M-066: Microsoft Cumulative Patch for Internet Information Services (IIS) Vulnerabilities

[***** Start Microsoft Security Bulletin MS02-018 *****]

Microsoft Security Bulletin MS02-018  

Cumulative Patch for Internet Information Services (Q319733)
Originally posted: April 10, 2002

Summary

Who should read this bulletin: Customers hosting web servers using 
Microsoft® Windows NT® 4.0, Windows® 2000, or Windows XP. 

Impact of vulnerability: Ten new vulnerabilities, the most serious of 
which could enable code of an attacker’s choice to be run on a server. 

Recommendation: Customers using any of the affected products should 
install the patch immediately. 

Maximum Severity Rating: Critical 

Affected Software: 

Microsoft Internet Information Server 4.0
 
Microsoft Internet Information Services 5.0
 
Microsoft Internet Information Services 5.1
  

Note: Beta versions of .NET Server after Build 3605 contains fixes for 
all of the vulnerabilities affecting IIS 6.0. As discussed in the FAQ, 
Microsoft is working directly with the small number of customers who 
are using the .NET Server beta version in production environments to 
provide immediate remediation for them. 

Technical description: 

This patch is a cumulative patch that includes the functionality of all 
security patches released for IIS 4.0 since Windows NT 4.0 Service Pack 6a, 
and all security patches released to date for IIS 5.0 and 5.1. A 
complete listing of the patches superseded by this patch is provided 
below, 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. 

In addition to including previously released security patches, this 
patch also includes fixes for the following newly discovered security 
vulnerabilities affecting IIS 4.0, 5.0 and/or 5.1: 

A buffer overrun vulnerability involving the operation of the chunked 
encoding transfer mechanism via Active Server Pages in IIS 4.0 and 5.0. 
An attacker who exploited this vulnerability could overrun heap memory 
on the system, with the result of either causing the IIS service to 
fail or allowing code to be run on the server. 
A Microsoft-discovered vulnerability that is related to the preceding 
one, but which lies elsewhere within the ASP data transfer mechanism. 
It could be exploited in a similar manner as the preceding vulnerability, 
and would have the same scope. However, it affects IIS 4.0, 5.0, and 5.1. 
A buffer overrun involving how IIS 4.0, 5.0 and 5.1 process HTTP header 
information in certain cases. IIS performs a safety check prior to parsing 
the fields in HTTP headers, to ensure that expected delimiter fields are 
present and in reasonable places. However, it is possible to spoof the 
check, and convince IIS that the delimiters are present even when they 
are not. This flaw could enable an attacker to create an URL whose HTTP 
header field values would overrun a buffer used to process them. 
A Microsoft-discovered buffer overrun vulnerability in IIS 4.0, 5.0 and 
5.1 that results from an error in safety check that is performed during 
server-side includes. In some cases, a user request for a web page is 
properly processed by including the file into an ASP script and 
processing it. Prior to processing the include request, IIS performs an 
operation on the user-specified file name, designed to ensure that the 
file name is valid and sized appropriately to fit in a static buffer. 
However, in some cases it could be possible to provide a bogus, 
extremely long file name in a way that would pass the safety check, 
thereby resulting in a buffer overrun. 
A buffer overrun affecting the HTR ISAPI extension in IIS 4.0 and 5.0. 
By sending a series of specially malformed HTR requests, it could be 
possible to either cause the IIS service to fail or, under a very 
difficult operational scenario, to cause code to run on the server. 
A denial of service vulnerability involving the way IIS 4.0, 5.0, and 
5.1 handle an error condition from ISAPI filters. At least one ISAPI 
filter (which ships as part of FrontPage Server Extensions and ASP.NET), 
and possibly others, generate an error when a request is received 
containing an URL that exceeds the maximum length set by the filter. 
In processing this error, the filter replaces the URL with a null value. 
A flaw results because IIS attempts to process the URL in the course of 
sending the error message back to the requester, resulting in an access 
violation that causes the IIS service to fail. 
A denial of service vulnerability involving the way the FTP service in 
IIS 4.0, 5.0 and 5.1 handles a request for the status of the current 
FTP session. If an attacker were able to establish an FTP session with 
an affected server, and levied a status request that created a particular 
error condition, a flaw in the FTP code would prevent it from correctly 
reporting the error. Other code within the FTP service would then 
attempt to use uninitialized data, with an access violation as the result. 
This would result in the disruption of not only FTP services, but also 
of web services.
 
