There is increased discussion around threats that adopt so-called “living off the land” tactics. Attackers are increasingly making use of tools already installed on targeted computers or are running simple scripts and shellcode directly in memory. Creating fewer new files on the hard disk, or being completely fileless, means less chance of being detected by traditional security tools and therefore minimizes the risk of an attack being blocked. Using simple and clean dual-use tools allows the attacker to hide in plain sight among legitimate system administration work.
Attackers are increasingly living off the land (https://www.symantec.com/connect/blogs/attackers-are-increasingly-living-land)
Living off the land and fileless attack techniques (https://www.symantec.com/content/dam/symantec/docs/security-center/white-papers/istr-living-off-the-land-and-fileless-attack-techniques-en.pdf)