ActiveX is a software technology developed by Microsoft for its Windows series of operating systems. It provides an API, or "Application Programming Interface," that allows web developers to write software, or "controls," that have direct access to the computer's software and hardware. While most web pages are only able to interact in a very limited way with the user, by way of the web browser, those that are enhanced with an ActiveX Download Control have much greater flexibility.

Through the use of an ActiveX Download Control, a web page is able to access files on the user's computer, communicate directly with hardware and interact with core Windows components. This allows for more advanced functionality to be supported, such as running games that occupy the full screen, providing a front end for cloud based Internet applications and other innovative features.

Because of the level of power that the ActiveX software provides, it is viewed as a double edged sword. The software allows for a great deal of control over the user's computer. This can be highly useful and also highly dangerous. An ActiveX Download Control is capable of causing serious damage to the operating system and install malicious programs on its own. These controls can also capture user data such as keystrokes, confidential files and network transmissions. Because of this, it is the user's responsibility to ensure that any ActiveX Download Control that they download and run is from a trusted source.

ActiveX Download Controls are signed with an encryption key that identifies the author and publisher of the content. These certificates must be up to date and recognized by a trustworthy authority before they can be safely used. It is important to note that, in some cases, these certificates can be stolen and forged. Due to security concerns, ActiveX Download Controls are currently only supported in Internet Explorer.