A trio of Cross-Site Scripting (CSS) vulnerabilities affecting IIS 4.0, 
5.0 and 5.1: one involving the results page that’s returned when searching 
the IIS Help Files, one involving HTTP error pages; and one involving the 
error message that’s returned to advise that a requested URL has been 
redirected. All of these vulnerabilities have the same scope and effect: 
an attacker who was able to lure a user into clicking a link on his 
web site could relay a request containing script to a third-party web 
site running IIS, thereby causing the third-party site’s response 
(still including the script) to be sent to the user. The script would 
then render using the security settings of the third-party site rather 
than the attacker’s. 

Mitigating factors:
Buffer overrun in Chunked Encoding transfer: 

On default installations of IIS 5.0 and 5.1, exploiting the vulnerability 
to run code would grant the attacker the privileges of the IWAM_computername 
account, which has only the privileges commensurate with those of an 
interactively logged-on unprivileged user.
 
The vulnerability requires that Active Server Pages (ASP) be enabled on 
the system in order to be exploited. Version 1.0 of the IIS Lockdown Tool 
removes ASP by default, and the current version (version 2.1) removes it 
by default if Static Web Server has been selected. 
The URLScan tool can be configured to prevent chunked encoding requests. 
If this has been done, the vulnerability could not be exploited.
 

Microsoft-discovered variant of Chunked Encoding buffer overrun: 

This vulnerability is subject to exactly the same mitigating factors as 
the buffer overrun in the Chunked Encoding transfer, with one exception. 
The URLScan tool could not be used to protect against the vulnerability. 

Buffer Overrun in HTTP header handling:
 
On default installations of IIS 5.0 and 5.1, exploiting the vulnerability 
to run code would grant the attacker the privileges of the IWAM_computername 
account, which has only the privileges commensurate with those of an 
interactively logged-on unprivileged user. 
The vulnerability requires that Active Server Pages (ASP) be enabled on 
the system in order to be exploited. Version 1.0 of the IIS Lockdown Tool 
removes ASP by default, and the current version (version 2.1) removes it 
by default if Static Web Server has been selected. 
The URLScan tool’s default ruleset would likely limit the attacker to 
using this vulnerability for denial of service attacks only. 

Buffer Overrun in ASP Server-Side Include Function: 

On default installations of IIS 5.0 and 5.1, exploiting the vulnerability 
to run code would grant the attacker the privileges of the IWAM_computername 
account, which has only the privileges commensurate with those of an 
interactively logged-on user. 
The vulnerability requires that Active Server Pages (ASP) be enabled on 
the system in order to be exploited. Version 1.0 of the IIS Lockdown Tool 
removes ASP by default, and the current version (version 2.1) removes it 
by default if Static Web Server has been selected.
 
The URLScan tool’s default ruleset would likely limit the attacker to 
using this vulnerability for denial of service attacks only. 

Buffer overrun in HTR ISAPI extension: 

Microsoft has long recommended disabling the HTR ISAPI extension. Systems 
on which this has been done would be at no risk from the vulnerability. 
(All versions of the IIS Lockdown Tool disable HTR support by default).
 
