Killbit is a term that refers to a security feature in Microsoft's Internet Explorer web browsers. A killbit instructs the browser to never use a specific piece of ActiveX control software whether third-party or Microsoft, identifying it by a number. The main purpose of a killbit is to close security holes. If a vendor discovers that there is a security hole in a specific version of an ActiveX control, they can request that Microsoft put out a "killbit" for it. Killbit updates are typically deployed to Microsoft Windows operating systems via Windows Update.