What is Fileless Malware? Definition, Examples & Prevention

Fileless malware has emerged as one of the most serious challenges security teams face today. Such attacks move beyond dropping files to disk or installing traditional payloads. Instead, they exploit trusted tools already in the system memory, abuse legitimate features like Windows Management Instrumentation or scripting interfaces, execute code in memory, and avoid leaving evident traces. As a result, many conventional security tools fail to detect them until damage is done.

In the first quarter of 2025, the global cybersecurity landscape has shifted in favour of stealthy threats. A recent report from WatchGuard observed a 171 percent increase in unique malware detections compared to the previous quarter and a substantial rise in zero-day threats and living off the land techniques.  

Fileless attacks represent more than an academic danger for companies protecting data and digital privacy. These threats have been used in Asian espionage campaigns and military targets in other regions. Recently, a new fileless malware framework called EggStreme was deployed against the Philippine military. It operates entirely in memory with multiple stages and six distinct components. 

This guide will explain fileless malware, how it works, what standard techniques are used, and how you can recognize its presence. You will also learn how to remove fileless malware and defend your systems proactively so these invisible threats do not compromise your privacy or business integrity.

What is Fileless Malware?

Fileless malware is a cyber threat that operates in memory rather than relying on traditional files stored on a system’s hard drive. Unlike conventional malware, which installs malicious executables or programs that can be scanned and removed, fileless malware leverages trusted, built-in system tools such as PowerShell, Windows Management Instrumentation (WMI), or macros within documents. Exploiting these legitimate processes blends into regular system activity and avoids detection by many signature-based antivirus solutions.

Think of it as an intruder who slips into your home without breaking a lock or leaving fingerprints. Instead of leaving behind a clear trail, fileless malware moves through your environment quietly, running commands directly in memory, which makes it extremely difficult to identify. This stealthy approach is sometimes called “living off the land,” as it abuses tools already part of the operating system.

The danger of fileless malware lies in its persistence and adaptability. Since it does not rely on traditional files, security tools that scan for suspicious downloads or stored executables often fail to detect it. Attackers frequently use it to establish backdoors, exfiltrate sensitive data, or deploy additional payloads while keeping their presence hidden for extended periods.

How does fileless malware work?

Fileless malware operates by turning the host system against itself. Instead of dropping a malicious file to disk, an attacker uses legitimate tools and trusted processes to run code directly in memory. 

The overall goal is the same as traditional malware: gain access, maintain a foothold, move laterally, and exfiltrate data, but the method is stealthier and harder to detect. Below is a practical, step-by-step explanation of how a typical fileless attack unfolds and what each stage looks like from a defender’s perspective.

1. Initial access

Attackers gain entry through common vectors that seem ordinary at first glance. Common entry points include a phishing email with a malicious document, a compromised website that delivers a weaponized script, or stolen credentials to authenticate into remote services. 

The payload at this stage often relies on scripts or macros rather than executable files. Because these mechanisms use built-in capabilities like Office macros or browser script engines, they bypass file-based detection.

2. Execution using trusted tools

Once the attacker has a way to run commands, they invoke legitimate system tools to fetch or execute code in memory. Typical tools include PowerShell, Windows Management Instrumentation, or built-in Windows utilities such as rundll32 and certutil. 

These tools can download encoded payloads or execute base64-encoded commands that expand directly into memory. Because the tools are legitimate and commonly used by administrators, their activity can easily blend with normal operations unless telemetry is monitored closely.

3. In-memory payload and process injection

The core payload often never touches disk. Instead, the attacker loads code into the memory space of a trusted process. Process injection techniques allow malicious code to run under the context of a legitimate process. This provides two advantages for the attacker. One it uses the privileges and context of the host process, which can simplify later actions. Two it hides the malicious code from scanners that look for suspicious files on disk. Standard techniques include reflective DLL loading, direct code injection, and run-time assembly loading in managed runtimes.

4. Establishing persistence without files

Even though the payload runs in memory, it is common for attackers to implement persistence so they can survive reboots or reestablish access. Persistence techniques do not need files. Examples include scheduled tasks that run benign-appearing commands, registry entries that execute code via built-in tools, WMI event subscriptions, and abusing remote management or scripting features. Attackers sometimes store encrypted or encoded fragments in registry keys or in less monitored configuration stores and retrieve them on demand.

5. Credential access and lateral movement

After executing code in memory, attackers typically attempt to harvest credentials or tokens to expand their access. They may dump credentials from memory, use built-in credential APIs or simply forward session tokens. With credentials, the attacker moves laterally across the network using legitimate remote management protocols. Because the movement leverages standard administrative tools, the activity can appear legitimate in ordinary logs.

