Researchers at Sophos Group plc have released details of new ransomware written in Python that attackers used to compromise and encrypt virtual machines hosted on an ESXi hypervisor.

The ransomware attack was first discovered at 12:30 a.m. on Sunday when those behind the attack broke into a TeamViewer account running on a computer that belonged to a user who also had domain administrator access credentials. Ten minutes later, the attackers used the Advanced IP Scanner tool to look for targets on the network.

The researchers believe the ESXi Server on the network was vulnerable because it had an active Shell programming interface used for commands and updates. That allowed the attackers to install a secure network communications tool called Bitvise on the machine belonging to the domain administrator, which gave them remote access to the ESXi system, including the virtual disk files used by the virtual machines.

By 3:40 a.m., the attackers deployed the ransomware and encrypted the virtual hard drives hosted on the ESXi server.

“This is one of the fastest ransomware attacks Sophos has ever investigated and it appeared to precision-target the ESXi platform,” explained Andrew Brandt, a principal researcher at Sophos. “Python is a coding language not commonly used for ransomware. However, Python is pre-installed on Linux-based systems such as ESXi, and this makes Python-based attacks possible on such systems.”

Brandt further noted that ESXi servers represent an attractive target for ransomware threat actors because they can attack multiple virtual machines at once, and each of the virtual machines could be running business-critical applications or services.

“Attacks on hypervisors can be both fast and highly disruptive,” Brandt added. “Ransomware operators including DarkSide and REvil have targeted ESXi servers in attacks.”

The researchers noted that administrators who operate ESXi or other hypervisors on their networks should follow security best practices. Recommended practices include using unique, difficult-to-brute-force passwords and enforcing the use of multifactor authentication wherever possible. In addition, the ESXi Shell can and should be disabled whenever it’s not being used by staff for routine maintenance.

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