The URLScan tool, if using its default ruleset, would prevent this 
vulnerability from being exploited to run code on the server even if 
HTR support was enabled. 
The vulnerability could only be used to run code on the server if the 
attacker knew the locations of certain information in memory. In practice, 
the most likely such situation would occur if the web server had never 
served any web content since being rebooted. In all other cases, it would 
only be possible to use the vulnerability for denial of service attacks. 
On default installations of IIS 5.0 and 5.1, exploiting the vulnerability 
to run code would grant the attacker the privileges of the IWAM_computername 
account, which has only the privileges commensurate with those of an 
interactively logged-on user. 
If the vulnerability were used in a denial of service attack, normal 
operation could be restored on an IIS 4.0 server by restarting the IIS 
service; on IIS 5.0 and higher, the service would automatically restart 
itself. 

Access violation in URL error handling: 

An IIS 4.0 server could be put back into normal operation by restarting 
the service. An IIS 5.0 or 5.1 server would automatically restart the 
service. 
The vulnerability could only be used for denial of service attacks. There 
is no capability to use the vulnerability to gain privileges on the system.
 
The sole ISAPI filter known to generate the error that results in the access 
violation ships only as part of FrontPage Server Extensions and ASP.NET. 
ASP.NET is not installed by default, and FPSE can be uninstalled if desired. 
 

Denial of service via FTP Status request: 

The IIS Lockdown Tool disables FTP support by default. 
An IIS 4.0 server could be put back into normal operation by restarting 
the service. An IIS 5.0 or 5.1 server would automatically restart the 
service. 
The vulnerability could only be used for denial of service attacks. There 
is no capability to use the vulnerability to gain privileges on the system. 

Cross-site Scripting in IIS Help File search facility, HTTP Error Page, 
and Redirect Response message: 

The vulnerabilities could only be exploited if the attacker could entice 
another user into visiting a web page and clicking a link on it, or 
opening an HTML mail. 
The Redirect Response vulnerability could only be exploited if the user 
was running a browser other than Internet Explorer. IE does not actually 
render the text in the Redirect Response, but instead recognizes it by 
its response header and processes the redirect without displaying any text. 

Severity Rating: 
Buffer overrun in ASP Chunked Encoding data transfer mechanism:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     Critical          Critical            None 
IIS 5.0     Critical          Critical            None 
IIS 5.1     None              None                None 

Microsoft-discovered variant of ASP Chunked Encoding buffer overrun:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     Critical          Critical            None 
IIS 5.0     Critical          Critical            None 
IIS 5.1     Critical          Critical            None 

Buffer Overrun in HTTP Header Handling:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     Critical          Critical            None 
IIS 5.0     Critical          Critical            None 
IIS 5.1     Critical          Critical            None 

Buffer Overrun in ASP Server-Side Include Function:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     Critical          Critical            None 
IIS 5.0     Critical          Critical            None 
IIS 5.1     Critical          Critical            None 

Buffer overrun in HTR ISAPI extension:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     Moderate          Moderate            None 	
IIS 5.0     Moderate          Moderate            None 
IIS 5.1     None              None                None 

Access violation in URL error handling:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     Moderate          Moderate            None 
IIS 5.0     Moderate          Moderate            None 
IIS 5.1     Moderate          Moderate            None 

Denial of service via FTP status request:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     Moderate          Moderate            None 
IIS 5.0     Moderate          Moderate            None 
IIS 5.1     Moderate          Moderate            None 

Cross-site Scripting in IIS Help File search facility:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     None              None                Moderate 
IIS 5.0     None              None                Moderate 
IIS 5.1     None              None                Moderate 

Cross-site Scripting in HTTP Error Page:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     None              None                Moderate 
IIS 5.0     None              None                Moderate 
IIS 5.1     None              None                Moderate 

Cross-site Scripting in Redirect Response message:  
          Internet Servers  Intranet Servers  Client Systems 
IIS 4.0     None              None                Low 
IIS 5.0     None              None                Low 
IIS 5.1     None              None                Low 

The above assessment is based on the types of systems affected by the 
vulnerability, their typical deployment patterns, and the effect that 
exploiting the vulnerability would have on them. 

