The request filtering module is configured to deny a request that contains a double escape sequence

In IIS 7.5 I have a site that contains a page that takes an encrypted part of a URL. This encrypted string includes a plus sign '+' which causes IIS to throw a "HTTP Error 404.11 - Not Found" error stating "The request filtering module is configured to deny a request that contains a double escape sequence."

The problem is that a + sign used to be acceptable in earlier versions of IIS so these URLs need to remain for legacy reasons. So, I need to make them allowed again in IIS.

The quick fix

This can be easily achieved with a simple web.config change:

<system.webServer>
    <security>
        <requestFiltering allowDoubleEscaping="true" />
    </security>
</system.webServer>

This allows URLs to contain this plus symbol '+'.

The warning

There are consequences to this which unsurprisingly are security related so please read Double Encoding to familiarise yourself with the risk for your situation. If it is a risk to you maybe the best solution is to redesign those URLs?