Internet Explorer Prompts for Credentials When Using a .pac File

If Internet Explorer is configured to use a .pac file to access different Web addresses (or URLs) that are hosted by the same host name through different means (for example, one through a proxy and another directly), Internet Explorer may return "Authentication failure" messages, prompt for credentials, and not allow access to certain Web sites.

By default, Internet Explorer caches the connection (whether by a proxy or direct connection) that is used to access a specific host name. The EnableAutoproxyResultCache registry value was introduced to allow administrators to disable this proxy caching capability so that Internet Explorer can use different proxy servers to connect to different Web sites that are hosted on the same host name. Without this registry value, Internet Explorer always uses the proxy server through which the first connection to a specific host name is made, regardless of the settings in the .pac file. With this caching disabled, Internet Explorer reads the .pac file for each URL, but if Internet Explorer connects to a Web site on a specific host directly and then connects to a different Web site on the same host through a proxy server, it correctly tries to connect through the proxy server, but sends incorrect proxy authorization 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 are experiencing this specific problem. This fix may receive additional testing. Therefore, if you are not severely affected by this problem, Microsoft recommends that you wait for the next Internet Explorer 5.5 service pack that contains this hotfix.

To resolve this problem immediately, 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: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 typical 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 fix should have the following file attributes or later:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.

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