An anonymous reader writes "Microsoft remotely deleted old versions of Tor anonymizing software from Windows machines to prevent them from being exploited by Sefnit, a botnet that spread through the Tor network. It's unclear how many machines were affected, but the total number of computers on the Tor network ballooned from 1 million to 5.5 million as Sefnit spread. 'By October, the Tor network had dropped two million users thanks to Sefnit clients that had been axed. No one, not even the Tor developers themselves, knew how Microsoft had gone on a silent offensive against such a big opponent and won a decisive battle,' the Daily Dot reported. In a blog post, Microsoft claimed it views Tor as a 'good application,' but leaving it installed presented a severe threat to the infected machines."