Abstract: Modern Intel, AMD, and ARM processors translate complex instructions into simpler internal micro-ops that are then cached in a dedicated on-chip structure called the micro-op cache. This work presents an in-depth characterization study of the micro-op cache, reverse-engineering many undocumented features, and further describes attacks that exploit the micro-op cache as a timing channel to transmit secret information. In particular, this paper describes three attacks – (1) a same thread cross-domain attack that leaks secrets across the user-kernel boundary, (2) a cross-SMT thread attack that transmits secrets across two SMT threads via the micro-op cache, and (3) transient execution attacks that have the ability to leak an unauthorized secret accessed along a misspeculated path, even before the transient instruction is dispatched to execution, breaking several existing invisible speculation and fencing-based solutions that mitigate Spectre.