6. Command and control and data exfiltration

The in-memory agent will contact a command and control server for further instructions. Communication can be disguised using legitimate protocols such as HTTPS, DNS tunnelling, or services generally allowed by firewalls. The agent may stage stolen data in memory before exfiltration or relay it through living off-the-land tools. Since the data is not necessarily written to disk, typical data loss prevention tools can miss these actions.

7. Evasion and cleanup

Attackers use several evasion techniques to minimize traces. They may clear logs, disable forensic artifacts, or execute only short-lived tasks to minimize traces. Some fileless campaigns are multistaged with a small memory-resident loader that retrieves further components only when needed. 

In other cases, the entire operation is memory-resident for the session’s life. If defenders only rely on periodic file scans or endpoint signature checks, they are likely to miss these behaviors.

How to Detect Fileless Malware

Detecting fileless malware requires a shift in mindset from traditional file-based security to behavior-based observation and memory analysis. Because these attacks execute in memory and leverage trusted system tools, signature-based antivirus software often cannot identify them. 

Detection relies on monitoring anomalies, unusual tool usage, and unexpected system behaviors that indicate malicious activity.

Behavioral Monitoring

Fileless malware often manipulates legitimate system tools such as PowerShell, Windows Management Instrumentation, or Office macros. Monitoring how these tools are used is essential. For example, PowerShell commands that download or execute encoded scripts are a common red flag. Security teams should track command line arguments, unexpected process launches, and scripts running under unusual user contexts. Abnormal activity, even if using legitimate software, can indicate a fileless intrusion.

Endpoint Detection and Response (EDR/XDR)

Advanced endpoint detection and response platforms are crucial for spotting fileless threats. EDR systems monitor process behavior in real time and capture detailed telemetry such as parent-child process relationships, memory injection attempts, and anomalous system calls. XDR platforms extend visibility across multiple endpoints and network layers, allowing correlation of suspicious events that may appear normal in isolation but indicate coordinated malicious activity.

Memory Forensics

Since fileless malware lives in memory, analyzing system memory snapshots is one of the most reliable detection methods. Tools like Volatility or Rekall can identify injected code, abnormal processes, and other signs of in-memory attacks. Regular memory analysis, particularly on high-value systems, helps uncover malicious activity that would otherwise remain invisible.

Network Monitoring and Threat Hunting

Fileless malware often communicates with command and control servers using legitimate network protocols like HTTPS or DNS. Monitoring unusual outbound traffic patterns, especially encrypted connections to unknown or rarely contacted endpoints, can reveal hidden activity. Threat hunting teams should search for patterns such as repeated connections outside regular working hours, beaconing behavior, or unexpected data exfiltration attempts.

Log Analysis

Enabling detailed logging for administrative tools and scripting engines is vital. Windows Event Logs, Sysmon logs, and script block logging provide insight into potentially malicious behavior. Analysis should focus on unusual usage patterns, failed privilege escalations, or execution of administrative tools by users who typically would not have access.

Indicators of Attack (IOAs)

Unlike indicators of compromise that rely on known signatures, IOAs focus on suspicious actions. Examples include a non-administrative user invoking PowerShell to download encoded scripts, WMI event subscriptions executing unknown commands, or macros attempting to run encoded payloads. Security teams should establish rules and alerts around such behaviors to detect potential fileless activity quickly.

Fileless Malware Techniques

Fileless malware relies on techniques that allow attackers to execute malicious code without leaving traces on disk. These techniques exploit trusted system tools, scripting environments, and legitimate processes, complicating detection by conventional antivirus solutions. Understanding these techniques is essential for both prevention and detection. Below are the most common methods used in fileless attacks.

PowerShell and Script Abuse

PowerShell is a powerful scripting tool built into Windows that administrators frequently use for legitimate tasks. Attackers exploit PowerShell to execute encoded commands, download malicious scripts, and run them entirely in memory. Since PowerShell is a trusted tool, malicious activity often blends with normal system operations. Indicators include unusual command-line arguments, execution of base64-encoded commands, and network connections initiated by PowerShell.

Windows Management Instrumentation (WMI) Abuse

WMI allows administrators to manage systems remotely and query system information. Fileless malware abuses WMI to execute scripts or schedule tasks without creating files on disk. Attackers may create permanent or event-based subscriptions that trigger code execution when specific conditions are met. Because WMI activity is everyday in enterprise environments, abnormal use often goes unnoticed without dedicated monitoring.

DLL Injection and Reflective Loading

Malware often injects dynamic link libraries (DLLs) directly into the memory of trusted processes. Reflective DLL loading allows code to be executed in memory without ever writing the DLL to disk. This technique provides the attacker with the privileges of the host process while evading traditional file-based scanning. Suspicious process behavior and abnormal memory usage are key indicators of this technique.

Registry Manipulation for Persistence

Although fileless malware primarily resides in memory, attackers often use the Windows registry to maintain persistence. They may store encoded scripts, configuration data, or commands in registry keys. On system startup or at specific triggers, these registry entries allow the malware to reinject itself into memory. Regularly auditing unusual or newly created registry keys can help identify potential threats.

