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.
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