ActiveX is a Microsoft component model that lets software reuse controls and features inside Windows applications and older web environments. In practice, it was often used to embed browser-like functionality, document handling, or other interactive components into desktop programs and web pages. Because ActiveX controls can expose powerful local capabilities, they can blur the line between untrusted content and trusted code running on the machine.
That matters in cyber security because legacy ActiveX usage can expand attack surface in ways users do not expect. If a desktop app hosts old browser components, or if a page loads a dangerous control, attacker-controlled content may trigger code execution, privilege abuse, or other harmful behavior. Defenders look for embedded IE-era components, restrict where ActiveX is allowed, and remove or isolate controls that no longer need to exist. In modern environments, ActiveX is mostly a compatibility concern, but it remains relevant wherever old Windows integrations still survive.