An expansion card is an add-on board that plugs into a computer to extend what it can do, such as adding storage, network, graphics, or specialized input and output. Older systems often relied on expansion cards to gain functions the base machine did not include. In security terms, each card introduces extra hardware, firmware, drivers, and configuration that the host must trust and manage.
Expansion cards matter because they increase the attack surface of a system. A defective, modified, or malicious card can interfere with booting, expose data, impersonate a trusted device, or create hard-to-diagnose stability problems. Defenders reduce risk by inventorying installed hardware, verifying compatibility and firmware, limiting unnecessary add-ons, and testing changes carefully. In small or legacy computers, added cards can also make troubleshooting and hardening more difficult because every extra module changes how the whole system behaves.