Getting "The WebResource.axd handler must be registered in the configuration to process this request." error

I figured it out so I'm posting it here for search reasons. It is a bug in ASP.NET and it has to do with having ColdFusion installed. Microsoft hasn't yet released a fix.

There are two ways to fix this.

1. Change the AppPool in IIS7 to "Classic .NET AppPool". I'm not sure of any implications with this setting.

2. Remove all the references to ColdFusion from your applicationHost.config file in system32\inetsrv\config.

ColdFusion installs a global wildcard handler mapping which apparently overrides many of the standard .NET handlers. The solutions mentioned work just fine, but if you can't switch to Classic Mode and don't want to screw with your ColdFusion installation, you can remove the inherited handler mapping at the individual site level.

To do this, go to the site in question in IIS, double-click on "Handler Mappings" under the "IIS" section, and find the handler named something like "AboMapperCustom-XXXXXX" with "*" for the Path. Select the entry and click "Remove" in the sidebar. This will only remove the mapping for your application, so it won't break any existing CF sites on the server.

In IIS7 you need to add the <httpHandler> section to <system.webServer> instead of <system.web>. Here is an example.