Macro-Based Delivery

Attackers frequently use Microsoft Office documents with macros to deliver fileless payloads. When a user opens a malicious document and enables macros, the embedded script executes in memory, often invoking PowerShell or other system tools to continue the attack. This technique has been widely used in phishing campaigns due to its high success rate and ease of delivery.

Living Off the Land Binaries (LOLBins)

LOLBins are legitimate operating system binaries that attackers abuse to execute commands. Common examples include certutil, mshta, regsvr32, and rundll32. By leveraging these tools, malware can download additional code, execute scripts, or communicate with command and control servers without creating a new file. Monitoring unusual usage of these binaries is a key part of detecting fileless attacks.

Multi-Stage and In-Memory Frameworks

Advanced fileless attacks often use a small in-memory loader that retrieves additional payloads only when needed. These multi-stage frameworks minimize footprints in memory and on disk while maintaining flexibility for lateral movement and data exfiltration. Each stage can be tailored to the target environment, making attacks adaptable and difficult to detect.

How to Remove Fileless malware?

Removing fileless malware requires a different approach than traditional malware because it does not rely on files that can be easily scanned or deleted. Since the malware resides primarily in memory and may leverage legitimate system tools for persistence, remediation must be planned to contain the threat, eradicate malicious activity, and restore normal operations.

1. Immediate Containment

The first step in responding to a suspected fileless malware infection is containment. Isolate affected systems from the network to prevent lateral movement or data exfiltration. Disable compromised accounts or privileges that may have been exploited. If possible, stop processes identified as malicious, but do so carefully, as terminating critical system processes can disrupt normal operations.

2. Identify In-Memory Threats

Because fileless malware resides in memory, traditional file-based scanning is ineffective. Advanced endpoint detection and response (EDR) tools capture memory snapshots and analyze running processes. Look for suspicious command-line executions, injected modules in trusted processes, and anomalous parent-child process relationships. Memory forensics tools such as Volatility or Rekall can help uncover injected code and reveal the presence of malicious scripts.

3. Remove Persistence Mechanisms

Fileless malware often persists after a reboot using registry entries, scheduled tasks, or WMI event subscriptions. Carefully audit and remove these persistence mechanisms. Check for unusual registry keys, event subscriptions, and scheduled tasks that may trigger the execution of in-memory payloads. Ensure that any automated scripts or tools left by the malware are disabled.

4. System Cleanup and Recovery

After removing memory-resident malware and persistence mechanisms, review system integrity and restore any altered configurations. Apply patches to close vulnerabilities that the malware exploited and update security software to strengthen detection capabilities. Depending on the severity of the attack, consider performing a full system scan and using tools capable of detecting remnants of in-memory malware.

5. Credential and Access Management

Many fileless attacks focus on harvesting credentials for lateral movement. Change passwords for affected accounts, revoke tokens, and review access logs for unusual activity. Implement multi-factor authentication to prevent further unauthorized access.

6. Post-Incident Monitoring

Even after remediation, continuous monitoring is essential to ensure the malware does not reinfect systems. Use behavioral monitoring, endpoint telemetry, and network analysis to detect residual or repeated suspicious activity. Review logs and alerts to identify gaps in detection and improve defenses against future fileless attacks.

How to Protect Against Fileless Malware

Protecting against fileless malware requires a proactive and layered approach because these threats do not rely on traditional files that can be scanned or quarantined. Prevention focuses on reducing attack surfaces, monitoring unusual behavior, and implementing security controls that limit the ability of attackers to execute code in memory or abuse legitimate system tools.

Strengthen Endpoint Security

• Deploy advanced endpoint detection and response (EDR) solutions to analyze behavior, not just files.
• Memory scanning and script-blocking features catch malicious activity running without executables.
• Enable application whitelisting to restrict which programs can run on endpoints.
• Regularly review endpoint logs to identify anomalies like unusual PowerShell commands.
• Implement multi-factor authentication to make stolen credentials less useful.

Limit the Use of Trusted Tools

• Restrict administrative access to utilities such as PowerShell, WMI, and scripting engines to only those who truly need them.
• Configure PowerShell to run in constrained mode or with logging enabled to capture suspicious commands.
• Block or disable legacy scripting tools that are rarely needed but often abused by attackers.
• Monitor for excessive or unusual use of system binaries that attackers rely on for fileless attacks.
• Apply the principle of least privilege to reduce the potential damage if these tools are misused.

Regular Patching and Updates

• Outdated operating systems and applications are prime entry points for fileless attacks.
• Applying security patches quickly reduces the risk of attackers exploiting known vulnerabilities.
• Automating updates where possible ensures consistency across all endpoints and servers.
• Prioritizing critical patches for commonly abused software like browsers, Java, or Microsoft Office can block the initial foothold.
• Regular vulnerability scanning helps verify that systems are patched and secure.

