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Windows has just advised me to install a "Cumulative Security Update for ActiveX Killbits." What are Killbits? How will I/my computer be harmed if I do not install a Killbit update?

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up vote 5 down vote accepted

From Wikipedia:

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.

A killbit is a flag in the Windows Registry that marks a CLSID as being unsafe. The CLSID acts as a serial number for the software in question — a unique GUID that must exist for each piece of software that behaves as an ActiveX control. If an ActiveX container finds that the CLSID of a killbit entry matches the CLSID of the software, then the software is blocked from running in the ActiveX container. If a vendor wants to release an updated version then they release it with a different CLSID.

So basically, a the KillBits list is a list of ActiveX components that will never be allowed to run on your system due to them having security holes. This list needs regular updates.

You could also check here for some more elaborate information about KillBits than Wikipedia offers: http://blogs.technet.com/b/srd/archive/2008/02/06/the-kill_2d00_bit-faq_3a00_-part-1-of-3.aspx

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Killbits within ActiveX is a security feature. Basically if someone find a security hole in an ActiveX control/plugin microsoft can issue a Killbit that can disable the control/plugin.

Oops beaten to it, the Wikipedia article explains it much better than I have :D

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That's ok! We're all here to complement each other aren't we? :D –  BloodPhilia Sep 16 '10 at 22:32
    
Yep, we are. I'm going post a link to the TechNet explanation in a little while, if nobody else puts it up first. (This is one of those "I was able to figure it out for myself, but I thought it would benefit SU to have the question in the database" questions.) –  Pops Sep 16 '10 at 22:35
    
@Lordtorgamus Haha! Anyway, just a tip, if you type "@blood" (using the first 5 letters or more of someone's username) the person it's directed at will receive a notification of your comment! ;) And I couldn't resist updating my answer! =P –  BloodPhilia Sep 16 '10 at 22:37
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