Counter-Strike: Global Offensive players will no longer be able to trade container keys between accounts because the trade was part of a massive worldwide fraud network. Players earned cases in Counter-Strike containing weapons and cosmetic upgrades, but had to purchase the keys to open the boxes. Developer Valve runs an internal marketplace on Steam where it allowed players to trade the boxes and the keys. Valve patched the game on October 28 and explained the problem in its patch notes.

“In the past, most key trades we observed were between legitimate customers,” the statement said. “However, worldwide fraud networks have recently shifted to using CS:GO keys to liquidate their gains. At this point, nearly all key purchases that end up being traded or sold on the marketplace are believed to be fraud-sourced.”

This isn’t the first time Counter-Strike’s microtransactions were at the center of fraud. In September, 2017, the Federal Trade Commission settled with two YouTubers who ran popular websites that allowed fans to gamble their Counter-Strike skins. The influencers advertised the gambling site to fans on YouTube with video titles like HOW TO WIN $13,000 IN 5 MINUTES CS GO Betting without disclosing that they owned it.

Valve did not immediately respond to request for comment.