sentences: 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 eficiency. This method is eficient 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 remotelyexploitable 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 h t tp : //www.rstcorp. com/its4.