Based on a study of material being downloaded by botnets to victim machines during the 12 months cited, Kaspersky Lab researcher Alexander Ermin said that backdoors made up the bulk of downloads, showing that the botnet operators wanted to gain the best possible control over infected devices.
The study did not count update files that were downloaded by bots and also excluded configuration files. Only files that were judged to be unique by their MD5 hash files were counted and the study covered more than 60,000 different command and control servers associated with 150 bot families and their variants.
Ermin also found that the number of download droppers — programs that secretly install malicious programs, built into their code, on a computer — was also increasing, an indication that multi-stage and more complex attacks were on the rise.
Another trend noticed was that botnets appeared to be increasingly leased by people depending on their needs. In many cases, the actual specialist task that the botnet was built for could not be identified.
The total unique malicious files downloaded was 16,213 in the second half of 2017 and 13,858 for the first half of 2018. A chart of the top 10 malware for each period shows that a backdoor known as njRAT made up 5.2% of downloads in the first half of 2018. njRAT also had the most command and control servers, with a presence in 99 countries.
For the second half of 2017, a botnet known as Lethic was the top download, with 17%.
Graphic: courtesy Kaspersky Lab