[NT] Microsoft IIS ssinc.dll Over-long Filename Buffer Overflow Vulnerability

From: SecuriTeam (support_at_securiteam.com)
Date: 05/30/03

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    Date: 30 May 2003 23:07:35 +0200

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      Microsoft IIS ssinc.dll Over-long Filename Buffer Overflow Vulnerability


    NSFOCUS Security Team has found a buffer overflow vulnerability in the way
    a dynamic linking library (ssinc.dll) that is shipped with Microsoft IIS
    4.0/5.0 handles the files it parsers. Exploiting the vulnerability would
    allow local attackers to gain SYSTEM privileges.


    Microsoft IIS supports SSI (Server Side Include) functionality. The ssinc
    DLL is a SSI interpreter. By default the extended names .stm,.shtm and
    shtml will be mapped to interpreter (ssinc.dll).

    SSI supports the "#include" command. Typically, it is used in the
    following manner:

    <!--#include file="filename"-->

    The interpreter's handling of the "#include" command first requires it to
    attempt to gain the physical path of the shtml file, to do so it will copy
    the URI request to a buffer of a fixed size.

    For example, when requesting the following shtml file:
    http://iishost/abc/test.shtml, it will copy the string "/abc/test.shtml"
    to the buffer.

    Because the length of the shtml filename is not been checked during the
    copy procedure, it can cause a stack overflow.

    NSFocus Security Team has found a similar problem in the past. See:
    http://www.nsfocus.net/index.php?act=advisory&do=view&adv_id=17, and the
    corresponding Microsoft Security Bulletin is found at:

    Microsoft's pervious patch to IIS has involved checking for an over-long
    shtml filename in order to avoid a buffer overflow. Whenever an overly
    long shtml filename is found, it will shorten the filename to a legal
    length, and then attempt to open it. However, after it has been successful
    in opening the file, the ssinc.dll will revert back to the original
    filename (which can be overly long).

    Therefore, by creating a WEB file with a special length, local attackers
    could bypass the check, then request a shtml file with an overly long
    filename to cause a buffer overflow, this in turn causes IIS cease to
    respond. By carefully constructing the overflow data, attackers could run
    arbitrary code with local SYSTEM privileges.

    However, two conditions are required to carry out the attack:
    1. Attackers need to have the privilege to create files on web directory.
    2. Attackers need to be able to access the created files via the web site.

    1. Disable untrusted users' writing privileges to web directory.
    2. If SSI functionality is not required, remove .shtml, .shtm, .stm
    3. Install IIS Lockdown tools provided by Microsoft:
    <http://www.microsoft.com/downloads/details.aspx?familyid=dde9efc0-bb30-47eb-9a61-fd755d23cdec&displaylang=en> http://www.microsoft.com/downloads/details.aspx?familyid=dde9efc0-bb30-47eb-9a61-fd755d23cdec&displaylang=en

    Vendor Status:
    2002.11.05 Informed vendor about the issue
    2003.05.28 Microsoft has issued a Security Bulletin (MS03-018) and the
    related patch.

    Detailed Microsoft Security Bulletin is available at:

    Patches are available at:
    <http://microsoft.com/downloads/details.aspx?FamilyId=2F5D9852-4ADD-44F8-8715-AC3D7D7D94BF> http://microsoft.com/downloads/details.aspx?FamilyId=2F5D9852-4ADD-44F8-8715-AC3D7D7D94BF


    The information has been provided by <mailto:security@nsfocus.com>
    NSFOCUS Security Team.


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