Killbit

Killbit is a term that refers to a 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. Consequently, killbits are frequently seen as downloadable items within Windows Update.

Killbits are simply a flag in the Windows Registry that marks a GUID as being unsafe. The GUID acts as a serial number for the software in question - a unique GUID must exist for each potential piece of software that behaves as an ActiveX control. If Internet Explorer finds that the GUID of a killbit entry matches the GUID of the software, then the software cannot be run by Internet Explorer. If a vendor wants to release an updated version, they simply release it with a different GUID.

References

# [http://support.microsoft.com/kb/240797 Microsoft KB240797] : "How to stop an ActiveX control from running in Internet Explorer" (August 24, 2007)
# [http://blogs.technet.com/swi/archive/2008/02/06/The-Kill_2D00_Bit-FAQ_3A00_-Part-1-of-3.aspx Microsoft Technet] : "The Kill-Bit FAQ"