Windows 10 doesn't read a PAC file referenced by a file protocol

Important

The Internet Explorer 11 desktop application will be retired and go out of support on June 15, 2022 (for a list of what's in scope, see the FAQ). The same IE11 apps and sites you use today can open in Microsoft Edge with Internet Explorer mode. Learn more here.

This article solves an issue where Internet Explorer or Microsoft Edge can't read a proxy auto-configuration (PAC) file referenced by a file protocol.

Original product version:   Windows 10, Microsoft Edge, Internet Explorer 11
Original KB number:   4025058

Symptoms

Consider the following scenario:

  • You configured Microsoft Internet Explorer 11 or Microsoft Edge on Windows 10 to use the Use Automatic Configuration script option.

  • You're testing or deploying a PAC file. You're storing the file in a local location, such as C:\temp\proxy.pac.

  • You specified the file location in the Use Automatic Configuration script option in Internet Setting by using the file protocol.

  • You created the following registry subkey:

    HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\  
    Value: EnableLegacyAutoProxyFeature  
    Type:REG_DWORD  
    Data:1
    

In this scenario, the PAC file isn't read by Internet Explorer or Microsoft Edge.

Cause

This issue occurs because Internet Explorer and Microsoft Edge on Windows 10-based computers use the WinHttp proxy service to retrieve proxy server information. The WinHttp Proxy service doesn't support using the ftp:// or file:// protocol for a PAC file.

Resolution

To resolve this issue, host the PAC file on a web server, and then refer to it by using the http protocol.

Note

The application/x-ns-proxy-autoconfig MIME type has to be specified for the PAC file on the web server in order for the WinHttp proxy service to consume the PAC file.
If not, the extension of the PAC file has to be .dat, .js, .pac, or .jvs.