In the suit, they argue that the Treasury Department overstepped its authority in what it can sanction, claiming that "Tornado Cash software, including the smart contracts, consists of immutable open-source software code, which is not property, a foreign country or a national thereof, or a person of any kind." They've also argued that the designation is unconstitutional under both the free speech protections of the First Amendment and the due process protections of the Fifth Amendment.
In the wake of OFAC adding Tornado Cash to the U.S. sanctions list in early August, Coinbase has announced they will fund a lawsuit against the Treasury Department to challenge the decision. Coinbase itself is not a plaintiff in a lawsuit, though two of the plaintiffs are Coinbase employees, who along with four other individuals filed suit in a Texas court. They say they previously used Tornado Cash for licit purposes, and are now suffering financial damages because they can't legally use the service.