Vulnerability identifiers: 

Buffer overrun in Chunked Encoding mechanism: CAN-2002-0079 

Microsoft-discovered variant of Chunked Encoding buffer overrun: 
CAN-2002-0147 

Buffer Overrun in HTTP Header handling: CAN-2002-0150

Buffer Overrun in ASP Server-Side Include Function: CAN-2002-0149 

Buffer overrun in HTR ISAPI extension: CAN-2002-0071 

Access violation in URL error handling: CAN-2002-0072 

Denial of service via FTP status request: CAN-2002-0073 

Cross-site Scripting in IIS Help File search facility: CAN-2002-0074 

Cross-site Scripting in HTTP Error Page: CAN-2002-0148 

Cross-site Scripting in Redirect Response message: CAN-2002-0075 

Tested Versions:

The following table indicates which of the currently supported versions 
of IIS are affected by the vulnerabilities. Versions prior to IIS 4.0 
are no longer supported and may or may not be affected by these 
vulnerabilities. IIS 6.0 is a beta product and is therefore not intended 
for use in production systems. A small number of customers are deploying 
IIS 6.0 servers in product environments as part of a joint program with 
Microsoft, and patches will be delivered directly to them. 
 
                                              IIS 4.0 IIS 5.0 IIS 5.1 
Buffer overrun in Chunked Encoding mechanism    Yes     Yes     No 
Microsoft-discovered variant of Chunked Encoding 
buffer overrun                                  Yes     Yes     Yes 
Buffer Overrun in HTTP Header handling          Yes     Yes     Yes 
Buffer Overrun in ASP Server-Side Include 
Function                                        Yes     Yes     Yes 
Buffer overrun in HTR ISAPI extension           Yes     Yes     No 
Access violation in URL error handling          Yes     Yes     Yes 
Denial of service via FTP status request        Yes     Yes     Yes 
Cross-site Scripting in IIS Help File search    No      Yes     Yes 
Cross-site Scripting in HTTP Error Page         Yes     Yes     Yes 
Cross-site Scripting in Redirect Response 
message                                         Yes     Yes     Yes 

Patch availability

Download locations for this patch 

Microsoft IIS 4.0:
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=37931 

Microsoft IIS 5.0:
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=37824 

Microsoft IIS 5.1:
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=37857 

Additional information about this patch

Installation platforms: 

The IIS 4.0 patch can be installed on systems running Windows NT 4.0 
Service Pack 6a. 

The IIS 5.0 patch can be installed on systems running Windows 2000 
Service Pack 1 or Service Pack 2. 

The IIS 5.1 patch can be installed on systems running Windows XP 
Professional Gold. 

Inclusion in future service packs: 

No additional service packs are planned for Windows NT 4.0.
 
The IIS 5.0 fixes will be included in Windows 2000 Service Pack 3.
 
The IIS 5.1 fixes will be included in Windows XP Service Pack 1. 

Reboot needed: 

- IIS 4.0: Yes 
- IIS 5.0: No 
- IIS 5.1: No. (In some cases, a pop-up dialogue may say that the system 
needs to be rebooted in order for the patch installation process to be 
completed. This dialogue, if it appears, can be ignored) 

Superseded patches: 

- IIS 4.0 and 5.0: This patch supersedes the one provided in Microsoft 
Security Bulletin MS01-044. (This is the previous cumulative patch for 
IIS 4.0 and 5.0, and supersedes additional patches) 

- IIS 5.1: None 

Verifying patch installation:

IIS 4.0: 

To verify that the patch has been installed on the machine, confirm that 
the following registry key has been created on the machine: 
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Hotfix\Q319733. 

To verify the individual files, consult the file manifest in Knowledge Base 
article Q319733. 

IIS 5.0: 

To verify that the patch has been installed on the machine, confirm that 
the following registry key has been created on the machine: 
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Updates\Windows 2000\SP3\Q319733. 

To verify the individual files, use the date/time and version information 
provided in the following registry key: 
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Updates\Windows 2000\SP3\Q319733\Filelist. 

