We describe ITS4, a tool for statically scanning security-critical C source code for vulnerabilities. Compared to other approaches, our scanning technique stakes out a new middle ground between accuracy and efficiency. This method is efficient enough to offer real-time feedback to developers during coding while producing few false negatives. Unlike other techniques, our method is also simple enough to scan C++ code despite the complexities inherent in the language. Using ITS4 we found new remotely-exploitable vulnerabilities in a widely distributed software package as well as in a major piece of e-commerce software. The ITS4 source distribution is available at http://www.rstcorp.com/its4.