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.

The FBI has alerted the over 1.8 billion Gmail users about a serious ransomware threat that could compromise their private data. Investigations have revealed that the Medusa ransomware group has targeted more than 300 victims using phishing tactics to exploit vulnerabilities in users’ software.