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Bug Fix: Error 404 when a URL contains a plus symbol (+) with IIS7

Due to a change in IIS 7’s security model, it is much stricter on what is and what isn’t a valid URL – that includes rejecting URLs that have a plus symbol (+) in the path (not the query string).  I’ve found this problem normally occurs when you are using URL re-writing to turn ugly URLs like ‘default.aspx?my+value’ into pretty, human friendly URLs like ‘/my+value’.

To work around this new feature you can put the following in your web.config:

        <requestFiltering allowDoubleEscaping="true" />

Be warned though, you are disabling a security feature so you are opening yourself up to attacks using malicious URLs.  Ensure that you really do need to have + symbols in the URL before doing this.  For example, if the plus sign is coming from have spaces in a string, could you replace the spaces with a hyphen (-)?