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Fortinet Zero-Day and Custom Malware Used by Suspected Chinese Actor in Espionage Operation


Cyber espionage threat actors continue to target technologies that do not support endpoint detection and response (EDR) solutions such as firewalls, IoT deviceshypervisors and VPN technologies (e.g. FortinetSonicWallPulse Secure, and others). Mandiant has investigated dozens of intrusions at defense industrial base (DIB), government, technology, and telecommunications organizations over the years where suspected China-nexus groups have exploited zero-day vulnerabilities and deployed custom malware to steal user credentials and maintain long-term access to the victim environments.

We often observe cyber espionage operators exploiting zero-day vulnerabilities and deploying custom malware to Internet-exposed systems as an initial attack vector. In this blog post, we describe scenarios where a suspected China-nexus threat actor likely already had access to victim environments, and then deployed backdoors onto Fortinet and VMware solutions as a means of maintaining persistent access to the environments. This involved the use of a local zero-day vulnerability in FortiOS (CVE-2022-41328) and deployment of multiple custom malware families on Fortinet and VMware systems. Mandiant published details of the VMware malware ecosystem in September 2022.

In mid-2022, Mandiant, in collaboration with Fortinet, investigated the exploitation and deployment of malware across multiple Fortinet solutions including FortiGate (firewall), FortiManager (centralized management solution), and FortiAnalyzer (log management, analytics, and reporting platform). The following steps generally describe the actions the threat actor took:

  1. Utilized a local directory traversal zero-day (CVE-2022-41328) exploit to write files to FortiGate firewall disks outside of the normal bounds allowed with shell access.
  2. Maintained persistent access with Super Administrator privileges within FortiGate Firewalls through ICMP port knocking
  3. Circumvented firewall rules active on FortiManager devices with a passive traffic redirection utility, enabling continued connections to persistent backdoors with Super Administrator privileges
  4. Established persistence on FortiManager and FortiAnalyzer devices through a custom API endpoint created within the device
  5. Disabled OpenSSL 1.1.0 digital signature verification of system files through targeted corruption of boot files

Mandiant attributes this activity to UNC3886, a group we suspect has a China-nexus and is associated with the novel VMware ESXi hypervisor malware framework disclosed in September 2022. At the time of the ESXi hypervisor compromises, Mandiant observed UNC3886 directly connect from FortiGate and FortiManager devices to VIRTUALPITA backdoors on multiple occasions.

Mandiant suspected the FortiGate and FortiManager devices were compromised due to the connections to VIRTUALPITA from the Fortinet management IP addresses. Additionally, the FortiGate devices with Federal Information Processing Standards (FIPS) compliance mode enabled failed to boot after it was later rebooted. When FIPS mode is enabled, a checksum of the operating system is compared with the checksum of a clean image. Since the operating system was tampered by the threat actor, the checksum comparison failed, and the FortiGate Firewalls protectively failed to startup. With assistance from Fortinet, Mandiant acquired a forensic image of these failing devices, prompting the discovery of the ICMP port knocking backdoor CASTLETAP.

Charles Carmakal, CTO, Mandiant Consulting says: “Chinese espionage operators’ recent victims include DIB, government, telecoms, and technology. Given how incredibly difficult they are to find, most organizations cannot identify them on their own. It’s not uncommon for Chinese campaigns to end up as multi-year intrusions. We hope this information and the accompanying hardening steps help more organizations to uncover these long standing breaches sooner.” 

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