Similar to criminals in the physical world, cyber-criminals use a variety of illegal and immoral means to achieve monetary gains. Recently, malware known as ransomware started to leverage strong cryptographic primitives to hold victims' computer files "hostage" until a ransom is paid. Victims, with no way to defend themselves, are often advised to simply pay. Existing defenses against ransomware rely on ad-hoc mitigations that target the incorrect use of cryptography rather than generic live

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... tection. To fill this gap in the defender's arsenal, we describe the approach, prototype implementation, and evaluation of a novel, automated, and most importantly proactive defense mechanism against ransomware. Our prototype, called PayBreak, effectively combats ransomware, and keeps victims' files safe. PayBreak is based on the insight that secure file encryption relies on hybrid encryption where symmetric session keys are used on the victim computer. PayBreak observes the use of these keys, holds them in escrow, and thus, can decrypt files that would otherwise only be recoverable by paying the ransom. Our prototype leverages low overhead dynamic hooking techniques and asymmetric encryption to realize the key escrow mechanism which allows victims to restore the files encrypted by ransomware. We evaluated PayBreak for its effectiveness against twenty hugely successful families of real-world ransomware, and demonstrate that our system can restore all files that are encrypted by samples from twelve of these families, including the infamous CryptoLocker, and more recent threats such as Locky and SamSam. Finally, PayBreak performs its protection task at negligible performance overhead for common office workloads and is thus ideally suited as a proactive online protection system.