What is "killbit"

Killbit is a security feature in web browsers based on Microsoft's Trident engine (such as Internet Explorer) and other ActiveX containers that respect the killbit (such as Microsoft Office). A killbit instructs an ActiveX control container never to use a specific piece of ActiveX software, whether third-party or Microsoft, as identified by its class identifier (CLSID).

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.