1. A method for detecting at least one security vulnerability, comprising: providing test instructions for an application being a web application or a client server application installed at a tested unit; adding test code, at runtime of said application on said tested unit, to a code of said application, said code is executed on a program code execution environment of said tested unit at the runtime of said application, said test code being added according to said test instructions; sending via a network at least one message to said application according to said test instructions during runtime of said application; wherein said test code is executed on said program code execution environment of said tested unit to gather at least one reaction of said application to said at least one message during runtime of said application; wherein said test code is executed on said program code execution environment to forward said at least one gathered reaction from said tested unit to a testing unit which performs an analysis of said at least one reaction; and detecting by said testing unit a presence or an absence of at least one security vulnerability according to said analysis.