The US Court of Appeals for the DC Circuit has dealt a blow to the legal strategy used by Prenda Law, a "copyright troll" that sued thousands of users over allegations of downloading porn movies over BitTorrent.

The decision is more than just another setback for Prenda, which has been on the defensive since last year when it was sanctioned for its conduct and referred to criminal investigators. It's a ruling from an appeals court against the joining of many defendants together in a single copyright lawsuit based on their use of BitTorrent. As such, it will likely have effects on other companies that are still active in the mass-lawsuit space, like Malibu Media.

Prenda shell company AF Holdings previously argued that it should be allowed to sue more than 1,000 people in the same lawsuit, because they were all part of the same BitTorrent "swarm" sharing the same file, an adult movie called Popular Demand.

"We are unconvinced," wrote Judge David Tatel, penning today's opinion for a unanimous three-judge panel. He continued:

AF Holdings has provided no reason to think that the Doe defendants it named in this lawsuit were ever participating in the same swarm at the same time. Instead, it has simply set forth snapshots of a precise moment in which each of these 1,058 Does allegedly shared the copyrighted work—snapshots that span a period of nearly five months. Two individuals who downloaded the same file five months apart are exceedingly unlikely to have had any interaction with one another whatsoever. Their only relationship is that they used the same protocol to access the same work.

The opinion goes on to use an analogy offered by Electronic Frontier Foundation lawyer Corynne McSherry, describing a casino blackjack table that has different players come and go over time. Two BitTorrent users who download a file months apart are like players at the same blackjack table at different times.

The judges made clear that if AF Holdings identified users who were in the same swarm at the same time, it would have been amenable to joinder—but not otherwise.

In addition to the joinder issue, the judges found separately that Prenda shouldn't have even been allowed to use this Washington DC-based lawsuit to gather information about Internet subscribers outside the District of Columbia. Prenda was looking for information for 188 Verizon subscribers, even though only 18 lived in DC. Only one of 400 Comcast subscribers identified by Prenda lived in the city.

"Here, we think it quite obvious that AF Holdings could not possibly have had a good faith belief that it could successfully sue the overwhelming majority of the 1,058 John Doe defendants in this district," they wrote. "In seeking such information, AF Holdings clearly abused the discovery process."