Fortinet recently warned that a FortiOS zero-day vulnerability has been exploited in attacks aimed at government organizations. Google-owned cybersecurity firm Mandiant reported on Thursday that those attacks were likely conducted by a Chinese state-sponsored threat actor.

The vulnerability in question is tracked as CVE-2022-41328 and it has been described as a medium-severity path traversal issue in FortiOS that can lead to command execution. Fortinet announced patches on March 7 and made public some details about attacks exploiting the flaw two days later.

Fortinet said CVE-2022-41328 was exploited by a sophisticated threat actor in highly targeted attacks aimed at governmental or government-related entities. The company launched an investigation after a firmware integrity check failed on a device.

Fortinet published indicators of compromise (IoCs), but did not share too much information on the attacker, other than that it appeared to have advanced capabilities — based on the use of custom implants — and a deep understanding of FortiOS and the underlying hardware.

In a blog post published on Thursday, Mandiant revealed that the attack was conducted by a cyberespionage group it tracks as UNC3886. The UNC classification is used by Mandiant for uncategorized groups, but the company believes this threat actor is working in support of the Chinese government’s goals. The company is still looking for links between this and other Chinese threat actors.

Mandiant also believes UNC3886 was behind attacks observed last year that involved the installation of persistent backdoors on VMware ESXi hypervisors.

The security firm said it collaborated with Fortinet last year to investigate the deployment of malware on various Fortinet products, including the FortiGate firewall, the FortiAnalyzer log management and analytics platform, and the FortiManager management solution. 

In the case of attacks involving CVE-2022-41328, Mandiant explained, the attackers exploited the flaw to write files to FortiGate firewalls. They used super administrator privileges to maintain persistent access, and bypassed firewall rules using a passive traffic redirection utility. 

The attackers also established persistence on FortiAnalyzer and FortiManager devices using a custom API endpoint, and disabled OpenSSL digital signature verification of system files. 

“We believe the targeting of these devices will continue to be the go to technique for espionage groups attempting to access hard targets,” said ​​Ben Read, head of Mandiant Cyber Espionage Analysis at Google Cloud. 

“This is due to their being accessible from the internet allowing actors to control the timing of the intrusion, and in the case of VPN devices and routers – the large amount of regular inbound connections makes blending in easier. Organizations, especially those in industries historically targeted by Chinese espionage, should take steps to both harden these devices and monitor them for suspicious activity,” Read added.

Mandiant recently also linked the exploitation of a FortiOS SSL-VPN vulnerability tracked as  CVE-2022-42475 to Chinese threat actors. 

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