Original URL: http://www.theregister.co.uk/2008/08/15/ms_kill_bit/
Microsoft ramps up vuln ActiveX controls cull
Third-party fire and exploit block
This week's Patch Tuesday update was nearly as difficult to digest as a Michael Phelp's breakfast. It contained 11 bulletins covering 26 underlying vulnerabilities, the most in two years.
With all this high-calorie content to chew through two important points about the update have gone largely overlooked. Firstly, a promised Media Player update failed to make it through quality control.
Secondly Microsoft issued ActiveX "kill-bit" controls for applications from HP and Aurigma that had been the subjects of security alerts over recent months.
A kill bit involves a registry change that prevents an ActiveX control from being loaded by Internet Explorer. The technique blocks a popular exploitation route without addressing the underlying vulnerability, which needs to be separately patched.
Microsoft is making greater use of the approach, which it only applies with the blessing of third-party developers, in what amounts to an attempt to protect slow-to-patch customers from themselves.
August's update included kill bits for two third-party applications - Aurigma Image Uploader and HP Instant Support - and constitutes the third time Microsoft has disabled third-party add-ons using the approach since April. An update in April included a kill bit for the Yahoo! Music Jukebox product and problems in the BackWeb Client embedded in the Logitech Desktop Manager were nixed by a kill bit in June.
In all four cases patches from the third-party developers themselves had been available months before the Microsoft kill-bit, so this isn't just a workaround pending the availability of updates.
More information on how kill bits work, along with their uses and limitations, can be found in an FAQ from Microsoft here.
Microsoft's basic line is that if a third-party developer finds out it has produced a vulnerable ActiveX control, it will issue a kill bit that disables the control.
Earlier this month Microsoft launched a scheme to help third-party developers to find and fix bugs in their software. The Microsoft Vulnerability Research programme, while not directly tied to the kill-bit scheme, illustrates a growing recognition in Microsoft and elsewhere that the combination of two of more vulnerabilities, which alone aren't that bad, can create a serious risk from blended threats. ®