Every program needs some input and some logic to translate such input into structured data in memory. Language parsers, in particular, are notoriously error-prone to write,so programmers often define a high-level grammar and use parsergenerators to make the parsing process easier. What does this entail for a security analyst? In this talk you will learn what to do when you encounter GNU bison-generated code in a binary, what kind of code it creates, and how to exploit its structure to analyze targets faster and improve your fuzzers in the process. You will also walk away with a free (as in freedom) tool to automate your reverse engineering efforts.