Computerized methods and systems inspect data packets received from a web server for the presence of a value from a list of prohibited values. If a prohibited value is absent, a gateway injects at least one JavaScript code segment for execution by a web browser. The at least one JavaScript code segment includes a plurality of JavaScript functions which include at least one security analysis JavaScript function and a plurality of modified JavaScript functions. Each of the modified JavaScript functions is created from a respective native JavaScript function to include at least one code segment that when executed inspects for at least one of: a dynamic modification of at least one JavaScript function from a prohibited list of JavaScript functions, a dynamic creation of at least one JavaScript function from the prohibited list of JavaScript functions, or a dynamic reference to a value from the list of prohibited values.