It's like a standard padding oracle attack, but 4 billion times slower (on average) and requires 4 billion times more CPU work and bandwidth, and you have to be

It's like a standard padding oracle attack, but 4 billion times slower (on average) and requires 4 billion times more CPU work and bandwidth, and you have to be able to distinguish between a HMAC failure and a padding error. (a.k.a. isn't happening)