A URL may contain special character that need special syntax handling in order to be interpreted. Special characters are represented using a percentage character followed by two digits representing the octet code of the original character (%HEX-CODE).

For instance US-ASCII space character would be represented with %20. This is often referred as escaped ending or percent-encoding. Since the server decodes the URL from the requests, it may restrict the access to some URL paths by validating and filtering out the URL requests it received. An adversary will try to craft an URL with a sequence of special characters which once interpreted by the server will be equivalent to a forbidden URL.

It can be difficult to protect against this attack since the URL can contain other format of encoding such as UTF-8 encoding, Unicode-encoding, etc. The adversary could also subvert the meaning of the URL string request by encoding the data being sent to the server through a GET request. For instance an adversary may subvert the meaning of parameters used in a SQL request and sent through the URL string (See Example section).