Apparently, hackers succeeded in creating and deploying a modified version of the Modern Warfare 3 lobby, which enabled some players playing in God mode to gain a significant advantage, especially in moving speed.
While Infinity Ward originally had promised its players double experience points over the weekend, the company had to fix the problem by initially simply shutting down servers. However, the strategy was only partially successful as the modified lobbies had already been deployed to Xbox consoles client side and resulted in a reset of experience points on those devices. Some reports suggest that a similar hack could be targeting PCs at this time. Infinity ward recommends all players not to accept game invites from private parties they do not know.
It is unclear whether the resignation of Robert Bowling, Infinity Ward's Creative Strategist, and one of the most public faces of the franchise, was triggered by the hack attacks. Bowling had repeatedly criticized attacks against Call of Duty and described the responsible hackers as "dirtbags". He announced his departure from Infinity Ward shortly after the first attacks.