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IIS: Malformed URL Can Cause Service Failure in IIS 5.0 and Exchange 2000 Server

The information in this article applies to:
  • Microsoft Internet Information Services version 5.0


Microsoft Internet Information Services (IIS) 5.0 contains a flaw that affects the way that a URL is handled if it has a specific construction and its length is within a very narrow range of values. If such a URL were repeatedly sent to an affected system, a confluence of events could cause a memory allocation error that would result in the failure of the IIS service.

IIS 5.0 is affected by the same vulnerability as the Microsoft Exchange 2000 Server vulnerability that is described in the following article in the Microsoft Knowledge Base:

Q287678 XWEB: Malformed URL Can Cause Service Failure in IIS 5.0 and Exchange 2000 Server
Mitigating factors:
  • The vulnerability would not enable the attacker to gain any administrative control over the server or to alter any data on it.

  • The affected services automatically restart in the event of a failure; therefore, an affected system would resume service almost immediately.

  • A successful attack against an Exchange server would only disrupt Web-based mail clients' use of the server. The server would continue to be available for MAPI-based clients such as Microsoft Outlook.

  • The ISAPI involved in this vulnerability authenticates the user before servicing the request; therefore, a properly configured Exchange server would be at less risk than an IIS server.


A supported fix is now available from Microsoft, but it is only intended to correct the problem described in this article and should be applied only to systems that are determined to be at risk of attack. Please evaluate your computer's physical accessibility, network and Internet connectivity, and other factors to determine the degree of risk to your computer. Please see the associated Microsoft Security Bulletin to help make this determination. This fix may receive additional testing at a later time, to further ensure product quality. If your computer is sufficiently at risk, Microsoft recommends that you apply this fix now. Otherwise, wait for the next Windows 2000 service pack that contains this fix.

To resolve this problem immediately, download the fix as instructed below or contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:
NOTE: In special cases, charges that are normally incurred for support calls may be canceled, if a Microsoft Support Professional determines that a specific update will resolve your problem. Normal support costs will apply to additional support questions and issues that do not qualify for the specific update in question.

IMPORTANT: Because the flaw occurs in two different code modules, one of which is installed as part of IIS 5.0 and both of which are installed as part of Exchange 2000, it is important for Exchange 2000 administrators to install both the IIS and Exchange patches below.

The following files are available for download from the Microsoft Download Center:
IIS 5.0:
[GRAPHIC: Download]Download Q286818_W2K_SP3_x86_en.exe now

Exchange 2000 Server:
[GRAPHIC: Download]Download Q287678engi386.exe now
For additional information about how to download Microsoft Support files, click the article number below to view the article in the Microsoft Knowledge Base:
Q119591 How to Obtain Microsoft Support Files from Online Services
Microsoft used the most current virus detection software available on the date of posting to scan this file for viruses. Once posted, the file is housed on secure servers that prevent any unauthorized changes to the file.

The English version of this fix should have the following file attributes or later:
   Date        Time    Version      Size     File name
   02/01/2001  07:21p  0.9.3940.16  428,816  Httpext.dll 


Microsoft has confirmed this to be a problem in Microsoft Internet Information Services 5.0.


For more information about this issue, see the following Microsoft Web site:
For additional information about how to install Microsoft Windows 2000 and Microsoft Windows 2000 hotfixes at the same time, click the article number below to view the article in the Microsoft Knowledge Base:
Q249149 Installing Microsoft Windows 2000 and Windows 2000 Hotfixes

Additional query words: dos denial of service

Keywords : kbWin2000PreSP3Fix kbgraphxlinkcritical
Issue type : kbbug
Technology : kbiisSearch kbiis500

Last Reviewed: June 1, 2001
© 2001 Microsoft Corporation. All rights reserved. Terms of Use.

Article ID: Q286818

Last Reviewed:
June 1, 2001

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