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Symbiote Malware Poses Stealthy, Linux-Based Threat to Financial Industry

A Linux-based banking Trojan is a master at staying under the radar.

A stealthy Linux threat called Symbiote is targeting financial institutions in Latin America, with all file, processes, and network artifacts hidden by the malware, making it virtually invisible to detection by live forensics.

The malware was first uncovered in November, according to a blog post by BlackBerry Research. What sets Symbiote apart from other Linux malware is its approach to infecting running processes, rather than using a stand-alone executable file to inflict damage.

It then harvests credentials to provide remote access for the threat actor, exfiltrating credentials as well as storing them locally.

"It operates as a rootkit and hides its presence on the machine. Once it has infected the machine fully, it allows you to see only what it wants you to see," Joakim Kennedy, security researcher at Intezer and author of the BlackBerry blog post, explains. "Essentially, you can't trust what the machine is telling you."

However, it can be detected externally, he says, since it exfiltrates stolen credentials via the DNS requests.

Kennedy says the domain names the malware uses impersonate big banks in Brazil, which also helps it stay under the radar.

"While we couldn't tell based on only what we found, attackers targeting financial institutions are often motivated by potential monetary gain," he says.

Shared Object Library

Nicole Hoffman, senior cyber threat intelligence analyst at Digital Shadows, points out that unlike most malware variants, the Symbiote malware is a shared object library, instead of an executable file.

Symbiote uses the LD_PRELOAD variable that allows it to be pre-loaded by applications before other shared object libraries.

"This is a sophisticated and evasive technique that can help the malware blend in with legitimate running processes and applications, which is one of the reasons Symbiote is difficult to detect," she says.

The malware also has Berkeley Packet Filter (BPF) hooking functionality. Packet capture tools intercept, or capture, network traffic typically for the purposes of an investigation.

BPF is a tool embedded within several Linux operating systems that allows users to filter out certain packets depending on the type of investigation they are performing, which can reduce the overall results, making analysis easier.

"The Symbiote malware is designed to essentially filter its traffic out of the packet capture results," Hoffman explains. "This is just another layer of stealth used by the attackers to cover their tracks and fly under the radar."

Kennedy adds that this is the first time the BPF hooking functionality has been observed operating in this way, and points out that other malware variants have typically used BPF to receive commands from their command-and-control server.

"This malware instead uses this method to hide network activity," he says. "It's an active measure used by the malware to prevent being detected if someone investigates the infected machine — like covering up its footsteps so it's harder to track down.”

Easier to Attack?

Mike Parkin, senior technical engineer at Vulcan Cyber, says there may be a perception on the attacker's part that the targets in Latin America have a less mature security infrastructure and would thus be easier to attack.

He explains that the attackers went out of their way to hide their malware from anything that's running on the infected system, leveraging BPF to hide their communications traffic.

"While this will work on the local host, other network-monitoring tools will be able to identify the hostile traffic and the infected source," he says.

He explains that there are several endpoint tools available that should identify changes on a victim system.

"There are also forensic techniques that can use the malware's own behavior against it to reveal its presence," he notes. "The authors who created Symbiote went to great lengths hide their malware. They leveraged a combination of techniques, though in so doing delivered some indicators of compromise that defenders could use to identify an infection in-situ."

Kennedy says that the most important action is to focus on the techniques used by this malware to ensure that you can detect and/or protect against those, whether you're protecting against Symbiote or another attack that uses the same technique.

"I would say Symbiote, and other recently discovered undetected Linux malware, shows that operating systems other than Windows are not immune to highly evasive malware," he says. "Since it doesn’t get as much attention as Windows malware, we don't know what else is out there that hasn’t been discovered yet."