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There has been a resurgent trend in the industry to enforce a variety of security policies in hardware. The current trend for developing dedicated hardware security extensions is an imperfect, lengthy, and costly process. In contrast to this trend, a flexible hardware monitor can efficiently enforce and enhance a variety of security policies as security threats evolve. Existing hardware monitors typically suffer from one (or more) of the following drawbacks: a restricted set of monitoring actions, considerable performance and power overheads, or an invasive design. In this paper, we propose a minimally-invasive and efficient implementation of a Programmable Hardware Monitor (PHMon) with expressive monitoring rules and flexible fine-grained actions. PHMon can enforce a variety of security policies and can also assist with detecting software bugs and security vulnerabilities. Our prototype of PHMon on an FPGA includes the hardware monitor and its interface with a RISC-V Rocket processor as well as a complete Linux software stack. We demonstrate the versatility of PHMon and its ease of adoption through four different use cases: a shadow stack, a hardware-accelerated fuzzing engine, an information leak prevention mechanism, and a hardware-accelerated debugger. Our prototype implementation of PHMon incurs 0.9% performance overhead on average, while the hardware-accelerated fuzzing engine improves fuzzing performance on average by 16× over the state-of-the art software-based implementation. Our ASIC implementation of PHMon only incurs a 5% power overhead and a 13.5% area overhead.