Fileless Malware: The Complete Guide

Posted on Sep 29, 2022 by Devaang Jain

If you think viruses can only infect your devices via malicious files, think again. Fileless malware can unleash horror on your digital devices if you aren’t prepared. 

A recent study indicated a whopping 900% increase in the number of attacks in just over a year. And that’s very likely an understatement of the real numbers, as this type of malware is designed to not leave any footprints.

So, what exactly are fileless viruses and how do you combat a fileless malware attack? Stay tuned as we walk you through the dark world of invisible malware ‘living off the land’.

What’s so ‘Fileless’ About Fileless Malware?

Let’s first understand how conventional malware attacks a system. The target individual often downloads a malicious file on their device via a network they’re connected to. The file then executes the programmed activity. In contrast, fileless malware does not rely on downloading any files at all, making it ‘fileless’.

Every device has a number of apps and software installed. Fileless malware preys upon these built-in utilities and exploits their vulnerabilities. A cybercriminal can analyze the configuration of software and hardware on a system, and launch a fileless malware attack without ever touching the device.

Fileless malware is so insidious because it doesn’t leave any footprint at all. Antivirus software hunts for malware signatures on system disks, but fails to identify fileless malware because it resides in the RAM. 

Are Fileless Malware Attacks a New Breed?

While it might come as a surprise, fileless malware has been with us for several years. Some cybersecurity experts believe it’s been steadily evolving from viruses focused on targeted attacks. 

Among the first fileless viruses was SQL Slammer, a buffer overflow exploit in Microsoft’s SQL Server, first surfacing in 2003. The “Sapphire Worm”, as it’s also called, disrupted hundreds of thousands of network devices in minutes.

A distinctive feature of Slammer is its minuscule memory footprint. It does not write itself to the system disk, but stays in the RAM. It’s also easy to remove, since the user needs to simply restart their system to get rid of the virus. 

“Netscape may be a thing of the past, but malware isn’t.”

However, Slammer wasn’t the first fileless malware. In 2001, a new computer worm infected systems worldwide. The attack targeted a vulnerability in the Microsoft IIS web server and compromised even large-scale enterprise networks. This malware, called “Code Red”, is one of the earliest fileless malware codes on record

The Full Spectrum of Fileless Malware Attacks

Fileless malware comes in several flavors. While the specific types differ in how they infect a system, all malware retains a common core functionality: the virus infects the host without leaving a trace. 

The fileless attack spectrum is vast, and more often than not, an attacker will execute a sophisticated attack combining two or more different fileless malware types. These include:

  • Exploit kit-based malware: An exploit kit is a selection of exploits, i.e., vulnerabilities in the OS or app code. An attacker commonly scans the target system for the installed apps and native software, and places dedicated exploits to take full advantage. The attacker remotely controls the way these exploits function over a central management console. They typically send out a phishing email to lay the trap and lure the victim in.
  • Memory-only fileless malware: This malware confines itself to the system memory at all times. It will discover and open backdoors for further attacks and system control. Many viruses can capture user credentials and use it to create new accounts to take the attack one step further. They can hide themselves and place malicious code in the OS kernel or the system registry to remain undetected. 
  • Ransomware attacks: This malware uses an exploit or malicious code to gain access to a system file and encrypt it. For instance, an attacker can use a Microsoft Office macro-based script to lock down a file until the victim pays a ransom to regain access.
  • Resident registry-based malware: An attacker can inject fileless malware directly into the Windows registry where it resides. The virus thus averts all detection and will remain persistent which makes it notoriously difficult to remove. This is definitely one of the most insidious variants of fileless malware.

How Do You Detect a Fileless Malware Infection?

Fileless malware can inject itself into the RAM and execute malicious code

Here’s the unfortunate part. Fileless malware is notoriously difficult to detect. Most people never realize a memory-only malware has compromised their devices. Digital forensics have a hard time dealing with the malware, as it leaves a practically non-existent trail for anyone to follow and know what they’re dealing with. 

The best approach is to look out for Indicators of Attack (IOAs). These are signs indicating the presence of a fileless attack carrier. 

