A recently identified stealthy Linux implant has allowed Chinese cyberespionage group Winnti to conduct targeted attacks under the radar, French cybersecurity firm ExaTrack warns.
Dubbed ‘Melofee’ and targeting Linux servers, the malware is accompanied by a kernel mode rootkit and is installed using shell commands, a behavior like that of other Winnti Linux rootkits.
The identified Melofee samples are likely dated April/May 2022 and share a common code base, but show small changes in communication protocol, encryption, and functionality. The main change between observed samples is the inclusion of a kernel mode rootkit in the newest version.
The rootkit is a modified version of an open-source project called Reptile and has limited functionality, mainly installing a hook to hide itself and another to ensure communication with the userland component.
The infection chain involves the use of shell commands to fetch an installer and a custom binary from an attacker-controlled server. Written in C++, the installer deploys both the rootkit and the server implant and ensures that both are executed at boot time.
The implant supports commands to kill its process and remove persistence, update itself and relaunch, create a new socket for interaction, harvest and exfiltrate system information, read/write files, launch a shell, and list/create/delete directories.
The malware supports communication over TCP, using a custom packet format, can use a TLS encrypted channel to exchange data with the command-and-control (C&C) server, and can send data using the KCP protocol.
Analysis of the Melofee infrastructure revealed connections with C&C servers used by ShadowPad, Winnti, and HelloBot, and with domains that PlugX, Spark, Cobalt Strike, StowAway, and the toDesk remote control tool have used as C&C servers.
ExaTrack also discovered a Linux implant called ‘AlienReverse’, which showed code similarities with Melofee, but which they consider a different malware family.
“The capabilities offered by Melofee are relatively simple, but may enable adversaries to conduct their attacks under the radar. These implants were not widely seen, showing that the attackers are likely limiting its usage to high value targets,” ExaTrack concludes.
Also known as APT41, Barium, Blackfly, Bronze Atlas, Double Dragon, Wicked Spider, and Wicked Panda, and believed to be sponsored by the Chinese government, Winnti has been actively launching cyberespionage and financially motivated attacks since at least 2007.
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