The FBI has warned people who use Google’s Gmail about a dangerous ransomware scheme. More than 1.8 billion people could be at risk from hackers who hold your private data hostage if you make this one easy mistake.

The FBI and the U.S. Cybersecurity and Infrastructure Security Agency are urging users of popular email services like Gmail and Outlook to be on the lookout for a dangerous and potentially costly ransomware scheme. A bulletin released this week detailed a warning for the Medusa ransomware gang, a group that’s been active since 2021.

The FBI, MS-ISAC and CISA released a joint cybersecurity advisory about the Medusa ransomware variant, advising people and organizations to use multifactor authentication and other strategies to prevent attacks from the ransomware-as-a-service provider that has targeted hundreds since 2021.