At the beginning of this year, at Black Hat Federal Conference, I proposed a simple taxonomy that could be used to classify stealth malware according to how it interacts with the operating system. Since that time I have often referred to this classification as I think it is very useful in designing system integrity verification tools and to talk about malware in general. Now I decided to explain this classification a bit more as well as extend it of a new type of malware - the type III malware. The article is available as a PDF document here.