Identifying fileless files: a complete guide to detecting and stopping malware in memory

Attacks that operate without leaving a trace on disk have become a major headache for many security teams because they execute entirely in memory and exploit legitimate system processes. Hence the importance of knowing how to identify fileless files and defend themselves against them.

Beyond headlines and trends, understanding how they work, why they are so elusive, and what signs allow us to detect them makes the difference between containing an incident and regretting a breach. In the following lines, we analyze the problem and propose solutions.

What is fileless malware and why does it matter?

 

Fileless malware is not a specific family, but rather a way of operating: Avoid writing executables to disk It uses services and binaries already present in the system to execute malicious code. Instead of leaving an easily scannable file, the attacker abuses trusted utilities and loads its logic directly into RAM.

This approach is often encompassed in the 'Living off the Land' philosophy: attackers instrumentalize native tools such as PowerShell, WMI, mshta, rundll32 or scripting engines like VBScript and JScript to achieve their goals with minimal noise.

Among its most representative features we find: execution in volatile memory, little or no persistence on disk, use of system-signed components and high evasion capacity against signature-based engines.

Although many payloads disappear after a reboot, don't be fooled: adversaries can establish persistence by leveraging Registry keys, WMI subscriptions, or scheduled tasks, all without leaving suspicious binaries on the disk.

Why do we find it so difficult to identify fileless files?

The first barrier is obvious: There are no anomalous files to inspectTraditional antivirus programs based on signatures and file analysis have little room for maneuver when execution resides in valid processes and malicious logic resides in memory.

The second is more subtle: the attackers camouflage themselves behind legitimate operating system processesIf PowerShell or WMI are used daily for administration, how can you distinguish normal use from malicious use without context and behavioral telemetry?

Furthermore, blindly blocking critical tools is not feasible. Disabling PowerShell or Office macros across the board can break operations and It does not completely prevent abusesbecause there are multiple alternative execution paths and techniques to circumvent simple blocks.

To top it all off, cloud-based or server-side detection is too late to prevent problems. Without real-time local visibility into the issue... command lines, process relationships, and log eventsThe agent cannot mitigate on the fly a malicious flow that leaves no trace on disk.

How a fileless attack works from start to finish

Initial access usually occurs with the same vectors as always: phishing with office documents that ask to enable active content, links to compromised sites, exploitation of vulnerabilities in exposed applications, or abuse of leaked credentials to access via RDP or other services.

Once inside, the opponent seeks to execute without touching the disc. To do this, they chain together system functionalities: macros or DDE in documents that launch commands, exploit overflows for RCE, or invoke trusted binaries that allow loading and executing code in memory.

If the operation requires continuity, persistence can be implemented without deploying new executables: startup entries in the RegistryWMI subscriptions that react to system events or scheduled tasks that trigger scripts under certain conditions.

With the execution established, the objective dictates the following steps: move laterally, exfiltrate dataThis includes stealing credentials, deploying a RAT, mining cryptocurrencies, or activating file encryption in the case of ransomware. All of this is done, when possible, by leveraging existing functionalities.

Removing evidence is part of the plan: by not writing suspicious binaries, the attacker significantly reduces the artifacts to be analyzed. mixing their activity between normal events of the system and deleting temporary traces when possible.

Techniques and tools that they usually use

The catalog is extensive, but it almost always revolves around native utilities and trusted routes. These are some of the most common ones, always with the goal of maximize in-memory execution and blur the trace:

• PowerShellPowerful scripting, access to Windows APIs, and automation. Its versatility makes it a favorite for both administration and offensive abuse.
• WMI (Windows Management Instrumentation)It allows you to query and react to system events, as well as perform remote and local actions; useful for persistence and orchestration.
• VBScript and JScript: engines present in many environments that facilitate the execution of logic through system components.
• mshta, rundll32 and other trusted binaries: the well-known LoLBins which, when properly linked, can execute code without dropping artifacts evident on disk.
• Documents with active contentMacros or DDE in Office, as well as PDF readers with advanced features, can serve as a springboard to launch commands in memory.
• Windows' register: self-boot keys or encrypted/hidden storage of payloads that are activated by system components.
• Seizure and injection into processes: modification of the memory space of running processes for host malicious logic within a legitimate executable.
• Operating kits: detection of vulnerabilities in the victim's system and deployment of tailored exploits to achieve execution without touching the disk.

The challenge for companies (and why simply blocking everything isn't enough)

A naive approach suggests a drastic measure: blocking PowerShell, prohibiting macros, preventing binaries like rundll32. The reality is more nuanced: Many of these tools are essential. for daily IT operations and for administrative automation.

In addition, attackers look for loopholes: running the scripting engine in other ways, use alternative copiesYou can package logic in images or resort to less monitored LoLBins. Brute blocking ultimately creates friction without providing a complete defense.

