threatpost[.]com/hard-target-fileless-malware/125054/
"Hard Target: Fileless Malware"
“There has been an unequivocal uptick in the use of fileless malware as a threat vector,” said Kevin Epstein, vice president of threat operations at Proofpoint. “We have seen more fileless malware since the beginning of 2017 than we saw in all of 2016 and 2015 combined.”
Basically by breaking the malware code into separate pieces, it's easier to get those pieces into a victim's copy of Windows -- there they reassemble those pieces, but only in memory, & then run the malware. Since the assembled malware is never written to a file, there's nothing file-based for security software to match against the known malware samples in its database.
Mitigation against these threats will take new tools and a shift in end-user awareness, Brumaghin said. For starters, security experts say disabling the use of PowerShell on networks is a good start. They also recommend monitoring more closely outbound traffic and tracing it back to applications making those requests. If Windows Notepad or Calculator are making network connections, you might have a problem, experts say.