Link on an early stage. Our technical requests were not answered nor confirmed. Therefore, not official statement, patch or upgrade is available. We suggest the use of another device for filtering forbidden web resources successfully. VII. VENDOR RESPONSE D-Link has been informed first via the unhandy web form at http://www.dlink.com (no public mail address for such cases could be found). The first responses claimed that the problem must be within a wrong configuration setting. Further discussions were initiated. The support was not able to understand the problem. Not even after several step-by-step guides and examples. They always suggest I have to upgrade to the latest firmware and they could not verify the problem. Therefore, no official solution, workaround or patch is available. 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=3808 computec.ch document data base (german) http://www.computec.ch/download.php IX. DISCLOSURE TIMELINE 2008/07/25 Identification of the vulnerability by Marc Ruef 2008/07/28 First information to D-Link via web form 2008/07/28 First reply by D-Link support via support (at) service.dlink (dot) biz [email concealed] (ticket id 1375981) 2008/07/29 Providing our config for further analysis 2008/08/06 Request for actual status (no reply) 2008/08/29 Another request for actual status 2008/08/29 Response could not verify the problem 2008/09/01 Detailed explanation of the exploitation 2008/09/01 Responder could still not understand the problem 2008/09/08 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.securityfocus.com/bid/10404 [2] http://seclists.org/bugtraq/2004/May/0263.html [3] 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.