ESET researchers uncovered a family of Linux malware named Linux/Mumblehard that stayed under the radar for more than 5 years. The main purpose of Mumblehard seems to be to send spam messages by sheltering behind the reputation of the legitimate IP addresses of the infected machines. During the first week of April, more than 3,000 machines were affected. The number of infected hosts is slowly decreasing, but the infection happens at specific times and that the botnet size has doubled over a 6-month period.”]
Source: https://informationsecuritybuzz.com/articles/spam-from-your-servers/