Microsoft Security Bulletin MS99-019
Patch Available for "Malformed HTR Request" Vulnerability
Patch Availability Information Updated: March 21, 2003
Originally Posted: June 15, 1999
Updated: July 22, 1999
This is an update to Microsoft Security Bulletin MS99-019. There are no changes in the status of the vulnerability or the patch; the purpose of this update is to clarify the products that are affected by the vulnerability, and the specific steps that customers should take. In particular, customers should review the Affected Software Versions and What Customers Should Do sections below.
Microsoft has released a patch that eliminates a vulnerability in Microsoft® Internet Information Server 4.0. (IIS 4.0 may be installed as a standalone product or as part of other Microsoft products). The vulnerability could allow denial of service attacks against an IIS server or, under certain conditions, could allow arbitrary code to be run on the server.
Microsoft has received reports of customer sites being attacked via this vulnerability, and strongly encourages all affected customers to download and install the patch, if appropriate. The patch is fully supported.
IIS supports several file types that require server-side processing. When a web site visitor requests a file of one of these types, an appropriate filter DLL processes it. A vulnerability exists in the way that .HTR, .STM and .IDC files are processed.
The vulnerability involves an unchecked buffer in the filter DLLs for these file types. This poses two threats to safe operation. The first is a denial of service threat. A malformed request for an .HTR, .STM or .IDC file could overflow the buffer, causing IIS to crash. The server would not need to be rebooted, but IIS would need to be rebooted in order to resume service. The second threat is that a carefully-constructed file request could cause arbitrary code to execute on the server via a classic buffer overrun technique. Neither attack could occur accidentally. The vulnerability is present regardless of whether .HTR, .STM or .IDC files are present on the server.
Microsoft is proactively releasing this patch to allow customers to take appropriate action to protect themselves against this vulnerability. In addition, web sites that do not require these file types can disable them altogether, as discussed in Microsoft's IIS Security Checklist.
Affected Software Versions
- Microsoft Internet Information Server 4.0
Note IIS 4.0 can be installed as part of other Microsoft products like BackOffice and Site Server. Instructions for determining whether IIS 4.0 is installed are provided below in What Customers Should Do.
Vulnerability Identifier: CVE-1999-0874
What Microsoft is Doing
Microsoft has released a patch that fixes the problem identified. The patches are available for download from the sites listed below in What Customers Should Do.
Microsoft also has sent this security bulletin to customers subscribing to the Microsoft Product Security Notification Service. See The Microsoft Product Security Notification Service for more information about this free customer service.
Microsoft has published the following Knowledge Base (KB) article on this issue:
- Microsoft Knowledge Base (KB) article 234905, An Improperly Formatted HTTP Request Can Cause The Inetinfo Process To Fail, http://support.microsoft.com/default.aspx?scid=kb;en-us;234905&sd=tech
(Note It might take 24 hours from the original posting of this bulletin for the KB article to be visible in the Web-based Knowledge Base.)
What Customers Should Do
After the original issuance of this security bulletin on June 15, 1999, Microsoft identified additional variants of this vulnerability. The comprehensive patch provided below contains a fix for the originally identified vulnerability in .HTR file processing, as well as a fix to similar vulnerabilities subsequently identified that affect .IDC and .STM files. All customers using IIS 4.0, even if installed as part of a different Microsoft product, should follow the steps below; they should do this even if they previously applied the workaround discussed in the original version of this bulletin.
Customers who are unsure whether they are potentially affected by this vulnerability should check their systems for any of the following files:
If any of these files are present, IIS 4.0 is installed and and the system is potentially vulnerable. The patch that eliminates this vulnerability is available at: http://www.microsoft.com/technet/security/patchavailability.mspx.
Customers who followed the administrative workaround provided in the original version of this bulletin do not need to back out the workaround prior to installing the patch, although they can if they would like to resume using .HTR files. However, per recommended security practices, as detailed in the IIS Security Checklist, unused script mappings should be removed as a matter of course.
Please see the following references for more information related to this issue.
- Microsoft Security Bulletin MS99-019, Patch Available for "Malformed HTR Request" Vulnerability, (The Web-posted version of this bulletin), http://www.microsoft.com/technet/security/bulletin/ms99-019.mspx.
- Microsoft Knowledge Base (KB) article 234905, An Improperly Formatted HTTP Request Can Cause The Inetinfo Process To Fail http://support.microsoft.com/default.aspx?scid=kb;en-us;234905&sd=tech.
- IIS Security Checklist, http://www.microsoft.com/technet/security/chklist/iischk.mspx
Obtaining Support on this Issue
If you require technical assistance with this issue, please contact Microsoft Technical Support. For information on contacting Microsoft Technical Support, please see http://support.microsoft.com/contactussupport/?ws=support.
- June 15, 1999: Bulletin Created.
- June 17, 1999: Bulletin updated to advise availability of patch for .HTR, .STM, and .IDC files
- June 18, 1999. Provided additional clarification regarding affected software and recommended actions.
- V2.0 (March 21, 2003): Introduced versioning and updated patch availability information
For additional security-related information about Microsoft products, please visit http://www.microsoft.com/technet/security
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