The zero-day exploitation of a now-patched medium-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.

Also employed by UNC3886 at this stage is a utility dubbed TABLEFLIP, a network traffic redirection software to connect directly to the FortiManager device regardless of the access-control list (ACL) rules put in place.

This is far from the first time Chinese adversarial collectives have targeted networking equipment to distribute bespoke malware, with recent attacks taking advantage of other vulnerabilities in Fortinet and SonicWall devices.

The revelation also comes as threat actors are developing and deploying exploits faster than ever before, with as many as 28 vulnerabilities exploited within seven days of public disclosure — a 12% rise over 2021 and an 87% rise over 2020, according to Rapid7.

This is also significant, not least because China-aligned hacking crews have become "particularly proficient" at exploiting zero-day vulnerabilities and deploying custom malware to steal user credentials and maintain long-term access to target networks.

"The activity [...] is further evidence that advanced cyber espionage threat actors are taking advantage of any technology available to persist and traverse a target environment, especially those technologies that do not support EDR solutions," Mandiant said.