A Basic web.config for ColdFusion on Windows IIS

Here’s a basic web.config I use with most of my Windows IIS + ColdFusion sites. It allows ColdFusion errors to pass through and sets a few helpful defaults.

When running ColdFusion on Windows IIS, one common issue is that IIS will block or override ColdFusion’s own error output. You might expect to see a <cfthrow> or your custom error template, but instead IIS shows its own 500 error page.

To fix this, you need to tell IIS to pass through errors generated by ColdFusion. The key is this line in your web.config:

<httpErrors errorMode="DetailedLocalOnly" existingResponse="PassThrough" />

Below is a simple, safe example of a web.config you can use for most ColdFusion applications:

<?xml version="1.0" encoding="UTF-8"?> <configuration> <system.webServer> <!-- Allow ColdFusion errors to pass through IIS --> <httpErrors errorMode="DetailedLocalOnly" existingResponse="PassThrough" /> <!-- Optional: Allow larger uploads (example: 100 MB) --> <security> <requestFiltering> <requestLimits maxAllowedContentLength="104857600" /> </requestFiltering> </security> <!-- Optional: Enable compression --> <urlCompression doStaticCompression="true" doDynamicCompression="true" /> </system.webServer> </configuration> 

This is all you need for a basic setup. The most important part is the existingResponse="PassThrough", which ensures ColdFusion errors are not hidden by IIS.

You can copy this file to the root of your ColdFusion application. Adjust the maxAllowedContentLength if your application handles file uploads.

That’s it — a simple web.config you can safely use in general situations.