I like to play active-defense. Every day, I extract attacker's IP addresses from my SSH honeypots and perform a quick Nmap scan against them. The goal is to gain more knowledge about the compromised hosts. Most of the time, hosts are located behind a residential broadband connection. But sometimes, you find more interesting stuff. When valid credentials are found, the classic scenario is the installation of a botnet client that will be controlled via IRC to launch multiple attacks or scans. Malicious binaries are pre-compiled for many architectures but, this time, I felt lucky and got access to the source code! I found a compromised host (located in the Seychelles) that was hosting pre-compiled binaries and the source code of the botnet client itself. I had a quick look of course...
Honestly, the client is not very complex and only basic features are implemented but it helps to understand how to code malicious software. First of all, only one C&C server was hardcoded in the source code (also located in the Seychelles) but the client can handle multiple servers. I presume that binaries are compiled with a new C&C every time a new campaign is started. The connection occurred on an unusual port: 9271 (the default one being 6667 - IRC).
Once started, the client forks itself, tries to connect to its C&C. If it does not work, it sleeps for five seconds and tries the next one (if configured).
This is not a very complex example but it shows how a badly protected Linux box can be infected and integrated into a botnet to generate malicious activity. The fact that the main feature is a Telnet scanner and the presence of binaries for multiple architectures tend to think for the botnet targets residential routers or small embedded Linux like storage devices. In the mean time, the server hosting the source code and binaries is offline for 24 hours. The hardcoded C&C server is still alive.