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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.
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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the patches superseded by this patch is provided in the web-hosted 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. 

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