Speaker: Ben H. Using the same stream cipher key twice is known to be a Very Bad Idea, but keystream-resuse vulnerabilities are still very much a thing of the present - both in legitimate software and in the malware landscape. We describe a heuristic algorithm which can detect vulnerabilities of this kind. We explain the inner workings of the algorithm and demonstrate a proof-of-concept attack on sevreral examples of vulnerable data, including files encrypted by the DirCrypt malware and encrypted traffic generated by malware such as variants of Zeus and Ramnit.