Microsoft is urging users of VMware’s ESXi hypervisor to take immediate action to ward off ongoing attacks by ransomware groups that give them full administrative control of the servers the product runs on.

The vulnerability, tracked as CVE-2024-37085, allows attackers who have already gained limited system rights on a targeted server to gain full administrative control of the ESXi hypervisor. Attackers affiliated with multiple ransomware syndicates—including Storm-0506, Storm-1175, Octo Tempest, and Manatee Tempest—have been exploiting the flaw for months in numerous post-compromise attacks, meaning after the limited access has already been gained through other means.

Admin rights assigned by default

Full administrative control of the hypervisor gives attackers various capabilities, including encrypting the file system and taking down the servers they host. The hypervisor control can also allow attackers to access hosted virtual machines to either exfiltrate data or expand their foothold inside a network. Microsoft discovered the vulnerability under exploit in the normal course of investigating the attacks and reported it to VMware. VMware parent company Broadcom patched the vulnerability on Thursday.

“Microsoft security researchers identified a new post-compromise technique utilized by ransomware operators like Storm-0506, Storm-1175, Octo Tempest, and Manatee Tempest in numerous attacks,” members of the Microsoft Threat Intelligence team wrote Monday. “In several cases, the use of this technique has led to Akira and Black Basta ransomware deployments.”

The post went on to document an astonishing discovery: Escalating hypervisor privileges on ESXi to unrestricted admin was as simple as creating a new domain group named “ESX Admins.” From then on, any user assigned to the group—including newly created ones—automatically became admin, with no authentication necessary. As the Microsoft post explained:

Further analysis of the vulnerability revealed that VMware ESXi hypervisors joined to an Active Directory domain consider any member of a domain group named “ESX Admins” to have full administrative access by default. This group is not a built-in group in Active Directory and does not exist by default. ESXi hypervisors do not validate that such a group exists when the server is joined to a domain and still treats any members of a group with this name with full administrative access, even if the group did not originally exist. Additionally, the membership in the group is determined by name and not by security identifier (SID).

Creating the new domain group can be accomplished with just two commands:

• net group “ESX Admins” /domain /add
• net group “ESX Admins” username /domain /add

They said over the past year, ransomware actors have increasingly targeted ESXi hypervisors in attacks that allow them to mass encrypt data with only a “few clicks” required. By encrypting the hypervisor file system, all virtual machines hosted on it are also encrypted. The researchers also said that many security products have limited visibility into and little protection of the ESXi hypervisor.

The ease of exploitation, coupled with the medium severity rating VMware assigned to the vulnerability, a 6.8 out of a possible 10, prompted criticism from some experienced security professionals.

ESXi is a Type 1 hypervisor, also known as a bare-metal hypervisor, meaning it’s an operating system unto itself that’s installed directly on top of a physical server. Unlike Type 2 hypervisors, Type 1 hypervisors don’t run on top of an operating system such as Windows or Linux. Guest operating systems then run on top. Taking control of the ESXi hypervisor gives attackers enormous power.

The Microsoft researchers described one attack they observed by the Storm-0506 threat group to install ransomware known as Black Basta. As intermediate steps, Storm-0506 installed malware known as Qakbot and exploited a previously fixed Windows vulnerability to facilitate the installation of two hacking tools, one known as Cobalt Strike and the other Mimikatz. The researchers wrote:

Earlier this year, an engineering firm in North America was affected by a Black Basta ransomware deployment by Storm-0506. During this attack, the threat actor used the CVE-2024-37085 vulnerability to gain elevated privileges to the ESXi hypervisors within the organization.

The threat actor gained initial access to the organization via Qakbot infection, followed by the exploitation of a Windows CLFS vulnerability (CVE-2023-28252) to elevate their privileges on affected devices. The threat actor then used Cobalt Strike and Pypykatz (a Python version of Mimikatz) to steal the credentials of two domain administrators and to move laterally to four domain controllers.

On the compromised domain controllers, the threat actor installed persistence mechanisms using custom tools and a SystemBC implant. The actor was also observed attempting to brute force Remote Desktop Protocol (RDP) connections to multiple devices as another method for lateral movement, and then again installing Cobalt Strike and SystemBC. The threat actor then tried to tamper with Microsoft Defender Antivirus using various tools to avoid detection.

Microsoft observed that the threat actor created the “ESX Admins” group in the domain and added a new user account to it, following these actions, Microsoft observed that this attack resulted in encrypting of the ESXi file system and losing functionality of the hosted virtual machines on the ESXi hypervisor.   The actor was also observed to use PsExec to encrypt devices that are not hosted on the ESXi hypervisor. Microsoft Defender Antivirus and automatic attack disruption in Microsoft Defender for Endpoint were able to stop these encryption attempts in devices that had the unified agent for Defender for Endpoint installed.

Anyone with administrative responsibility for ESXi hypervisors should prioritize investigating and patching this vulnerability. The Microsoft post provides several methods for identifying suspicious modifications to the ESX Admins group or other potential signs of this vulnerability being exploited.