Threat actors are exploiting an old Solaris vulnerability to install BPFDoor malware.

The malicious software was discovered by researchers at PricewaterhouseCoopers (PwC) and linked to a China-based threat actor tracked as Red Menshen.

BPFDoor is a custom backdoor that has been in use for the last 5 years. It cannot be stopped by firewalls, it can work without opening any ports and does not require a command and control server as it can receive commands from any IP address on the web.

According to CrowdStrike, attackers are targeting Linux and Solaris systems that use the custom-built BPFDoor implant on telecommunications providers to steal personal user data.

The researchers pointed out that detecting BPFDoor/JustForFun implants on a Linux system can be very difficult, as the threat actors modifies existing SysVinit scripts on the host to achieve persistence. Therefore, reviewing the lines of code in SysVinit scripts is unlikely to reveal the reference to the implant.