A custom CPU is a processor designed and assembled from scratch instead of being bought as a standard off-the-shelf chip. It may use a unique instruction set, timing model, memory interface, or control logic, so software and tools cannot assume the behavior of mainstream x86 or ARM systems.
In cyber security, custom CPUs matter because they change the trust model. New hardware increases the chance of design bugs, weak isolation, and undocumented behavior, while also making analysis harder for defenders. Attackers may benefit from the obscurity of a rare architecture, but they also face a smaller tool ecosystem and more brittle code. Defenders use validation, emulation, hardware review, and careful testing to confirm that the processor executes code consistently and that security-critical functions such as memory protection and privilege separation work as intended.