Purely server-side or cloud-based analysis doesn't solve the problem either. Without rich endpoint telemetry and without responsiveness in the agent itselfThe decision comes late and prevention is not feasible because we have to wait for an external verdict.

Meanwhile, market reports have long pointed to a very significant growth in this area, with peaks where the Attempts to abuse PowerShell nearly doubled in short periods, which confirms that it is a recurring and profitable tactic for adversaries.

Modern detection: from file to behavior

The key is not who executes, but how and why. Monitoring the process behavior and its relationships It is decisive: command line, process inheritance, sensitive API calls, outbound connections, Registry modifications, and WMI events.

This approach drastically reduces the evasion surface: even if the binaries involved change, the attack patterns are repeated (scripts that download and execute in memory, abuse of LoLBins, invocation of interpreters, etc.). Analyzing that script, not the 'identity' of the file, improves detection.

Effective EDR/XDR platforms correlate signals to reconstruct the complete incident history, identifying the root cause Instead of blaming the process that 'showed up', this narrative links attachments, macros, interpreters, payloads, and persistence to mitigate the entire flow, not just an isolated piece.

The application of frameworks such as MITRE ATT & CK It helps map observed tactics and techniques (TTPs) and guide threat hunting towards behaviors of interest: execution, persistence, defense evasion, credential access, discovery, lateral movement and exfiltration.

Finally, the endpoint response orchestration must be immediate: isolate the device, end processes involved, revert changes in the Registry or task scheduler and block suspicious outgoing connections without waiting for external confirmations.

Useful telemetry: what to look at and how to prioritize

To increase the probability of detection without saturating the system, it is advisable to prioritize high-value signals. Some sources and controls that provide context. critical for fileless are:

• Detailed PowerShell Log and other interpreters: script block log, command history, loaded modules, and AMSI events, when available.
• WMI RepositoryInventory and alert regarding the creation or modification of event filters, consumers, and links, especially in sensitive namespaces.
• Security events and Sysmon: process correlation, image integrity, memory loading, injection, and creation of scheduled tasks.
• Red: anomalous outbound connections, beaconing, payload download patterns, and use of covert channels for exfiltration.

Automation helps separate the wheat from the chaff: behavior-based detection rules, allowlists for legitimate administration and enrichment with threat intelligence limits false positives and accelerates the response.

Prevention and reduction of surface

No single measure is sufficient, but a layered defense greatly reduces risk. On the preventative side, several lines of action stand out for crop vectors and make life more difficult for the adversary:

• Macro management: disable by default and allow only when absolutely necessary and signed; granular controls via group policies.
• Restriction of interpreters and LoLBins: Apply AppLocker/WDAC or equivalent, control of scripts and execution templates with comprehensive logging.
• Patching and mitigations: close exploitable vulnerabilities and activate memory protections that limit RCE and injections.
• Strong authenticationMFA and zero trust principles to curb credential abuse and reduce lateral movement.
• Awareness and simulationsPractical training on phishing, documents with active content, and signs of anomalous execution.

These measures are complemented by solutions that analyze traffic and memory to identify malicious behavior in real time, as well as segmentation policies and minimal privileges to contain the impact when something slips through.

Services and approaches that are working

In environments with many endpoints and high criticality, managed detection and response services with 24/7 monitoring They have proven to accelerate incident containment. The combination of SOC, EMDR/MDR, and EDR/XDR provides expert eyes, rich telemetry, and coordinated response capabilities.

The most effective providers have internalized the shift to behavior: lightweight agents that correlate activity at the kernel levelThey reconstruct complete attack histories and apply automatic mitigations when they detect malicious chains, with rollback capability to undo changes.

In parallel, endpoint protection suites and XDR platforms integrate centralized visibility and threat management across workstations, servers, identities, email, and the cloud; the goal is to dismantle the chain of attack regardless of whether or not files are involved.

Practical indicators for threat hunting

If you have to prioritize search hypotheses, focus on combining signals: an office process that launches an interpreter with unusual parameters, WMI subscription creation After opening a document, modifications to startup keys followed by connections to domains with poor reputation.

Another effective approach is to rely on baselines from your environment: what is normal on your servers and workstations? Any deviation (newly signed binaries appearing as parents of interpreters, sudden spikes in performance (of scripts, command strings with obfuscation) deserves investigation.

Finally, don't forget memory: if you have tools that inspect running regions or capture snapshots, the findings in RAM They can be the definitive proof of fileless activity, especially when there are no artifacts in the file system.

The combination of these tactics, techniques, and controls does not eliminate the threat, but it puts you in a better position to detect it in time. cut the chain and reduce the impact.

When all of this is applied judiciously—endpoint-rich telemetry, behavioral correlation, automated response, and selective hardening—the fileless tactic loses much of its advantage. And, although it will continue to evolve, the focus on behaviors Rather than in files, it offers a solid foundation for your defense to evolve with it.