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The U.S. Department of Energy
Computer Incident Advisory Center
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INFORMATION BULLETIN
Microsoft CGI Filename Decode Error Vulnerability in IIS
[Microsoft Security Advisory MS01-026]
May 16, 2001 16:00 GMT
Number L-083
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________
PROBLEM: The IIS CGI filename decoder will decode the filename twice
in error. This presents a vulnerability that could allow an
intruder several means of attack.
PLATFORM: Microsoft IIS 4.0 Microsoft IIS 5.0
DAMAGE: An intruder could get around security checks or be able to run
arbitrary system commands. This could result in code execution
or unauthorized file disclosure.
SOLUTION: Apply the patch described below.
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VULNERABILITY The risk is MEDIUM to HIGH; attacker would gain access at IUSR
ASSESSMENT: privilege level and be able to execute programs. IIS is
currently one of the most targeted applications for hackers.
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[****** Start Microsoft Advisory ******]
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- --
Title: Superfluous Decoding Operation Could Allow Command
Execution via IIS
Date: May 14, 2001
Software: IIS 4.0 and 5.0
Impact: Three vulnerabilities: Code execution; denial of
service, information disclosure.
Bulletin: MS01-026
Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-026.asp.
- ----------------------------------------------------------------------
- --
Issue:
======
This patch is a cumulative patch that includes the functionality of all
security patches released to date for IIS 5.0, and all patches released
for IIS 4.0 since Windows NT(r) 4.0 Service Pack 5. A complete listing
of the patches superseded by this patch is provided in the web-hosted
security bulletin, in the section titled "Additional information about
this patch". Before applying the patch, system administrators should
take note of the caveats discussed in the same section.
The patch also eliminates three newly discovered vulnerabilities:
- A vulnerability that could enable an attacker to run
operating system commands on an affected server. When
IIS receives a user request to run a script or other
server-side program, it performs a decoding pass to
render the request in a canonical form, then performs
security checks on the decoded request. A vulnerability
results because a second, superfluous decoding pass is
performed after the security checks are completed. If an
attacker submitted a specially constructed request, it could
be possible for the request to pass the security checks, but
then be mapped via the second decoding pass into one that
should have been blocked -- specifically, it could enable
the request to execute operating system commands or programs
outside the virtual folder structure. These would be executed
in the security context of the IUSR_machinename account which,
by virtue of its membership in the Everyone group, would grant
the attacker capabilities similar to those of a non-administrative
user interactively logged on at the console.
- A vulnerability that could enable denial of service attacks
against the FTP service. A function that processes wildcard
sequences in FTP commands doesn't always allocate sufficient
memory when performing pattern matching. Under unusual
circumstances, it could be possible for an attacker to levy an
FTP command containing a wildcard sequence that, when expanded,
would overrun the allocated memory and cause an access violation.
This would cause the IIS service (which provides both the web and
FTP functionality) to fail. As a result, all web or FTP sessions
in progress at the time would be severed, and no new sessions
could be established until the IIS service was restarted. In IIS
5.0, the service would restart automatically. In IIS 4.0, operator
intervention would be required to restart the service.
- A vulnerability that could make it easier for an attacker to find
Guest accounts that had been inadvertently exposed via FTP. By
design, if a user wishes to log onto an FTP server using a domain
user account, rather than a local one, he should be required to
precede it with the name of the domain. However, if an attacker
preceded an account name with a particular set of characters, the
FTP service would search the domain, and all trusted domains, for
the user account. The account would need to be enabled, and the
attacker would still need to know the correct password in order
to log into the account. For all practical purposes, this would
limit the attacker to attacking the Guest account, as it is the
only account with both a well-known account name and a well-known
default password.
The patch also corrects errors in three previous patches:
- The patch originally provided in Microsoft Security Bulletin
MS00-060 successfully eliminated the vulnerability at issue there,
but created an opportunity to cause the server to expend an
inordinate amount of time processing a particular type of invalid
request.
- The patches originally provided in Microsoft Security Bulletins
MS01-014 and MS01-016 (which superseded MS01-014) successfully
eliminated the vulnerabilities at issue there, but created a
potential denial of service condition via a memory leak.
Mitigating Factors:
====================
IIS vulnerability:
- The vulnerability does not provide a way for the attacker to
learn the folder structure on the server. As a result, if the
operating system were installed on a separate drive from the
web root or in non-standard folders, it could prevent an
attacker from locating programs of interest.
- The vulnerability does not provide administrative access to
the server. If the recommendations in the IIS 4.0 and IIS 5.0
security checklists have been followed, sensitive programs
will have been moved to folders that can only be accessed by
the Administrator, and non-administrative access to server
resources will be have been severely restricted.
FTP denial of service vulnerability:
- The attacker would require the ability to start an FTP
session in order to exploit the vulnerability.
FTP user account vulnerability:
- The vulnerability could only be exploited if the FTP server
was a domain member. However, this is usually not appropriate
for Internet-connected FTP servers.
- The vulnerability could only be exploited if the Guest account
on the local machine was disabled, but the Guest account on a
trusted domain was enabled. By default, the Guest account is
disabled in both Windows NT 4.0 and Windows 2000.
Patch Availability:
===================
- A patch is available to fix this vulnerability. Please read the
Security Bulletin
http://www.microsoft.com/technet/security/bulletin/ms01-026.asp
for information on obtaining this patch.
Acknowledgment:
===============
- NSfocus (http://www.nsfocus.com) for reporting the vulnerability
affecting IIS.
- Lukasz Luzar of Developers.of.PL and Aiden ORawe for reporting
the FTP denial of service.
- Kevin Kotas of eSecurityOnline (http://www.esecurityonline.com)
for reporting the problem in the fixes that were provided in
MS00-060, MS01-014 and MS01-016.
- -
- ---------------------------------------------------------------------
THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED
"AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL
WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT
SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY
DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO
THE FOREGOING LIMITATION MAY NOT APPLY.
[****** End Microsoft Advisory ******]
______________________________________________________________________
_________
CIAC wishes to acknowledge the contributions of Microsoft for the
information contained in this bulletin.
______________________________________________________________________
_________
CIAC, the Computer Incident Advisory Center, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and
coordination
among computer security teams worldwide.
CIAC services are available to DOE, DOE contractors, and the NIH.
CIAC
can be contacted at:
Voice: +1 925-422-8193 (7x24)
FAX: +1 925-423-8002
STU-III: +1 925-423-2604
E-mail: ciac@ciac.org
Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.
World Wide Web: http://www.ciac.org/
Anonymous FTP: ftp.ciac.org
PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins. If you are not part of these
communities, please contact your agency's response team to report
incidents. Your agency's team will coordinate with CIAC. The Forum of
Incident Response and Security Teams (FIRST) is a world-wide
organization. A list of FIRST member organizations and their
constituencies can be obtained via WWW at http://www.first.org/.
This document was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor the University of California nor any of their
employees, makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, product, or process
disclosed, or represents that its use would not infringe privately
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otherwise, does not necessarily constitute or imply its endorsement,
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University of California. The views and opinions of authors expressed
herein do not necessarily state or reflect those of the United States
Government or the University of California, and shall not be used for
advertising or product endorsement purposes.
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