It depends. In WoWs case they're reverse engineered, but there are lots of examples of games that had their servers leaked. Cabal Online for example had a serve

It depends. In WoWs case they're reverse engineered, but there are lots of examples of games that had their servers leaked. Cabal Online for example had a server leaked in 2009, and pirates have since then been modifying it to add current content. It's fascinating.