[UNIX] DansGuardian Hex Encoding URL Banned Extension Filter Bypass Vulnerability

From: SecuriTeam (support_at_securiteam.com)
Date: 08/01/04

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      DansGuardian Hex Encoding URL Banned Extension Filter Bypass Vulnerability
    ------------------------------------------------------------------------

    SUMMARY

     <http://dansguardian.org> DansGuardian is a web Open Source content
    filter available for various Unix based operating systems, including
    Linux. It filters the actual content of pages based on many methods
    including phrase matching, PICS filtering and URL filtering.

    DansGuardian allows malicious users to bypass the extension filter rules
    when processing URLs which contain an hex encoded filename.

    DETAILS

    Vulnerable Systems:
     * DansGuardian version 2.8 and prior

    Immune Systems:
     * DansGuardian version 2.8.0.1

    DansGuardian employs a content filter that is used to filter out certain
    unwanted type of files among other content. Using a hex encoded filename,
    a malicious user is able to fool the content filter, thus bypassing it
    completely. An example of such a case can be seen below:
    http://server/file.%65%78%65 or http://server/file%2eexe

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

    Patch Availability:
    There is already a newer version available which addresses this
    vulnerability. Users are encouraged to upgrade to version 2.8.0.1. As a
    temporary workaround, the following patch can be used:
    --- FOptionContainer.cpp.diff ---
    806d805
    < url.hexDecode();
    ---------------------------------

    Disclosure Timeline
    28/07/2004 Notification to the main developer (author at dansguardian dot
    org)
    28/07/2004 DansGuardian 2.8.0.1 released
    29/07/2004 Public Security Advisory.

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:ruben@udea.edu.co> Rub n
    Molina.

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