Scripts can be run in the error page that is returned by ISA Server (295389)

SYMPTOMS

If you click a link (or URL) to a page that includes script code that for any reason generates an error, the error message from Internet Security and Acceleration (ISA) Server contains the original script from the link and it runs in your Web browser.

This is a cross-site scripting vulnerability that affects the error page that ISA Server generates in response to a request for a non-existent page or an unsuccessful connection attempt to a page. As with all cross-site scripting vulnerabilities, this vulnerability could enable an attacker to either run script in the security domain of another, presumably trusted, Web site, or to access cookies that a site had written to a your computer.

CAUSE

This problem occurs because ISA Server returns the complete original requested URL to the browser in the error message along with the description of the reason why the URL could not be accessed. Because the original request contains a script, the browser runs the script on receipt.

The fix corrects the problem by not returning the URLLOCATION token in the error that is returned to the browser. The Web Proxy service has a list of templates for the error pages. In these templates, there are places for tokens that the service populates before returning the page to the client. One of those tokens is URLLOCATION, which contains the URL. In the template it appears as:

URL: [URLLOCATION]

The page that is returned to the client appears as:

URL: http://www.samplename.com/path

ISA Server supports editing of the error pages. Administrators can change the text and format of these error pages and use the tokens as they want in the pages; the Web Proxy service returns the pages accordingly.

This fix stops the support for the URLLOCATION token. ISA Server ignores the token in the generic error pages or in pages that administrators have edited. ISA Server also removes the "URL" prefix before this token. This change will be in effect for all ISA Server versions in all languages.

RESOLUTION

Service Pack Information

This problem was first corrected in Microsoft Internet Security and Acceleration Server 2000 Service Pack 1.
For additional information about how to obtain the latest ISA Server service pack, click the following article number to view the article in the Microsoft Knowledge Base:

313139 How to obtain the latest Internet Security and Acceleration Server 2000 service pack

Hotfix information

A supported fix is now available from Microsoft, but it is only intended to correct the problem that is described in this article. Apply it only to computers that you determine are at risk of attack. Evaluate your computer's physical accessibility, network and Internet connectivity, and other factors to determine the degree of risk to your computer. See the associated Microsoft Security Bulletin to help determine the degree of risk. This fix may receive additional testing. If your computer is sufficiently at risk, Microsoft recommends that you apply this fix now. Otherwise, wait for the next ISA Server 2000 service pack that contains this fix.

To resolve this problem immediately, download the fix by following the instructions later in this article or contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information about support costs, visit the following Microsoft Web site:

http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS

NOTE: In special cases, charges that are ordinarily incurred for support calls may be canceled if a Microsoft Support Professional determines that a specific update will resolve your problem. The usual support costs will apply to additional support questions and issues that do not qualify for the specific update in question.

The English version of this hotfix has the file attributes (or later) that are listed in the following table. The dates and times for these files are listed in coordinated universal time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.

   Date         Time   Version         Size     File name   
   -----------------------------------------------------
   15-Aug-2001  16:48  3.0.1200.68 shp 381,200  W3proxy.exe

MORE INFORMATION

For more information about this vulnerability, see the following Microsoft Web site:

http://www.microsoft.com/technet/security/bulletin/MS01-045.mspx

Cross-site scripting is a type of security vulnerability that results when Web content does not adequately filter its inputs. In most cases, cross-site scripting occurs when a Web page accepts some kind of user input (for example, a phrase to search for) and then creates a page using that input. If the page does not check for the presence of script within the input, the script, when it is processed as part of the Web page, runs within the Web site's domain.

Such a condition is not dangerous when you provide the input (you could have performed the actions directly rather than performing them via the script). However, there are cases in which it is possible for a third party to "inject" input that contains script into your Web session. This does pose a hazard because it could enable the third party's script to run in your browser by using the security settings appropriate to the Web page. For additional information, see the following Microsoft Web site:

http://www.microsoft.com/technet/itsolutions/security/topics/crsstfaq.asp

The fix that is described in this article also applies to the following Microsoft Knowledge Base article:

289503 Memory Leak in ISA Server H.323 Gatekeeper Service and Winsock Proxy Service When Decoding Malformed Packets