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KB: Server Returns HTTP 404 Error When Allow Unlisted File Name Extensions Is Disabled in IIS

Updated: April 22, 2026
Altium On-Prem Enterprise Server Altium On-Prem Enterprise Server
When the IIS security setting "Allow unlisted file name extensions" is disabled, the Altium On‑Prem Enterprise Server web interface may become inaccessible and return HTTP 404 errors. Although IIS and Altium services remain running, requests to the server are blocked because the application depends on multiple file types that are not permitted unless explicitly allowed. By manually adding the required file name extensions to the IIS Request Filtering configuration for the Altium Enterprise Server site, access can be restored while keeping the restrictive IIS setting enabled. This article describes the symptoms, explains the cause, and outlines the steps required to resolve the issue.

Solution Details

Web Interface inaccessible after IIS change

Users attempting to access the Altium On‑Prem Enterprise Server (AES) web interface receive the following error:

“No web page was found for the address: <Server Address>. HTTP error 404.”

The issue occurs immediately after disabling Allow unlisted file name extensions in Internet Information Services (IIS). The server appears unavailable, even though IIS and the Altium services are running.

IIS request filtering blocks required file types

AES serves its web interface and APIs using multiple file types such as .aspx, .ashx, .json, .css. When Allow unlisted file name extensions is disabled, IIS blocks any request that does not explicitly match an allowed extension, resulting in HTTP 404 errors across the application

Explicitly allow required extensions

Manually add the required file extensions to the Allowed File Name Extensions list in IIS for the Altium Enterprise Server site.

To resolve the issue while maintaining a restrictive IIS security posture, the required file name extensions must be manually added to the Allowed File Name Extensions list for the AES site in IIS.

Add extension in IIS Request Filtering

  1. Open IIS Manager
  2. Navigate to: Sites » Altium On‑Prem Enterprise Server
  3. Open Request Filtering

Internet Information Services (IIS) Manager home view showing a server with Application Pools and Sites listed, ASP.NET and IIS feature icons in the main panel, and Request Filtering available among server features. 

  1. In the Actions pane, select Allow File Name Extension...
  2. Add each of the following extensions individually: .ashx .ico .asmx .aspx .axd .css .js .json .cshtml .html .png .jpg .gif .woff .woff2 .ttf .eot .svc .git .metadata .zip .xml

IIS Manager Request Filtering page with the “Allow File Name Extension” dialog open, prompting for a file name extension to add to the allowed list while file name extensions are otherwise blocked.  

  1. Ensure “Allow unlisted file name extensions” remains disabled

IIS Manager Request Filtering feature with Edit Request Filtering Settings open, showing the General options and the “Allow unlisted file name extensions” checkbox unchecked, alongside other request limit and security settings. 

  1. Refresh the AES URL and verify that the web interface is accessible.

Additional Notes

  • This approach maintains a stricter IIS security posture while allowing AES to function correctly.
  • A full IIS restart is typically not required; however, recycling the application pool may help if changes do not take effect immediately.
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