Someone figured out an exploit to unlock unlimited ammo, however now it's spreading across Xbox Live like a virus.
There's something fishy going on with Call of Duty: Modern Warfare 2. A few sources deem the situation as a quasi-virus infestation, spreading across Xbox Live and infecting gamers playing Modern Warfare 2 multiplayer matches online. Like an epidemic, it started at one source--presumably a modder--and is now taking over the network, living in the console's cache as it follows player after player, infecting other unsuspecting gamers in every joined match.
But don't worry: this virus won't brick your console's HDD. In fact, some gamers may take it as an early Christmas present, granting unlimited ammo for all weapons, and providing nonexistent loading times. That's definitely not something to cry about, however this "gift" won't go away: it remains no matter what private match you join. Obviously, this type of "cheat" can create a problem in overall gameplay balance, perhaps even making the matches unplayable.
However Infinity Ward community manager Robert Bowling said on his Twitter page that the problem is being addressed, and that patches are currently in the works. "It'll be posted on the forums in the new Status Update forum at the top," Bowling said, referring to the latest update on the fix. "I'll link to it here though so you know when it's posted."
Players who come across this "infectious" exploit will need to disconnect from Xbox Live and reboot the console in order to get rid of the problem, according to Kotaku. Currently there's no information on how this exploit works, or if its causing any real underlying damage or security breaches.