Chinese hackers used VMware ESXi zero-day to backdoor VMs

VMware patched today a VMware ESXi zero-day vulnerability exploited by a Chinese-sponsored hacking group to backdoor Windows and Linux virtual machines and steal data.

The cyber espionage group-tracked as UNC3886 by cybersecurity firm Mandiant who discovered the attacks-abused the CVE-2023-20867 VMware Tools authentication bypass flaw to deploy VirtualPita and VirtualPie backdoors on guest VMs from compromised ESXi hosts where they escalated privileges to root.

“A fully compromised ESXi host can force VMware Tools to fail to authenticate host-to-guest operations, impacting the confidentiality and integrity of the guest virtual machine,” VMware said in today’s security advisory.

The attackers installed the backdoor malware using maliciously crafted vSphere Installation Bundles, packages designed to help admins create and maintain ESXi images.

A third malware strain that Mandiant spotted during the investigation acted as a memory-only dropper that deobfuscated second-stage DLL payloads on the hijacked VMs. “This open communication channel between guest and host, where either role can act as client or server, has enabled a new means of persistence to regain access on a backdoored ESXi host as long as a backdoor is deployed and the attacker gains initial access to any guest machine,” Mandiant said.

In March, Mandiant also revealed that the Chinese UNC3886 hackers abused a zero-day vulnerability in the same mid-2022 campaign to compromise FortiGate firewall devices and deploy previously unknown Castletap and Thincrust backdoors.

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