[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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To: list@securiteam.com Date: 30 May 2003 23:07:35 +0200
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Microsoft IIS ssinc.dll Over-long Filename Buffer Overflow Vulnerability
------------------------------------------------------------------------
SUMMARY
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.
DETAILS
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>
http://www.nsfocus.net/index.php?act=advisory&do=view&adv_id=17, and the
corresponding Microsoft Security Bulletin is found at:
<http://www.microsoft.com/technet/security/bulletin/ms01-044.asp>
http://www.microsoft.com/technet/security/bulletin/ms01-044.asp
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.
Workaround:
1. Disable untrusted users' writing privileges to web directory.
2. If SSI functionality is not required, remove .shtml, .shtm, .stm
mapping.
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:
<http://www.microsoft.com/technet/security/bulletin/ms03-018.asp>
http://www.microsoft.com/technet/security/bulletin/ms03-018.asp
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
ADDITIONAL INFORMATION
The information has been provided by <mailto:security@nsfocus.com>
NSFOCUS Security Team.
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