Unintended emissions are signals a device leaks as a byproduct of normal operation. They can be electromagnetic, acoustic, thermal, or even optical, and they may carry information about the system’s activity. In cyber security, these leaks matter because an attacker does not always need network access to observe them; sometimes they can recover data by measuring the physical behavior of hardware.
These emissions are a core concern in TEMPEST-style security, where defenders try to limit information leakage from radios, cables, power circuits, displays, and other components. In attacks, an adversary may passively listen for leakage or, in some setups, inject a signal that interacts with nonlinear hardware and makes the emissions easier to analyze. Defenses include shielding, filtering, careful equipment placement, and testing for exploitable side channels.