China-linked APT UNC3886 exploits VMware zero-day since 2021

China-linked group UNC3886 has been exploiting vCenter Server zero-day vulnerability CVE-2023-34048 since at least late 2021.

Mandiant researchers reported that China-linked APT group UNC3886 has been exploiting vCenter Server zero-day vulnerability CVE-2023-34048 since at least late 2021.

vCenter Server is a critical component in VMware virtualization and cloud computing software suite. It serves as a centralized and comprehensive management platform for VMware’s virtualized data centers.

In October, VMware addressed a critical out-of-bounds write vulnerability, tracked as CVE-2023-34048 (CVSS score 9.8), that impacts vCenter Server.

The company updated its advisory on January 18, 2023, revealing that it is aware of exploitation “in the wild.”

“As of January 18, 2024 VMware is aware of exploitation “in the wild.”” reads the advisory.

In June 2023, Mandiant researchers observed the cyberespionage group UNC3886 exploiting a VMware ESXi zero-day vulnerability tracked as CVE-2023-20867.

Researchers from Mandiant first detailed the activity of the group in September 2022 when they discovered a novel malware persistence technique within VMware ESXi Hypervisors.

The technique was used by malware authors to achieve administrative access within VMware ESXi Hypervisors and take over vCenter servers and virtual machines for Windows and Linux.

The highly targeted and evasive nature of this attack leads the experts to believe that the attack was carried out for cyberespionage purposes by a China-linked actor tracked as UNC3886.

In the attack investigated by Mandiant in September 2022, threat actors relied on malicious vSphere Installation Bundles (“VIBs”) to install two backdoors on the ESXi hypervisors, tracked as VIRTUALPITA and VIRTUALPIE. VIBs are collections of files that are designed to manage virtual systems, they can be used to create startup tasks, custom firewall rules, or deploy custom binaries upon the restart of an ESXi machine.

Further investigation conducted by Mandiant revealed additional techniques used by the group UNC3886 used to target multiple organizations avoiding EDR solutions. 

The cyberespionage group was observed harvesting credentials for service accounts from a vCenter Server for all connected ESXi hosts from the embedded vPostgreSQL server built into vCenter Server Appliance.  The threat actors are exploiting the zero-day vulnerability CVE-2023-20867 to execute privileged commands across Windows, Linux, and PhotonOS (vCenter) guest VMs without authentication of guest credentials from a compromised ESXi host and no default logging on guest VMs. 

The CVE-2023-20867 flaw is exclusively exploitable by an attacker with root access to the ESXi server.

Then the attackers deploy backdoors on ESXi hosts using an alternative socket address family, use VMCI, for lateral movement and maintain persistence.

In recent attacks, Chinese hackers were also spotted modifying and disabling logging services on compromised systems.

At the time, Mandiant had now evidence to discover how the attackers were deploying the backdoors to vCenter systems.

In late 2023, Mandiant noticed that a VMware vmdird service crashed minutes prior to the deployment of the backdoors being deployed.

“Analysis of the core dump of “vmdird” by both Mandiant and VMware Product Security showed that the process crashing is closely aligned with the exploitation of CVE-2023-34048, the out-of-bounds write vCenter vulnerability in the implementation of the DCE/RPC protocol patched in October 2023, which enables unauthenticated remote command execution on vulnerable systems.” reads the report published by Mandiant.

Mandiant observed crashes across multiple UNC3886 cases between late 2021 and early 2022.

The researchers also noticed that most environments where these crashes were observed had log entries preserved, however, the ‘vmdird’ core dumps were removed.

“VMware’s default configurations keep core dumps for an indefinite amount of time on the system, suggesting the core dumps were purposely removed by the attacker in an attempt to cover their tracks.” concludes the report. “As mentioned in the VMware advisory, this vulnerability has since been patched in vCenter 8.0U2 and Mandiant recommends VMware users updating to the latest version of vCenter to account for this vulnerability seeing exploitation in the wild.”

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Pierluigi Paganini

(SecurityAffairs – hacking, UNC3886)