A suspecting China-linked hacking campaign has been observed targeting unpatched SonicWall Secure Mobile Access (SMA) 100 appliances to drop malware and establish long-term persistence.
"The malware has functionality to steal user credentials, provide shell access, and persist through firmware upgrades," cybersecurity company Mandiant said in a technical report published this week.
The Google-owned incident response and threat intelligence firm is tracking the activity under its uncategorized moniker UNC4540.
The malware – a collection of bash scripts and a single ELF binary identified as a TinyShell backdoor – is engineered to grant the attacker privileged access to SonicWall devices.
The overall objective behind the custom toolset appears to be credential theft, with the malware permitting the adversary to siphon cryptographically hashed credentials from all logged-in users. It further provides shell access to the compromised device.
Mandiant also called out the attacker's in-depth understanding of the device software as well as their ability to develop tailored malware that can achieve persistence across firmware updates and maintain a foothold on the network.
The exact initial intrusion vector used in the attack is unknown, and it's suspected that the malware was likely deployed on the devices, in some instances as early as 2021, by taking advantage of known security flaws.
Coinciding with the disclosure, SonicWall has released updates (version 10.2.1.7) that come with new security enhancements such as File Integrity Monitoring (FIM) and anomalous process identification.
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In a statement shared with The Hacker News, SonicWall told that the campaign targeted "an extremely limited number of unpatched SMA 100 series appliances from the 2021 timeframe" and that it did not exploit a "new vulnerability."
The development comes nearly two months after another China-nexus threat actor was found exploiting a now-patched vulnerability in Fortinet FortiOS SSL-VPN as a zero-day in attacks targeting a European government entity and a managed service provider (MSP) located in Africa.
"In recent years Chinese attackers have deployed multiple zero-day exploits and malware for a variety of internet facing network appliances as a route to full enterprise intrusion," Mandiant said.