Malware
Malware authors target bugs, or loopholes, to exploit. A common method is exploitation of a buffer overrun vulnerability, where software designed to store data in a specified region of memory does not prevent more data than the buffer can accommodate being supplied. Malware may provide data that overflows the buffer, with malicious executable code or data after the end; when this payload is accessed it does what the attacker, not the legitimate software, determines.
Fileless malware
Fileless malware is sometimes considered synonymous with "in-memory" malware as both perform their core functionalities without writing data to disk during the lifetime of their operation. This has led some commentators to claim that this variant strain is nothing new and simply a “redefinition of the well-known term, memory resident virus”, whose pedigree can be traced back to the 1980s with the birth of the Lehigh Virus that was developed by the originator of the term, Fred Cohen, and became influential with his paper on the topic.