Learn in the textbookMalware Variants and Specialized Types
DefinitionFileless malware (ファイルレスマルウェア) is malware that runs through processing in memory and tools that come with the OS, without saving its body to disk as an executable file.
ExplanationUnlike the conventional kind, which runs a file written to storage, it loads its code into memory and runs it directly. Security software that looks for suspicious files struggles to find it, and its traces tend to vanish when the power goes off, which makes later investigation hard too. That is why file scanning is combined with defenses that watch how programs behave and flag anything abnormal.
AnalogyIt is like a burglar who also uses the tools lying around the house and leaves none of their own behind. With almost nothing left in the house, there is little to go on afterward.
Memory hook"-less" means "without", as in paperless and cashless. Here it means without a file.