What is Fileless Malware: Threats and Protection Strategies

Fileless malware is a type of cyberattack that runs directly in a system’s memory, without creating or storing files on disk. This makes it very difficult for traditional antivirus tools to detect.

These attacks typically begin with phishing emails and use legitimate system tools like PowerShell to execute malicious commands. Since no files are written to the hard drive, they leave little to no trace and often hide in memory or the registry, enabling long-term fileless malware persistence.

With 190,000 new malware attacks every second and 90% of them linked to phishing or social engineering, fileless malware has become one of the most dangerous and fastest-growing threats in cybersecurity.

This blog post delves into how fileless malware operates, why it poses a serious threat, the warning signs to look out for, and effective methods for fileless malware prevention.

What is Fileless Malware?

Fileless malware is a type of malicious software that doesn’t rely on files saved to a computer’s hard drive. Instead, it runs in the system’s memory (RAM), making it harder to detect and remove.

Unlike traditional malware, it doesn’t install any software. It uses legitimate system tools—such as PowerShell, WMI, or macros in Office documents—to execute malicious commands directly in memory.

Leaving no files behind, fileless malware is able to bypass most antivirus programs and security scans, allowing attackers to move undetected through systems and networks.

Why It's Harder to Detect

Fileless malware doesn’t create or store files on the system, so there’s nothing for traditional antivirus tools to scan or flag.

It runs in memory and uses trusted system processes, like PowerShell or WMI, which makes its activity look normal. As a result, it often goes unnoticed by signature-based detection tools.

To detect it, organizations need advanced security solutions that monitor system behavior, memory usage, and unusual activity in real time.

Why Attackers Use Fileless Malware

Attackers prefer fileless malware because it is fast, effective, and difficult to detect.

By using trusted system tools like PowerShell or WMI, they can carry out malicious actions without installing any files or triggering standard antivirus alerts.

This low-profile method helps them avoid detection, maintain access longer, and increase the overall success of the attack.

Why Is Fileless Malware a Growing Threat?

Fileless malware is becoming more common because it’s harder to detect and more likely to bypass traditional security tools. As organizations improve defenses against file-based threats, attackers shift to in-memory techniques that leave no files behind.

It also requires fewer resources to deploy and can spread quickly through phishing or compromised websites. Its use of legitimate system processes makes it harder to flag as suspicious.

These factors make fileless malware attacks highly effective, leading to their increased use in both targeted and widespread cyberattacks.

How Does Fileless Malware Work?

Fileless malware usually begins with a phishing email or a malicious link. When a user clicks or opens the content, a script is triggered that uses trusted system tools like PowerShell, WMI, or Office macros to execute malicious code directly in memory.

Because it doesn’t create or download files, the malware avoids detection by traditional antivirus programs that scan file systems.

Once it’s active in memory, the malware can collect sensitive data, spread to other devices on the network, and open hidden access points—often maintaining fileless malware persistence by embedding itself in system processes or the registry.

Fileless Malware Techniques

Attackers use several techniques to carry out fileless malware attacks, all designed to avoid detection and maintain access:

• PowerShell & WMI Abuse: Executes commands in memory using built-in Windows tools, avoiding the need for external files.
• Macro-Based Attacks: Embeds malicious scripts in Office documents that trigger when the file is opened.
• Registry-Based Persistence: Stores malicious code in the Windows registry to run automatically at startup.
• Living off the Land Binaries (LOLBins): Misuses legitimate system tools (like cmd.exe or mshta.exe) to execute harmful actions.
• Reflective DLL Injection: Loads malicious code directly into memory without touching the disk.

These techniques make detecting fileless malware more difficult and allow attackers to maintain long-term access through fileless malware persistence.

Signs of Fileless Malware Attacks

Fileless malware can be hard to spot, but certain system behaviors may indicate an active attack:

• Unusual PowerShell or WMI activity: Unexpected or frequent use of these tools, especially outside normal operations.
• High memory or CPU usage: Spikes in resource use without a clear cause may suggest in-memory malware.
• Unauthorized network connections: Strange outbound traffic or communication with unknown IPs.
• Modified registry entries or scheduled tasks: Changes to autostart entries can signal fileless malware persistence.
• Frequent system crashes or slowdowns: Disrupted performance can be a sign of malware running in the background.

Detecting these symptoms early is key to stopping fileless malware attacks before they cause serious damage.

Unusual System Behavior to Watch For

Fileless malware leaves no files behind, so it often reveals itself through subtle, unexpected system activity. Below are common signs that may indicate an active infection:

• PowerShell running without user action: Hidden scripts may be executing in the background.
• Unusual WMI activity: Processes running at odd times can signal malicious behavior.
• Malware persistence without installed software: The threat may reload after reboot via registry entries.
• Disabled security tools: Antivirus or alerts turning off without explanation.
• Consistent system slowdowns: Minor but ongoing performance issues may point to memory-based malware.

Spotting these behaviors early is key to identifying a fileless malware attack before it spreads.

