The U.S. Federal Bureau of Investigation (FBI) disclosed today that it infiltrated the world’s second most prolific ransomware gang, a Russia-based criminal group known as ALPHV and BlackCat. The FBI said it seized the gang’s darknet website, and released a decryption tool that hundreds of victim companies can use to recover systems. Meanwhile, BlackCat responded by briefly “unseizing” its darknet site with a message promising 90 percent commissions for affiliates who continue to work with the crime group, and open season on everything from hospitals to nuclear power plants.

A slightly modified version of the FBI seizure notice on the BlackCat darknet site (Santa caps added).

Whispers of a possible law enforcement action against BlackCat came in the first week of December, after the ransomware group’s darknet site went offline and remained unavailable for roughly five days. BlackCat eventually managed to bring its site back online, blaming the outage on equipment malfunctions.

But earlier today, the BlackCat website was replaced with an FBI seizure notice, while federal prosecutors in Florida released a search warrant explaining how FBI agents were able to gain access to and disrupt the group’s operations.

A statement on the operation from the U.S. Department of Justice says the FBI developed a decryption tool that allowed agency field offices and partners globally to offer more than 500 affected victims the ability to restore their systems.

“With a decryption tool provided by the FBI to hundreds of ransomware victims worldwide, businesses and schools were able to reopen, and health care and emergency services were able to come back online,” Deputy Attorney General Lisa O. Monaco said. “We will continue to prioritize disruptions and place victims at the center of our strategy to dismantle the ecosystem fueling cybercrime.”

The DOJ reports that since BlackCat’s formation roughly 18 months ago, the crime group has targeted the computer networks of more than 1,000 victim organizations. BlackCat attacks usually involve encryption and theft of data; if victims refuse to pay a ransom, the attackers typically publish the stolen data on a BlackCat-linked…