M-089: MS Heap Overrun in HTR Chunked Encoding Vulnerability

[***** Start Microsoft Security Bulletin MS02-028  *****]


Microsoft Security Bulletin MS02-028  


Heap Overrun in HTR Chunked Encoding Could Enable Web Server Compromise (Q321599)
Originally posted: June 12, 2002


Summary

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

Impact of vulnerability: Run code of an attacker's choice on the system 

Maximum Severity Rating: Moderate 

Recommendation: Customers who have a business-critical reason for retaining HTR 
scripting should apply the patch immediately. All others should ensure HTR is 
disabled. 


Affected Software: 

Microsoft Internet Information Server 4.0 
Microsoft Internet Information Services 5.0 



Technical details

Technical description: 


This patch eliminates a newly discovered vulnerability affecting Internet Information 
Services. Although Microsoft typically delivers cumulative patches for IIS, in this 
case we have delivered a patch that eliminates only this new vulnerability, while 
completing a cumulative patch. When the cumulative patch is customer-ready, we will 
update this bulletin with information on its availability. The FAQ provides 
information on the circumstances surrounding the vulnerability, and why we believe 
releasing a singleton patch immediately is in customers' best interests. To ensure 
that servers are fully protected against past as well as current vulnerabilities, we 
strongly recommend installing the previous cumulative patch (discussed in Microsoft 
Security Bulletin MS02-018) before installing this patch. 

The vulnerability is similar to the first vulnerability discussed in Microsoft 
Security Bulletin MS02-018. Like that vulnerability, this one involves a buffer 
overrun in the Chunked Encoding data transfer mechanism in IIS 4.0 and 5.0, and could 
likewise be used to 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. The chief 
difference between the vulnerabilities is that the newly discovered one lies in the 
ISAPI extension that implements HTR – an older, largely obsolete scripting technology 
– where the previous one lay in the ISAPI extension that implements ASP. 



Mitigating factors: 

Microsoft has long recommended disabling HTR functionality unless there is a business-
critical reason for retaining it. Systems on which HTR is disabled would not be at 
risk from this vulnerability. 

The IIS Lockdown Tool disables HTR by default in all server configurations. 

The current version of the URLScan tool provides a means of blocking chunked encoding 
transfer requests by default. 

On default installations of IIS 5.0, 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. 


Severity Rating:      Internet Servers   Intranet Servers   Client Systems 
IIS 4.0                   Moderate           Moderate          Moderate 
IIS 5.0                   Moderate           Moderate          Moderate 

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. Although the vulnerability would grant varying degrees of control 
to a successful attacker, depending on the particular version in use, a server 
configured using any of the Microsoft security checklists or security tools would not 
have the HTR functionality enabled. 


Vulnerability identifier: CAN-2002-0364 

Tested Versions:
Microsoft tested IIS 4.0, 5.0 and 5.1 to assess whether they are affected by these 
vulnerabilities. Previous versions are no longer supported, and may or may not be 
affected by these vulnerabilities. IIS 6.0 is a beta product, and beta products are 
typically not eligible for security patches; however, we have confirmed that no beta 
versions of IIS 6.0 are affected by the vulnerability.


Patch availability

Download locations for this patch 

Microsoft IIS 4.0: 
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=39579 
Microsoft IIS 5.0: 
http://www.microsoft.com/Downloads/Release.asp?ReleaseID=39217 



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. 


Inclusion in future service packs: 

No additional service packs are planned for Windows NT 4.0. 

The IIS 5.0 fix will be included in Windows 2000 Service Pack 3. 


Reboot needed: 

IIS 4.0: A reboot can be avoid by stopping the IIS service, installing the patch with 
the /z switch, then restarting the service. Knowledge Base article Q319733 provides 
additional information on this procedure. 

IIS 5.0: No.

 
Superseded patches: 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\Q321599. 


To verify the individual files, consult the file manifest in Knowledge Base article 
Q321599. 


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\Q321599. 


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\Q321599\Filelist. 


Caveats:
None 


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 eEye Digital Security for reporting this issue to us and working with 
us to protect customers. 


Support: 

Microsoft Knowledge Base article Q321599 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 (June 12, 2002): Bulletin Created. 


[***** End Microsoft Security Bulletin MS02-028  *****]

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