Although anti-virus software has significantly evolved over the last decade, classic signature matching based on byte patterns is still a prevalent concept for identifying security threats. Anti-virus signatures are a simple and fast detection mechanism that can complement more sophisticated analysis strategies... However, if signatures are not designed with care, they can turn from a defensive mechanism into an instrument of attack. In this paper, we present a novel method for automatically deriving signatures from anti-virus software and demonstrate how the extracted signatures can be used to attack sensible data with the aid of the virus scanner itself. We study the practicability of our approach using four commercial products and exemplarily discuss a novel attack vector made possible by insufficiently designed signatures. Our research indicates that there is an urgent need to improve pattern-based signatures if used in anti-virus software and to pursue alternative detection approaches in such products. (read more)