Since the advent of SPECTRE, a number of countermeasures have been proposed and deployed. Rigorously reasoning about their effectiveness, however, requires a well-defined notion of security against speculative execution attacks, which has been missing until now. In this paper (1) we put forward speculative non-interference, the first semantic notion of security against speculative execution attacks, and (2) we develop SPECTECTOR, an algorithm based on symbolic execution to automatically prove

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... eculative noninterference, or to detect violations. We implement SPECTECTOR in a tool, which we use to detect subtle leaks and optimizations opportunities in the way major compilers place SPECTRE countermeasures. A scalability analysis indicates that checking speculative non-interference does not exhibit fundamental bottlenecks beyond those inherited by symbolic execution.