Blind Spots: Automatically detecting ignored program inputs

A blind spot is any input to a program that can be arbitrarily mutated without affecting the program's output. Blind spots can be used for steganography or to embed malware payloads. If blind spots overlap file format keywords, they indicate parsing bugs that can lead to differentials. This paper formalizes the operational semantics of blind spots, leading to a technique that automatically detects blind spots based on dynamic information flow tracking. An efficient implementation is introduced an evaluated against a corpus of over a thousand diverse PDFs. There are zero false-positive blind spot classifications and the missed detection rate is bounded above by 11%. On average, at least 5% of each PDF file is completely ignored by the parser. Our results show promise that this technique is an efficient automated means to detect parser bugs and differentials. Nothing in the technique is tied to PDF in general, so it can be immediately applied to other notoriously difficult-to-parse formats like ELF, X.509, and XML.