Microsoft last week changed how it delivers an update that disables AutoRun, a Windows feature that big name worms, including Conficker and Stuxnet, have used to infect millions of PCs.
The company is now pushing the update to Windows XP and Vista users automatically.
When Microsoft first deployed the update Feb. 8, it said the patch would be offered as an optional download. To retrieve it, users had to manually checkmark the "KB971029" update in the "Software, Optional" section of Windows Update in XP, or in Vista's Windows Update panel under "Important."
But last week Microsoft changed those rules and began feeding users the update through the Automatic Updates feature of Windows Update, which automatically downloads and installs hotfixes and other software upgrades. In Windows XP, for example, users now see the AutoRun fix under the "High-priority updates" label, and the patch is pre-checked so it downloads and installs without any user action.
The "High-priority updates" section of XP's Windows Update is the same location where security-related patches appear.
Microsoft's move to cripple AutoRun is a response to malware's continued reliance on infection tactics that abuse AutoRun and AutoPlay, the technologies that automatically launch executable files on removable media, especially USB flash drives.
Both Conficker, a worm that spread widely in early 2009, and Stuxnet, the worm that analysts suspect was developed to sabotage Iran's nuclear programs, used AutoRun and flash drives to infect Windows PCs.
Microsoft changed AutoRun's behavior in Windows 7 to block automatic execution of files on a USB drive, and first backported the modifications to Windows XP and Vista in August 2009.
When the update is in place, flash drives inserted into a PC running XP or Vista no longer offer the option to run programs; the AutoRun extinction does not affect CDs or DVDs.
Microsoft confirmed the update reset, and said that it changed the delivery process to "minimize the user interaction required to install the updates on systems configured for automatic updating." However, the company did not respond to questions about why it did not communicate the change to users as it had in early February when it said the patch was optional.
The unannounced automatic deployment of the AutoRun update may cause confusion if users expect files -- in particular, setup executables that kick off software installation -- to launch when they insert a flash drive in their Windows XP or Vista PCs.
Microsoft noted that the update breaks the functionality of some USB drives. "Users who install this update will no longer receive a setup message that prompts them to install programs that are delivered by USB flash drives. Users will have to manually install the software," Microsoft warned in a security advisory.
To disable the update's changes and revert to Windows XP's and Vista's earlier behavior, users can run the "Enable Autorun" tool found on Microsoft's support site.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is email@example.com.