A steady increase in advanced persistent threat (APT) groups targeting Linux resources has prompted researchers to share the details of these attacks and explain the misconceptions around Linux security, as well as how organizations can better protect their Linux machines.

Kaspersky's Global Research & Analysis Team (GReAT) has observed this steady trend over the last eight years, says researcher Yury Namestnikov, More and more advanced groups are creating tools, and even platforms, to target devices running Linux software, he says.

There is a broadly held opinion that the Linux operating system is secure by default and not susceptible to malicious code. The misconception is rooted in the idea that cybercriminals have less malware for, and interest in, Linux desktops and servers. Much has been written about targeted attacks on Windows, the platform where Kaspersky finds the most APT attack tools.

While Linux hasn't faced the flood of viruses, worms, and Trojans that Windows systems have seen, researchers emphasize it's still an attractive target. Cyberattackers have PHP backdoors, rootkits, and exploit code written for Linux, but many businesses aren't very worried about it.

The implications are dangerous. As a result, information security departments and the security teams at software vendors are less focused on mitigations for this problem, creating a situation in which organizations have less visibility and tools to protect Linux desktops, servers, and IoT.

"It's not true for all organizations, but unfortunately, it's a common situation in many cases," Namestnikov explains. "And when we start to talk about targeted attacks we can easily see, almost every serious threat actor has some surprise tools for hacking in and getting control over Linux-running machines.

There are many reasons why APT groups target Linux over Windows, he says. A key factor is the trend toward containerization, which has driven Linux adoption. The machines can usually be accessed from the Internet and may serve as an initial entry point for attackers. A shift to virtualization and containerization means nearly all businesses use Linux in some daily tasks. 

Many organizations use more Linux and macOS devices than Windows systems, Namestnikov adds, giving attackers no other option. IT companies, telcos, and governments are among them. In some regions there is a move to adopt more Linux in desktop environments, especially in government and defense spheres such as Turkey and China, he notes.

Kaspersky's telemetry indicates servers are the most common target of these attacks, followed by corporate IT and network devices, then workstations. There have been in cases in which the attackers used compromised routers, running Linux, as a command-and-control for Windows implants in the same network, Namestnikov points out.

Servers should be a primary concern, researchers write in their full analysis. The strategic importance of Linux-based servers makes them a hot target. If an attacker can compromise a Linux server, they could both gain access to data on that server and target endpoints running Windows or macOS that may be connected. 

An Evolving Threat
Attackers have made changes to Linux malware and attacks targeting Linux devices. When they first began to write malware, their goal was to manipulate network traffic. This was clear in the case of Cloud Snooper, a threat actor that used a server-oriented Linux kernel rootkit designed to hook Netfilter traffic control functions and command-and-control communication that crossed the target's firewall.

Researchers note the same goal was evident for Barium/APT41. This group started out in 2013 targeting gaming companies for financial gain; over time, it developed new tools and went after more complicated targets. It employed Linux malware dubbed MessageTap, which attackers used to intercept SMS messages from the infrastructure of telecom providers.

APT attackers targeting Linux often use legitimate tools that are available on Linux-based servers and desktops — for example, the ability to compile code or run Python scripts — which ultimately leaves fewer traces in the logs, Namestnikov says. The most concerning things, he says, is their ability to hide on Linux devices, and leave then return as they please.

To do this, he explains, they can either infect IoT or network boxes, or replace legitimate files on compromised servers. Because these servers are not often updated, and in many cases don't have antivirus installed, these replacements are often seen too late — if they're seen at all. 

While Linux remains less frequently targeted than Windows, researchers advise businesses to take steps to protect their environments from these kinds of attacks. Their first tip is to keep a list of trusted sources for software and only install applications from official stores. Linux may provide more freedom, but it puts pressure on organizations to download software wisely.

Beyond this, they recommend checking network settings and avoiding unnecessary network applications. Organizations are also advised to properly configure their firewall from the Linux distributive to filter traffic and store the host's network activity. Other suggestions include protecting locally stored SSH keys used for network services, and setting up multifactor authentication for SSH sessions.