Network Monitoring and Segmentation

• Monitoring network traffic helps identify unusual activity such as unexpected outbound connections or abnormal use of trusted protocols.
• Segmenting networks limits lateral movement, so even if one machine is compromised the attacker cannot easily access the entire environment.
• Enforcing strict access controls between sensitive systems and general workstations reduces exposure.
• Intrusion detection and response tools can analyze traffic patterns to spot command and control communication.
• Maintaining visibility over east-west traffic inside the network is as important as monitoring inbound and outbound flows.

User Training and Awareness

Phishing emails and malicious documents remain common delivery methods for fileless malware. Educating employees about the risks of enabling macros, clicking unknown links, or downloading unverified attachments can prevent initial infections. Awareness programs should emphasize cautious behavior and reporting suspicious activity.

Access Control and Privilege Management

Fileless attacks often escalate privileges to execute code in memory or access sensitive data. Implementing the principle of least privilege ensures users and applications only have access to what is necessary for their roles. Regularly reviewing and revoking unnecessary privileges reduces the risk of compromise.

Using a VPN for Added Protection

While a VPN cannot directly prevent fileless malware from executing in memory, it adds an essential layer of security by encrypting your internet traffic and shielding your network communications. Many fileless attacks begin with phishing emails, malicious downloads, or exploitation of unprotected network connections. 

Using a trusted VPN like AstrillVPN encrypts sensitive data such as login credentials and administrative access tokens during transmission, reducing the likelihood that attackers can intercept and leverage them to execute in-memory attacks.

A VPN is especially valuable when working on public Wi-Fi or remote networks. It prevents attackers from eavesdropping or injecting malicious scripts over unsecured connections. Combined with endpoint security, strict privilege management, and behavioral monitoring, a VPN becomes a complementary layer that enhances defense against fileless and other sophisticated cyber threats.

Backup and Recovery Strategy

Although fileless malware primarily resides in memory, secondary payloads may affect files or databases. Regular, secure backups ensure systems can be restored quickly during an attack. Backups should be tested periodically to verify integrity and accessibility.

Fileless Malware Examples

Several recent, real-world incidents and case studies illustrate how fileless malware is being used today. These examples highlight different techniques, targets, and tradecraft.

EggStreme – Military Targeting via Fileless Espionage Framework

In September 2025, cybersecurity researchers uncovered a fileless malware framework called EggStreme, deployed by a China-linked threat actor against the Philippine military.

What stands out about EggStreme is that it operates almost entirely in memory. The attack uses DLL sideloading via a legitimate binary to load initial components. The multi-stage framework comprises six elements: a loader, a reflective loader, a backdoor with dozens of commands, a keylogger, and a secondary backdoor.

Why it matters: This case demonstrates how fileless malware can use “living off the land” tools (legitimate binaries) and in-memory payloads to achieve persistence and espionage without leaving noticeable disk artifacts.

DNSMessenger & POWERSOURCE Attacks on Government, Financial Institutions, Enterprises

In early 2025, researchers observed a cluster of fileless attacks tied to a shared framework. One component was DNSMessenger, which used malicious Word documents to deliver a PowerShell RAT with communication via DNS requests. Another was “POWERSOURCE,” a PowerShell-based backdoor linked to the group FIN7.

Both attacks avoided dropping conventional executable files. They relied on document macros, PowerShell scripting, and DNS-based communication to evade detection and move laterally within networks.

SecurityWeek

ReliaQuest Findings – Prevalence of Fileless Patterns in Critical Incidents

In the ReliaQuest 2024 Annual Threat Report, more than 86 percent of critical incident detections in customer environments involved fileless malware or living-off-the-land (LotL) techniques.

One frequently observed loader was SocGholish. It uses JavaScript payloads delivered through drive-by downloads, often masquerading as legitimate software updates. Once a victim visits a compromised site, the script executes, leading to in-memory execution without writing a persistent malicious file on disk.

Conclusion

Fileless malware represents one of the most advanced and stealthy forms of cyber threats today. Unlike traditional malware that relies on malicious files stored on a device, fileless attacks exploit trusted system tools and run directly in memory. This makes them extremely difficult to detect with conventional security solutions. 

Real-world incidents have shown that even well-defended organizations can fall victim if they do not adapt their defenses to this evolving threat.

Protecting against fileless malware requires a layered security strategy that includes regular patching, strong endpoint monitoring, network segmentation, and continuous user awareness. Modern behavioral detection tools and strict access controls are essential to reduce exposure. At the same time, ensuring a secure online environment with solutions like AstrillVPN adds another layer of defense by encrypting traffic and reducing opportunities for attackers to exploit unsecured connections.

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