UPDATE: Microsoft has issued Eurogamer the following statement on the Halo 4 Xbox Live bans.
"We are aware of isolated cases in which Halo 4 content has been propped on the web and are working closely with our security teams and law enforcement to address the situation immediately.
ORIGINAL STORY: Microsoft is issuing permanent Xbox Live bans to those it finds have pirated Halo 4 from Xbox Live.
Users on Reddit report Microsoft's Live enforcement team is sending emails notifying pirates that they've been permanently booted off Xbox Live for "prerelease title play" (thanks, carma!).
Over the weekend Halo 4 leaked onto torrent sites for use on hacked Xbox 360 consoles. Microsoft is investigating and taking down gameplay spoiler videos from YouTube.
For Microsoft to know you've been naughty and pirated the game, you'll have had your hacked Xbox 360 connected to Xbox Live.
"Your Xbox Live privileges will not be reinstated," reads the email. We've asked Microsoft for comment.
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