Fileless infections are exactly what their namesake says: they’re infections that don’t involve malicious files being downloaded or written to the system’s disk. While fileless infections are not necessarily new or rare, it presents a serious threat to enterprises and end users given its capability to gain privileges and persist in the system of interest to an attacker—all while staying under the radar. For instance, fileless infections have been incorporated in a targeted bot delivery, leveraged to deliver ransomware, infect point-of-sale (PoS) systems, and perpetrate click fraud. The key point of the fileless infection for the attacker is to be able to evaluate each compromised system and make a decision whether the infection process should continue or vanish without a trace.

The cybercriminal group Lurk was one of the first to effectively employ fileless infection techniques in large-scale attacks—techniques that arguably became staples for other malefactors.