In biology, a symbiote is an organism that lives in symbiosis with another organism. The symbiosis can be mutually beneficial to both organisms, but sometimes it can be parasitic when one benefits and the other is harmed. A few months back, we discovered a new, undetected malware that acts in this parasitic nature affecting Linux® operating systems. We have aptly named this malware Symbiote.

What makes Symbiote different from other Linux malware that we usually come across, is that it needs to infect other running processes to inflict damage on infected machines. Instead of being a standalone executable file that is run to infect a machine, it is a shared object (SO) library that is loaded into all running processes using LD_PRELOAD (T1574.006), and parasitically infects the machine. Once it has infected all the running processes, it provides the threat actor with rootkit functionality, the ability to harvest credentials, and remote access capability.

One interesting technical aspect of Symbiote is its Berkeley Packet Filter (BPF) hooking functionality. Symbiote is not the first Linux malware to use BPF. For example, an advanced backdoor attributed to the Equation Group has been using BPF for covert communication. However, Symbiote utilizes BPF to hide malicious network traffic on an infected machine.

When an administrator starts any packet capture tool on the infected machine, BPF bytecode is injected into the kernel that defines which packets should be captured. In this process, Symbiote adds its bytecode first so it can filter out network traffic that it doesn’t want the packet-capturing software to see.