UNC6148 Deploys Overstep Malware on SonicWall Devices, Possibly for Ransomware Operations

In a recent report by Google's Threat Intelligence Group (GTIG), a threat actor tracked as UNC6148 has been targeting SonicWall SMA appliances with new malware dubbed Overstep. The attack vector used by UNC6148 is a backdoor and rootkit designed to potentially enable data theft, extortion, or ransomware attacks.

Active since at least October 2024, the group uses a backdoor and user-mode rootkit to selectively remove log entries, hindering forensic investigation. It is likely that the initial infection vector was through the exploitation of known vulnerabilities. According to GTIG, UNC6148 leverages credentials and one-time password (OTP) seeds stolen during previous intrusions, allowing them to regain access even after organizations have applied security updates.

Overstep: A Sophisticated Backdoor and Rootkit

OVERSTEP is a sophisticated backdoor and user-mode rootkit written in C, designed to target SonicWall SMA 100 series appliances. The malware is compiled as a 32-bit ELF shared object for Intel x86 architecture and achieves persistence by placing itself in the /etc/ld.so.preload file.

The rootkit capabilities conceal its presence by blocking access to specific files and hiding associated processes and entries, including /etc/ld.so.preload, the malicious library itself, and processes related to bash, sh, or ssh. OVERSTEP uses a hijacked write function to inspect web server log data for embedded commands.

The researchers identified two main commands: dobackshell, which launches a reverse shell to a specified IP and port, and dopasswords, which creates a tar archive of sensitive system files and stores it in a publicly accessible web directory with open permissions.

Attack Vector: Exploiting Vulnerabilities

Google's Threat Intelligence Group assesses with moderate confidence that UNC6148 used a zero-day RCE vulnerability to deploy OVERSTEP malware on SonicWall SMA appliances. A May 2025 victim appeared on the “World Leaks” data leak site in June.

The attackers compromised a SonicWall SMA 100 series appliance by establishing a VPN session using stolen admin credentials. Once they gained access to the device, they launched a reverse shell, despite this not being supported by the device's design, likely via an unknown exploit.

Detection and Removal: A Challenge

The OVERSTEP backdoor and user-mode rootkit functions make detection and removal especially difficult. The malware's persistence is reinforced by locking the /etc/ld.so.preload file with the FS_IMMUTABLE_FL flag, making it nearly impossible to modify or delete.

GTIG recommends that all organizations with SMA appliances perform analysis to determine if they have been compromised. Organizations should acquire disk images for forensic analysis to avoid interference from the rootkit anti-forensic capabilities.

Conclusion

While these activities suggest financial motives, researchers have not yet confirmed them definitively. However, it is clear that UNC6148's tactics overlap with earlier reports from Truesec and dfir.ch involving Abyss-branded ransomware, implying a possible progression toward data extortion.

Google published indicators of compromise (IoCs) and Yara rules to detect UNC6148 attacks. Follow me on Twitter: @securityaffairs and Facebook and Mastodon for the latest security news and updates.