1. A method comprising: by one or more computing devices, marking a variable in source code of a software program written in an object-oriented scripting language, wherein marking the variable is performed with a keyword; marking one or more locations in the source code of the software program; constructing a control flow graph (CFG) for the software program; and tracking the marked variable through the CFG by determining a path in the CFG that leads from a first node corresponding to the marked variable to one of the marked locations in the source code of the software program.