A federal appeals court has ruled that computer programmers do not have the right to reverse-engineer Blizzard Entertainment’s video games to improve their playability.

The 8th Circuit Court of Appeals in St. Louis ruled Thursday that federal law—specifically, the Digital Millennium Copyright Act—disallows players from altering Blizzard games to link with servers other than the company’s official Battle.net site.

Affected games published by Blizzard, a division of Vivendi Universal, include titles in its “Diablo,” “Starcraft” and “Warcraft” lines.

In a 3-0 decision, the court upheld a trial judge’s ruling from October, concluding the programmers’ “circumvention in this case constitutes infringement.”

The DMCA broadly restricts circumventing, or bypassing, antipiracy measures. Blizzard had included such measures to tie its games to the Battle.net site and detect pirated copies.

The defendants in the case, Ross Combs and Rob Crittenden, reverse-engineered the Blizzard protocol using tools like “tcpdump” to listen to the software’s communications with a game server. Eventually, their “bnetd” project let Blizzard games connect with unofficial servers, yielding benefits like faster response times.

The 8th Circuit also cited a contractual agreement that Combs and Crittenden OK’d when installing Blizzard software. That agreement prohibits reverse-engineering.