Vulnerability Note VU#117394

Buffer Overflow in Core Microsoft Windows DLL

Overview

A buffer overflow vulnerability exists in the Win32 API libraries shipped with all versions of Microsoft Windows XP, Microsoft Windows 2000, Microsoft Windows NT 4.0, and Microsoft Windows NT 4.0 Terminal Server Edition. This vulnerability, which is being actively exploited on WebDAV-enabled IIS 5.0 servers, will allow a remote attacker to execute arbitrary code on unpatched systems. Sites running Microsoft Windows should apply a patch or disable WebDAV services as soon as possible.

I. Description

Microsoft Windows contains a dynamic link library (DLL) named ntdll.dll. This DLL is a core operating system component used to interact with the Windows kernel. A buffer overflow vulnerability exists in ntdll.dll, which is utilized by many different components in the Windows operating system.

The WebDAV (RFC2518) component of Microsoft IIS 5.0 is an example of one Windows component that uses ntdll.dll. The IIS WebDAV component utilizes ntdll.dll when processing incoming WebDAV requests. By sending a specially crafted WebDAV request to an IIS 5.0 server, an attacker may be able to execute arbitrary code in the Local System security context, essentially giving the attacker complete control of the system.

Because the vulnerable Win32 API component is utilized by many other applications, it is possible other exploit vectors exist. However, we have only been told of systems compromised running IIS 5.0 with WebDAV enabled. Sites using Windows but not running IIS 5.0 with WebDAV need to carefully weigh the trade-offs before applying patches to systems where the core vulnerability exists but may not be exploitable.

II. Impact

Any attacker who can reach a vulnerable web server can gain complete control of the system and execute arbitrary code in the Local System security context. Note that this may be significantly more serious than a simple "web defacement."

III. Solution

Apply a patch from your vendor

A patch is available from Microsoft at

http://microsoft.com/downloads/details.aspx?FamilyId=C9A38D45-5145-4844-B62E-C69D32AC929B&displaylang=en

Note that, according to MS03-007, "Microsoft was made aware that some customers who had received a hotfix from Product Support Services experienced stop errors on boot after applying the patch released for this bulletin." For more information, see the "Frequently asked questions" section of MS03-007.

Workarounds

Disable vulnerable service

Until a patch can be applied, you may wish to disable IIS:

http://support.microsoft.com/default.aspx?scid=kb;en-us;321141

If you cannot disable IIS, consider using the IIS lockdown tool to disable WebDAV (removing WebDAV can be specified when running the IIS lockdown tool). Alternatively, you can disable WebDAV by following the instructions located in Microsoft's Knowledgebase Article 241520, "How to Disable WebDAV for IIS 5.0":

http://support.microsoft.com/default.aspx?scid=kb;en-us;241520

Restrict buffer size

If you cannot use the IIS lockdown tool, consider restricting the size of the buffer IIS utilizes to process requests by using Microsoft's URL Buffer Size Registry Tool. This tool can be run against a local or remote Windows 2000 system running Windows 2000 Service Pack 2 or Service Pack 3. The tool, instructions on how to use it, and instructions on how to manually make changes to the registry are available here:

URL Buffer Size Registry Tool - http://go.microsoft.com/fwlink/?LinkId=14875
Microsoft Knowledge Base Article 816930 - http://support.microsoft.com/default.aspx?scid=kb;en-us;816930
Microsoft Knowledge Base Article 260694 - http://support.microsoft.com/default.aspx?scid=kb;en-us;260694

You may also wish to use URLScan, which will block web requests that attempt to exploit this vulnerability. Information about URLScan is available at

http://support.microsoft.com/default.aspx?scid=kb;[LN];326444

Systems Affected

Vendor	Status	Date Updated
Microsoft Corporation	Vulnerable	17-Mar-2003

References

http://www.microsoft.com/windows2000/technologies/web/default.asp
http://www.ietf.org/rfc/rfc2518.txt
http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/bulletin/ms03-007.asp
http://microsoft.com/downloads/details.aspx?FamilyId=C9A38D45-5145-4844-B62E-C69D32AC929B&displaylang=en
http://www.nextgenss.com/papers/ms03-007-ntdll.pdf
http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/bulletin/ms03-007.asp
http://www.microsoft.com/downloads/release.asp?ReleaseID=43955
http://support.microsoft.com/default.aspx?scid=kb;en-us;241520
http://support.microsoft.com/default.aspx?scid=kb;%5BLN%5D;326444
http://go.microsoft.com/fwlink/?LinkId=14875
http://support.microsoft.com/default.aspx?scid=kb;en-us;816930
http://support.microsoft.com/default.aspx?scid=kb;en-us;260694
http://www.lurhq.com/webdav.html

Credit

This document was written by Ian A Finlay.

Other Information

Date Public	03/17/2003
Date First Published	03/17/2003 03:26:26 PM
Date Last Updated	05/30/2003
CERT Advisory	CA-2003-09
CVE Name	CAN-2003-0109
US-CERT Technical Alerts
Metric	78.00
Document Revision	18

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