Open-source hardware is physical device design released with enough documentation for others to study, modify, manufacture, and repair it. That can include schematics, PCB layouts, bills of materials, mechanical drawings, and sometimes firmware. In cyber security, this matters because hardware is often a trust boundary: hidden components, undocumented interfaces, or locked-down repair paths can conceal vulnerabilities or make independent verification impossible.
Open designs help defenders inspect debug ports, firmware update paths, power and sensor circuitry, and other features that attackers may abuse. They also support audits for supply-chain tampering and make it easier to detect unsafe or backdoored implementations. But openness is not automatic security; it simply makes scrutiny possible. In real defenses, open-source hardware can improve transparency, reproducibility, and maintenance, while in attacks the same public documentation may help an adversary clone a device or study weak points faster.