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AUSCERT External Security Bulletin Redistribution
ESB-2001.203 -- Microsoft Security Bulletin MS01-026
Superfluous Decoding Operation Could Allow Command Execution via IIS
15 May 2001
AusCERT Security Bulletin Summary
Product: IIS 4.0
Operating System: Windows NT
Impact: Execute Arbitrary Code/Commands
Denial of Service
Access Required: Remote
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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.
Microsoft encourages customers to review the Security Bulletin at:
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This patch is a cumulative patch that includes the functionality of
security patches released to date for IIS 5.0, and all patches
for IIS 4.0 since Windows NT(r) 4.0 Service Pack 5. A complete
the patches superseded by this patch is provided in the web-hosted
security bulletin, in the section titled "Additional information
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
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
- 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.
- 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.
- A patch is available to fix this vulnerability. Please read the
for information on obtaining this patch.
- NSfocus (http://www.nsfocus.com) for reporting the vulnerability
- 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.
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CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY
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