IISW-SI-000229 - Double encoded URL requests must be prohibited by any IIS 8.5 website.
Request filtering enables administrators to create a more granular rule set with which to allow or reject inbound web content. By setting limits on web requests, it ensures availability of web services and mitigates the risk of buffer overflow type attacks. When the 'Allow double escaping' option is disabled it prevents attacks that rely on double-encoded requests.
Follow the procedures below for each site hosted on the IIS 8.5 web server: Open the IIS 8.5 Manager. Click the site name under review. Double-click the 'Request Filtering' icon. Click 'Edit Feature Settings' in the 'Actions' pane. Uncheck the 'Allow double escaping' check box.