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Category:   Application (Security)  >   DansGuardian Vendors:   Barron, Daniel
DansGuarding File Extension Filter Can Be Bypassed With Hex-Encoded URLs
SecurityTracker Alert ID:  1010817
SecurityTracker URL:
CVE Reference:   GENERIC-MAP-NOMATCH   (Links to External Site)
Date:  Jul 30 2004
Impact:   Host/resource access via network
Fix Available:  Yes  Vendor Confirmed:  Yes  Exploit Included:  Yes  
Version(s): 2.8 and prior versions
Description:   A vulnerability was reported in DansGuardian. A remote user can create a URL that will bypass the file extension filtering.

Ruben Molina reported that a remote user can create a URL that contains a hex-encoded character to bypass the file extension filter.

Some demonstration exploit URLs are provided:


The vendor was reportedly notified on July 28, 2004.

Impact:   A remote user can bypass filtering rules to access a file with a banned file extension.
Solution:   The vendor has released a fixed version (, available at:

Vendor URL: (Links to External Site)
Cause:   Input validation error, State error
Underlying OS:  Linux (Any), UNIX (Any)

Message History:   None.

 Source Message Contents

Subject:  DansGuardian Hex Encoding URL Banned Extension Filter Bypass

DansGuardian Hex Encoding URL Banned Extension Filter Bypass Vulnerability

Original Release Date: 2004-07-29
Author: Ruben Molina (a.k.a fradiavolo)


1. Systems affected:

All DansGuardian up to and including DansGuardian 2.8.

2. Overview:

DansGuardian ( is a web Open Source content filter
for various Unix based operating systems, including Linux. It filters the
content of pages based on many methods including phrase matching, PICS
filtering and
URL filtering.

DansGuardian may allow malicious users to bypass the extension filter
rules when
processing URLs which contain an hex encoded filename (e.g:
http://server/file.%65%78%65 or http://server/file%2eexe).

3. Impact:

Under some installations, this may violate security policy, or allow users to
inadvertantly access malicious web content.

4. Solution:

Upgrade to DansGuardian

5. Patch:

--- FOptionContainer.cpp.diff ---
<     url.hexDecode();

6. Timeline and credits:

28/07/2004 Notification to the main developer (author at dansguardian dot
28/07/2004 DansGuardian released
29/07/2004 Public Security Advisory.

7. Thanks to: people and Silence Team ;)



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