Stealthy, tricky to remove rootkit targets Linux systems on ARM and x86

The Umbreon rootkit runs from user mode but hijacks libc system calls

Security researchers have identified a new family of Linux rootkits that, despite running from user mode, can be hard to detect and remove.

Called Umbreon, after a Pokémon character that hides in the darkness, the rootkit has been in development since early 2015 and is now being sold on the underground markets. It targets Linux-based systems on the x86, x86-64 and ARM architectures, including many embedded devices such as routers.

According to malware researchers from antivirus firm Trend Micro, Umbreon is a so-called ring 3 rootkit, meaning that it runs from user mode and doesn't need kernel privileges. Despite this apparent limitation, it is quite capable of hiding itself and persisting on the system.

The rootkit uses a trick to hijack the standard C library (libc) functions without actually installing any kernel objects. Libc provides system call functions that other Linux programs can use for important operations like reading and writing files, spawning processes or sending network packets.

Umbreon hijacks these functions and forces other Linux executables to use its own libc-like library. This puts the rootkit in a man-in-the-middle position, capable of modifying system calls made by other programs and altering their output.

The rootkit also creates a hidden Linux account that can be accessed via any authentication method supported by Linux, including SSH (Secure Shell). This account does not appear in files like /etc/passwd because the rootkit can modify the output of such files when read, the Trend Micro researchers said in a blog post.

Umbreon also has a backdoor component called Espeon, named after another Pokémon character, that can establish a reverse shell to an attacker's machine when a TCP packet with special field values are received on the monitored Ethernet interface of an affected device. This means that attackers can open remote shells by simply sending a specially crafted packet to the infected device over the Internet.

It's hard to detect Umbreon using standard Linux tools, because most of them are written in C and rely on libc, whose output the rootkit hijacks, the Trend Micro researchers said. "One way is to develop a small tool to list the contents of the default Umbreon rootkit folder using Linux kernel syscalls directly."

Removing the rootkit from an infected system can also be tricky, especially for inexperienced users and attempts to do so could render the system unusable, the researchers said.

Trend Micro provided indicators of compromise in the form of file names and hashes, manual removal instructions and YARA detection rules for the new rootkit.

It seems that the rootkit was designed for manual installation, which means that attackers install it on systems manually after compromising them through other vulnerabilities.

While many desktop Linux systems receive automatic patches and are generally kept up to date by users, embedded devices like consumer routers and IP-based cameras are rarely updated.

As a result, there are hundreds of thousands of embedded devices out there that are vulnerable to known exploits and are routinely infected with malware. Just last week, Web security firm Sucuri blocked a massive DDoS attack that originated from two botnets, one made up of infected CCTV cameras and one made up of hijacked home routers.

Join the newsletter!

Error: Please check your email address.
Rocket to Success - Your 10 Tips for Smarter ERP System Selection
Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

Lucian Constantin

IDG News Service
Show Comments

Most Popular Reviews

Latest Articles

Resources

PCW Evaluation Team

Ben Ramsden

Sharp PN-40TC1 Huddle Board

Brainstorming, innovation, problem solving, and negotiation have all become much more productive and valuable if people can easily collaborate in real time with minimal friction.

Sarah Ieroianni

Brother QL-820NWB Professional Label Printer

The print quality also does not disappoint, it’s clear, bold, doesn’t smudge and the text is perfectly sized.

Ratchada Dunn

Sharp PN-40TC1 Huddle Board

The Huddle Board’s built in program; Sharp Touch Viewing software allows us to easily manipulate and edit our documents (jpegs and PDFs) all at the same time on the dashboard.

George Khoury

Sharp PN-40TC1 Huddle Board

The biggest perks for me would be that it comes with easy to use and comprehensive programs that make the collaboration process a whole lot more intuitive and organic

David Coyle

Brother PocketJet PJ-773 A4 Portable Thermal Printer

I rate the printer as a 5 out of 5 stars as it has been able to fit seamlessly into my busy and mobile lifestyle.

Kurt Hegetschweiler

Brother PocketJet PJ-773 A4 Portable Thermal Printer

It’s perfect for mobile workers. Just take it out — it’s small enough to sit anywhere — turn it on, load a sheet of paper, and start printing.

Featured Content

Product Launch Showcase

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?