1x1x1x1 is a famous ROBLOX hacker/exploiter best known for the April 2012 hack.
This hack, at the time, was the worst in ROBLOX history. ROBLOX was DDoSed, Builder's Club was made free, and Robux was given out for free. Not only that, but some official ROBLOX items were set as free and new items were created. The notification banner was changed and several admins were hacked into.
He has recently been spotted online as of 7/8/2017 as his character was changed. There is much speculation as to whether he is actually coming back or not.
1x1x1x1 may possibly be linked to the origins of Zero, A recently "leaked" hidden game thought to be operated by hackers, maybe bots, and other entities. This is a game operated by bharati, who is an infamous ROBLOX hacker/exploiter known for the clan named Anubis.
Screenshot 1 depicts 1x1x1x1 on the player list in-game around the time of 2007.
Screenshot 2 depicts one of 1x1x1x1's games, but due to one of the new ROBLOX updates, the place is now under review and inaccessible to regular players.
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