This can potentially reveal sensitive data introduced via virtual keyboards, which are used precisely to avoid data theft via another online threat: key stroke loggers. Spider.io found out that Internet Explorer versions 6 to 10 are vulnerable to this kind of exploitation. What's worse, hackers could potentially track your movements even if the I.E. window is minimized.
This vulnerability is also apparently very easy to take advantage of. All a hacker needs to do is buy a display ad on any webpage and wait until a user visits it. If the tab remains open, the hacker has continuous access to your mouse movements.
The issue was discovered by Spider.io, a web analytics company, when it was studying new ways to measure the effectiveness of ads placed in different parts of webpages. Microsoft was warned of this issue in October, but there's no apparent plan to patch it yet.