On Friday July 12th, 0x shut down their v2 Exchange because a flaw in the signature verification routine meant that a signature of 0x04 was treated as a valid signature for all non-smart-contract accounts. This blog post explains how this is possible.

0x is, when grossly oversimplified, a platform which allows users to trade with other users. If Alice wants to buy 1000 ZRX for 1 ETH, Alice can submit an order through the 0x protocol. If Bob then decides that he wants to take Alice's order, he can use 0x's Exchange contract to securely perform the exchange.