This is a public disclosure of a newly discovered vulnerability. Some affected parties have already been notified in a private disclosure that was sent out on November 13th.
When ETH is sent to an address, that address is able to perform arbitrary computations paid for by the originator of the transaction. This is a known vector for griefing. However, in some cases, at-risk systems such as exchanges did not put proper protections in place.
GasToken, which takes advantage of the refund mechanism on storage in Ethereum, allows users to store gas when the gas price is low and receive a gas refund when the gas price is high. By minting large amounts of GasToken when receiving ETH, the griefing vector mentioned above can now be a profitable attack.
Because it was unknown which exchanges did and did not have the protections in place, the private disclosure was made to as many exchanges as possible, many of which were not at risk. To our knowledge, all affected exchanges that received the disclosure have patched the vulnerability.
For more information the full disclosure can be found here.