This is an animated weapon camo that covers your weapon in flame, surrounded by an array of undead hands. It can be applied to any MW2 and MW3 weapon. Rotten Inferno is found under the "Events ...

Call of Duty's Ricochet anti-cheat team is cracking down on those who have earned gun camos through ill-gotten means in Call ...

In fact, due to this bug, players cannot unlock any of the weapon's Mastery Camos. While Call of Duty has acknowledged the issue, the bug persists even after a week since it was added to the game.