Identifying at least one custom parameter in a request uniform resource locator (URL). At least a first portion of source code of a Web application that typically consumes the custom parameter provided in the request URL can be identified. The Web application can be instrumented at the first portion of the source code. The Web application can receive the request URL and the Web application can be executed with the instrumented source code. At least one run-time value consumed by the second portion of the source code can be identified, and the run-time value can be compared to the request URL to determine whether the run-time value intersects with the request URL. Responsive to determining that the run-time value intersects with the request URL, the run-time value can be identified as the custom parameter. A custom parameter rule can be generated based on the comparison.