A method, a system, and a computer program product embodying computer readable code for configuring a rule file for a Web application firewall. The method includes: blocking a response created by a Web application; modifying the response by adding capturing code for capturing a regular expression and an associated parameter value embedded in the response while being executed; sending the modified response to the browser; receiving a request submitted by the browser and at least one regular expression and an associated parameter value captured by the capturing code; determining a parameter name and a regular expression associated with the same parameter value, 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.