IIS 5.1: 

To verify that the patch has been installed on the machine, confirm that 
the following registry key has been created on the machine: 
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Updates\Windows XP\SP1\Q319733. 

To verify the individual files, use the date/time and version information 
provided in the following registry key: 
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Updates\Windows XP\SP1\Q319733\Filelist. 

Caveats: 

1. The fixes for four vulnerabilities affecting IIS 4.0 servers are not 
included in the patch, because they require administrative action rather 
than a software change. Administrators should ensure that in addition to 
applying this patch, they also have taken the administrative action 
discussed in the following bulletins: 
    Microsoft Security Bulletin MS00-028 
    Microsoft Security Bulletin MS00-025 
    Microsoft Security Bulletin MS99-025 
       (which discusses the same issue as Microsoft Security Bulletin MS98-004) 
    Microsoft Security Bulletin MS99-013 

2. The patch does not include fixes for vulnerabilities involving non-IIS 
products like Front Page Server Extensions and Index Server, even though 
these products are closely associated with IIS and typically installed on 
IIS servers. At this writing, the bulletins discussing these 
vulnerabilities are: 
    Microsoft Security Bulletin MS01-043 
    Microsoft Security Bulletin MS01-025 
    Microsoft Security Bulletin MS00-084 
    Microsoft Security Bulletin MS00-018 
    Microsoft Security Bulletin MS00-006 
There is, however, one exception. The fix for the vulnerability affecting 
Index Server which is discussed in Microsoft Security Bulletin MS01-033 is 
included in this patch. We have included it because of the seriousness of 
the issue for IIS servers. 

3. Customers using IIS 4.0 should ensure that they 
have followed the correct installation order before installing this or any 
security patch. Specifically, customers should ensure that Windows NT 4.0 
Service Pack 6a has been applied (or re-applied) after installing the 
IIS 4.0 service. 

Localization:
Localized versions of this patch are available at the locations discussed 
in "Patch Availability". 

Obtaining other security patches: 
Patches for other security issues are available from the following 
locations: 

Security patches are available from the Microsoft Download Center, and can 
be most easily found by doing a keyword search for "security_patch". 

Patches for consumer platforms are available from the WindowsUpdate web site. 

All patches available via WindowsUpdate also are available in a 
redistributable form from the WindowsUpdate Corporate site. 

Other information: 

Acknowledgments
Microsoft thanks  the following people for reporting this issue to us and 
working with us to protect customers: 

- eEye Digital Security (http://www.eeye.com) for reporting the buffer 
overrun in the ASP chunked encoding implementation. 
- Entrust Technologies (http://www.entrust.com) for reporting the buffer 
overrun affecting the HTTP header handling. 
- Dave Aitel of @Stake (http://www.atstake.com) and Peter Grundl of KPMG 
for reporting the buffer overrun in the HTR ISAPI extension and the access 
violation in URL error handling. 
- Joe Smith (jsm1th@hotmail.com) and zenomorph ( admin@cgisecurity.com) of 
http://www.cgisecurity.com) for reporting the cross-site scripting 
vulnerability in the IIS Help File search facility. 
- Keigo Yamazaki of the LAC SNS Team (http://www.lac.co.jp/security/) for 
reporting the cross-site scripting vulnerability affecting redirect 
response messages. 
- Thor Larholm of Jubii A/S for reporting the cross-site 
scripting vulnerability affecting HTTP error pages. 

Support: 

Microsoft Knowledge Base article Q319733 discusses this issue and will be 
available approximately 24 hours after the release of this bulletin. 
Knowledge Base articles can be found on the Microsoft Online Support web 
site. 

Technical support is available from Microsoft Product Support Services. 
There is no charge for support calls associated with security patches. 

Security Resources: The Microsoft TechNet Security Web Site provides 
additional information about security in Microsoft products. 

Disclaimer: 
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. 

Revisions: 

V1.0 (April 10, 2002): Bulletin Created. 

[***** End Microsoft Security Bulletin MS02-018 *****]

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