Client requests to access a published Web site are blocked when you configure ISA Server 2006 to allow direct authentication to access a published Web server
You configure a computer that is running Microsoft Internet Security and Acceleration (ISA) Server 2006 to use pass-through authentication to access a published Web server. After you do this, all client requests to access the published Web site are blocked. Additionally, you may receive an error message that resembles the following:
Error Code: 403 Forbidden. ISA Server is configured to block HTTP requests that require authentication. (12250)
- You experience this issue when you use the No delegation, but client may authenticate directly (pass-through) authentication method.
- This issue may occur even if the ISA Server 2006 computer publishes a site that requires no authentication.
This issue may occur if the following conditions are true:
- The Allow client authentication over HTTP check box in the Web listener's Advanced Authentication Options dialog box is not selected.
- The Web listener is not enabled to listen for Secure Sockets Layer (SSL) requests.
For the solution, check out the KB article here: