1. A method for configuring a rule file for a Web application firewall computer, the method comprising the steps of: blocking, by the Web application firewall computer, a response created by a Web application of a Web server for a browser; modifying the response to produce a modified response having an added capturing code for capturing at least one regular expression and at least one associated parameter value embedded in the response when the response is executed by the browser; sending the modified response to the browser; receiving, by the Web application firewall computer, a request submitted by the browser, wherein the request contains at least one parameter name and at least one associated parameter value; receiving, by the Web application firewall computer, the at least one regular expression and the at least one associated parameter value captured by the capturing code; determining, by the Web application firewall computer, a parameter name and a regular expression which are associated with the same parameter value based on (i) the at least one parameter name and the at least one associated parameter value contained in the received request and (ii) the at least one regular expression and the at least one associated parameter value captured by the capturing code; and configuring the rule file of the firewall by use of the determined parameter name and regular expression associated with one another as a filtering rule; wherein the at least one associated parameter value is reset at the browser side after a previous parameter value fails to pass verification by the browser and therefore the response fails to be submitted.