Fileless malware is a type of malware that is designed to run in memory or within legitimate system processes, rather than as separate executable files, making it difficult to detect and remove.

What is fileless malware?

Fileless malware is a type of malicious code that operates directly in system memory (RAM) without writing files to disk. Instead of using traditional executable files, it leverages legitimate tools already built into the system—like PowerShell, WMI, or PsExec—to execute commands and spread across the network. This stealthy behavior makes it difficult to detect using conventional antivirus software, as there are no files to scan. Its goal is often to gain unauthorized access, exfiltrate data, or establish persistence.

How fileless malware appears in practice

Examples of real-world scenarios:

• An attacker sends a phishing email with a link that launches a malicious PowerShell script directly in memory.
• Malware abuses a legitimate tool like PsExec to execute code on remote machines without installing anything.
• A user opens a Word document with a macro that triggers a fileless payload to create a backdoor.
• WMI (Windows Management Instrumentation) is exploited to gather data from the corporate network silently.
• Temporary registry keys and volatile memory are used to execute malicious code without leaving a trace.

Because fileless malware often mimics typical system or administrative tasks, it’s especially dangerous in environments that rely solely on traditional signature-based defenses.

Fileless malware vs. related terms

• Fileless malware – Executes in memory, no files written to disk.
• Traditional Malware – Installed as files on disk (e.g., Trojans, viruses).
• Living-off-the-Land (LotL) Attacks – Use built-in system tools for malicious purposes (often part of fileless strategies).

The key difference lies in execution and detectability. Fileless attacks exploit trusted tools and don’t generate traditional malware artifacts, making them harder to find and stop.

How to detect or prevent fileless malwaru

Recommended actions:

1. Restrict PowerShell, WMI, and similar tools for non-admin users.
2. Monitor system behavior, not just files—focus on scripts, memory usage, and system tool activity.
3. Deploy advanced detection tools, such as EDR (Endpoint Detection & Response) or XDR (Extended Detection & Response).
4. Train employees to recognize phishing and suspicious attachments.
5. Regularly review unusual memory activity and transient processes.

Fileless malware won’t show up in standard virus scans. Without advanced monitoring and behavioral analysis, your organization may never notice the intrusion. Many attackers now favor this stealthy method—proactive defense is essential.