The zero-day exploitation of a now-patched medium-severity security flaw in the Fortinet FortiOS operating system has been linked to a suspected Chinese hacking group.

Threat intelligence firm Mandiant, which made the attribution, said the activity cluster is part of a broader campaign designed to deploy backdoors onto Fortinet and VMware solutions and maintain persistent access to victim environments.

The Google-owned threat intelligence and incident response firm is tracking the malicious operation under its uncategorized moniker UNC3886, a China-nexus threat actor.

“UNC3886 is an advanced cyber espionage group with unique capabilities in how they operate on-network as well as the tools they utilize in their campaigns,” Mandiant researchers said in a technical analysis.

“UNC3886 has been observed targeting firewall and virtualization technologies which lack EDR support. Their ability to manipulate firewall firmware and exploit a zero-day indicates they have curated a deeper-level of understanding of such technologies.”

It’s worth noting that the adversary was previously tied to another intrusion set targeting VMware ESXi and Linux vCenter servers as part of a hyperjacking campaign designed to drop backdoors such as VIRTUALPITA and VIRTUALPIE.

The latest disclosure from Mandiant comes as Fortinet revealed that government entities and large organizations were victimized by an unidentified threat actor by leveraging a zero-day bug in Fortinet FortiOS software to result in data loss and OS and file corruption.

The vulnerability, tracked as CVE-2022-41328 (CVSS score: 6.5), concerns a path traversal bug in FortiOS that could lead to arbitrary code execution. It was patched by Fortinet on March 7, 2023.

According to Mandiant, the attacks mounted by UNC3886 targeted Fortinet’s FortiGate, FortiManager, and FortiAnalyzer appliances to deploy two different implants such as THINCRUST and CASTLETAP. This, in turn, was made possible owing to the fact that the FortiManager device was exposed to the internet.

THINCRUST is a Python backdoor capable of executing arbitrary commands as well as reading and writing from and to files on disk.

The persistence afforded by THINCRUST is subsequently leveraged to deliver FortiManager scripts that weaponize the FortiOS path traversal flaw to overwrite legitimate files and modify firmware images.

This includes a newly added payload called “/bin/fgfm” (referred to as CASTLETAP) that beacons out to an actor-controlled server so as to accept incoming instructions that allow it to run commands, fetch payloads, and exfiltrate data from the compromised host.

“Once CASTLETAP was deployed to the FortiGate firewalls, the threat actor connected to ESXi and vCenter machines,” the researchers explained. “The threat actor deployed VIRTUALPITA and VIRTUALPIE to establish persistence, allowing for continued access to the hypervisors and the guest machines.”

Alternatively, on FortiManager devices that implement internet access restrictions, the threat actor is said to have pivoted from a FortiGate firewall compromised with CASTLETAP to drop a reverse shell backdoor named REPTILE (“/bin/klogd”) on the network management system to regain access.

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