Autorun is one way for malware to trick a user into installing some nefarious software from removable storage media. An executable could reside on a USB flash drive and, when inserted into a PC, would prompt the user to click the autorun file.
Back in February, Microsoft released an update that curbed autorun behavior for all supported Windows platforms stretching back to Windows XP SP3 to Windows Vista SP2 (Windows 7 already had a "fixed" autorun behavior and Windows XP SP2 was no longer supported through updates).
When comparing stats before and after the Microsoft update, we clearly see that blocking the autorun of removable storage had a significant effect on the infection rate. It may have been a tweak to stop a certain action on the users' part, but it was certainly effective.
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