Examples of Fileless Malware Attacks

Fileless malware operates by executing malicious code in memory, leaving no traditional file traces on disk. This hard-to-detect behavior allows it to bypass conventional antivirus solutions and exploit legitimate system tools like PowerShell. Below are several notable attacks that demonstrate these techniques:

Psloramyra Loader Delivering Quasar RAT – December 2024

Cybercriminals launched a phishing campaign using attachments that triggered PowerShell scripts to load the Quasar Remote Access Trojan (RAT) directly into memory. This fileless delivery method avoided leaving any artifacts on disk, allowing attackers to remotely control infected systems undetected. (Source)

Remcos RAT via Excel Exploit – November 2024

A phishing campaign exploited a known Microsoft Excel vulnerability (CVE-2017-0199) to run an HTML Application (HTA) file using mshta.exe. This HTA then executed obfuscated PowerShell commands in memory to install Remcos RAT, granting attackers full remote access without writing executable files. (Source)

APT37’s Shellcode Attack – Early 2025

The threat group APT37 (RoKRAT) targeted South Korean organizations using a fileless approach. The attack deployed a concealed 32-bit module via shellcode and PowerShell, which executed directly in memory using XOR-based logic. This technique left no traces on disk, complicating forensic analysis and detection. (Source)

Sectors Most Affected by Fileless Malware

Fileless malware attacks target industries with large attack surfaces, valuable data, and lower detection capabilities for memory-based threats. The sectors most affected include:

• Government and Public Sector: Often targeted due to sensitive data and outdated systems. Attackers exploit legacy infrastructure using tools like PowerShell to avoid detection.
• Healthcare: Holds extensive patient and insurance data. Fileless attacks can silently steal records or disrupt hospital systems without triggering alerts.
• Finance and Banking: High-value data and financial assets make this sector a prime target. Fileless malware helps criminals bypass defenses to access internal systems.
• Retail and Hospitality: Frequent phishing campaigns target employee devices and POS systems. Attackers use fileless techniques to steal payment data and customer information.

These industries continue to face rising risks as attackers increasingly use fileless malware to avoid detection and maximize damage.

Responding to a Fileless Malware Attack

Fileless malware requires a different response approach compared to traditional threats. Since there are no files to analyze, detection must focus on behavior and memory activity. Fast, coordinated action is critical to contain the breach and prevent lateral movement across the network.

Steps to Contain and Neutralize the Threat

Responding quickly and methodically is key to limiting damage from a fileless malware attack. Here are the essential steps:

• Isolate Affected Systems

Immediately disconnect compromised devices from the network to stop the malware from spreading.

• Analyze Memory and Running Processes

Use advanced endpoint detection tools to inspect memory, command-line activity, and suspicious processes.

• Identify the Entry Point

Trace back the attack to understand how the malware entered—often through phishing emails or exploited system tools.

• Terminate Malicious Activity

Kill any unauthorized processes or scripts running in memory and revoke associated user credentials.

• Apply Security Patches

Fix any exploited vulnerabilities and update software to close the entry point used by the attacker.

• Conduct a Forensic Investigation

Review logs and network behavior to assess the full scope of the attack and gather evidence for future prevention.

• Reinforce User Awareness

Train staff to recognize phishing and suspicious activity to reduce the risk of similar attacks in the future.

These steps help security teams contain the threat, restore system integrity, and harden defenses against future fileless attacks.

Best Practices for Fileless Malware Protection

To defend against fileless malware, organizations need to shift from traditional signature-based defenses to more dynamic, behavior-focused strategies. The following best practices can help strengthen your security posture and reduce the risk of successful attacks.

Table 1: Recommended Best Practices for Fileless Malware Protection

Keeping Software and Operating Systems Updated

Regular patching is one of the simplest yet most effective defenses against fileless malware. Attackers often exploit known vulnerabilities in outdated software to run malicious scripts in memory. By applying security updates promptly, organizations close these gaps before they can be used in an attack.

Automate patch management where possible, especially for widely-used tools like browsers, office applications, and operating systems. Also, monitor vendor advisories to stay ahead of critical vulnerabilities that are actively being exploited.

Keeping all systems current significantly reduces the attack surface and helps prevent initial compromise.

Utilizing Advanced Endpoint Protection Tools

Traditional antivirus tools often miss fileless malware because there are no files to scan. Instead, organizations should deploy advanced endpoint protection solutions like EDR (Endpoint Detection and Response) or XDR (Extended Detection and Response).

These tools monitor system behavior in real time, detect suspicious activities in memory, and provide visibility into command-line execution, PowerShell usage, and unusual network connections. They also support rapid response actions, such as isolating infected endpoints and terminating malicious processes.

Using advanced endpoint tools is essential for detecting and stopping fileless threats before they spread.

Role of Keepnet in Fileless Malware Detection

Keepnet’s Extended Human Risk Management platform defends against fileless malware by targeting the root cause—human error and social engineering. Since many fileless attacks begin with phishing emails, Keepnet offers advanced tools to identify, prevent, and respond to these threats:

• Phishing Simulator: Simulates realistic phishing attacks to test and strengthen employee awareness, reducing the risk of initial compromise.
• Security Awareness Training: Delivers AI-driven, adaptive training based on individual risk levels, helping users recognize phishing attempts and avoid risky behavior.
• Incident Responder: Automates the response to reported phishing emails, enabling swift containment and investigation of potential fileless entry points.
• Threat Intelligence: Provides up-to-date insights on active phishing campaigns and attack techniques that often lead to fileless malware infections.
• Threat Sharing: Enables collaboration with peers and industry groups to share indicators of compromise and strengthen collective defenses.

By combining proactive training, realistic simulations, and automated incident handling, Keepnet helps organizations stop fileless attacks before they start—by eliminating the human errors that open the door.