Red Menshen APT Group Deploying BPFDoor in Linux Kernel

APTs Red Menshen expands targets to Linux and cloud servers, as seen in ransomware attacks on VMware ESXi, Mirai botnet variations, and cloud-focused stealers and crypto miners.

APT groups extend focus beyond Windows, signified by Sandworm’s attacks on Linux-based routers. Unlike cybercrime malware with broad targets, APT malware prioritizes persistent stealth and routine maintenance.

Red Menshen, an APT group active in the Middle East and Asia, continuously enhances the BPFDoor backdoor, utilizing Berkeley Packet Filter (BPF) to evade Linux and Solaris OS firewalls. 

Cybersecurity researchers at Trend Micro identify the Linux and Solaris variants as Backdoor.Linux.BPFDOOR and Backdoor.Solaris.BPFDOOR.ZAJE, respectively, with added monitoring and detection patterns.

Red Menshen advances BPF filters, increasing instructions six-fold, indicating active development and successful deployment of BPFDoor.

Workflow of BPFDoor

The intriguing technical aspect of BPFDoor lies in its kernel-level loading of packet filters, commonly known as BPF or LSF in Linux, representing the same underlying technology.

BPFDoor’s BPF filters enable backdoor activation with a single network packet, bypassing firewalls by leveraging the kernel’s BPF engine, and this rootkit-like capability sets it apart from typical backdoors.

BPFDoor variants employ classic BPF filters, with Linux samples using SO_ATTACH_FILTER and Solaris samples utilizing libpcap functions for runtime filter loading.

When a packet with the magic number arrives, BPFDoor connects back to the source IP, establishing a distinct identifier-based communication.

A privileged reverse shell is established by BPFDoor, enabling remote command execution by the attacker through a pipe connection to the infected machine’s shell.

The samples of BPFDoor across 2018-2022 feature a uniform BPF program accepting unique magic numbers for the following protocols:-

• TCP
• UDP
• ICMP

The BPF program in these samples comprises 30 instructions, which measure the filter’s complexity, reads the report shared.

On the affected systems, there are three distinct packets that trigger the activation of the backdoor, and here below, we have mentioned them:-

• UDP packet containing the magic number 0x7255 at the data field
• ICMP ECHO (ping) packet containing the same 0x7255 magic number at the data field
• TCP packet containing the magic number 0x5293 at the data field

Experts identified four telfhash-supported samples introducing a 4-byte magic number for TCP packets, resulting in a new BPF program with 39 instructions.

In 2023, three samples utilized an enhanced BPF program with 229 instructions, specifically validating ICMP packets as ICMP ECHO requests.

Targets of Red Menshen APT

Here below, we have mentioned the countries targeted using BPFDoor:-

• Turkey
• Hong Kong
• Brazil

Here below, we have mentioned the industries targeted using BPFDoor:-

• Telecommunication services
• Financial services
• Other services

Incorporating BPF bytecode into malware poses a new complicated hurdle for security experts. So, the BPFDoor’s evolving filters indicate threat actors’ efforts to enhance stealth and evade detection.

Updating rules and diving into BPF filter analysis promptly is advised for network defenders and malware analysts.