Fileless malware is malicious behavior that runs mainly in memory or uses trusted tools already present on the system, instead of dropping an obvious executable file on disk. Attackers may launch code through scripts, PowerShell, WMI, macro chains, or injected processes, making the activity harder to spot with traditional file-based antivirus scans.
This matters because defenders often look for suspicious files, but fileless techniques can leave fewer artifacts while still creating remote access, credential theft, or lateral movement. In real attacks, a fileless RAT may start after a user opens a lure or runs a document, then keep its payload in memory and communicate over the network. Defenses focus on behavior: process trees, script logging, memory analysis, application control, and alerts for unusual living-off-the-land activity.