  • ⚠️ Lateral movement: If you discover an unintended movement of data across the network, examine the possibility of a fileless malware infection. For instance, look out for a sudden surge in your internet data traffic or bandwidth consumption. Once fileless malware executes on your system, it then spreads across other network components and transfers sensitive user data like login credentials across the internet. 
  • ⚠️ Suspicious behavior: Fileless malware often leads to infected devices behaving in unexpected ways. Should you find your devices running programs or apps you don’t recall installing or running yourself, it should make you question the possibility of filesless malware.
  • ⚠️ Employ AI: Invest in software harnessing AI-based behavioral analysis to monitor your system for common infection signs, like PIA’s antivirus. Such anti-malware software can even detect fileless malware exploiting built-in apps or legitimate accounts and are otherwise difficult to detect via sandboxing or signature-based scanning. 
  • ⚠️ Monitor the memory: Since fileless malware hides in the system memory, carefully watching the system RAM at regular intervals can help detect programs springing out of the blue. In particular, look out for processes elevating their privileges and seemingly unrelated to your activities on the system. If a system process appears to be running randomly with administrator privileges when it doesn’t need them, take a close look.

Is it Possible to Prevent Fileless Malware Attacks?

Anti-malware software with behavioral analysis can detect fileless malware

You can certainly keep fileless malware attacks at bay with simple precautionary steps. 

  1. Avoid installing apps you don’t need: This might seem trivial, but we’re all guilty of having our devices swamped with multiple apps we don’t remember installing. We use some apps just once and then forget about them. Uninstall any non-essential apps as this reduces the attack surface. Fewer apps and programs translates to fewer vulnerabilities an attacker can exploit. 

  1. Never click on suspicious links: Clicking on links in unsolicited emails can spell disaster. When you click on an innocent-looking link, a fileless virus might inject itself into your device memory. Use a URL scanner to expand shortened links and see their destination server address before clicking on them.

  1. Disable Flash and PowerShell: Most fileless malware exploit Flash, PowerShell, and WMI (Windows Management Instrumentation). Consider disabling these tools if you’re not using them.

  1. Restrict access privileges: Most apps and programs we install on our devices seek a number of permissions. We recommend only giving basic access permissions to any new apps – the lower an app’s access capabilities, the lower the probability of a fileless attack exploiting it to compromise the critical components of your system.

  1. Use macros wisely: If Microsoft Office is a part of your everyday toolkit, chances are, you use macros. Attackers have successfully used macros in the past to load malicious code into the device memory. We recommend avoiding macros you don’t trust and not opening documents from untrusted sources. 

  1. Keep your system updated: Install the latest updates for all your apps and software. You should also keep your OS up-to-date as updates patch exploits and vulnerabilities as and when the vendor discovers them.

  1. Use a VPN: Use a VPN to secure your web browsing. PIA VPN encrypts your internet traffic with 256-bit AES encryption, which stops cybercriminals from preying on your activity and latching onto your device. When you have an encrypted internet connection, an attacker has an increasingly hard time intercepting information and discovering ways to insert malicious code.

  1. Use a good antivirus: Many antiviruses have RAM-scanning capabilities that filter our malicious code executions. Private Internet Access has a state-of-the-art antivirus add-on that you can use to protect your PC from malware.

Infected with Fileless Malware? Here’s What to Do 

If you think you might be a victim of a fileless malware attack, don’t panic. It’s a good idea to reboot your devices as a first step. Restarting your system will flush out any memory-based malware. It’s possible for these viruses to infect your system again though.

Resetting your device to factory defaults wipes out any remaining fileless malware pieces. Since this deletes all your data, make sure to back up all files before you reset the system.

Use professional anti-malware software to identify fileless malware and eliminate it. PIA’s antivirus is regularly updated with the latest threats, so you can use it to identify and remove nasty fileless viruses. 

Stay One Step Ahead of Fileless Malware

Fileless malware is one of the most challenging attacks to detect and eliminate owing to one simple reason: the attack leaves no trail behind. An attacker can inject code into the system memory, registry, or the kernel, making the code virtually impossible to detect.

Behavioral monitoring is a feasible technique to detect and stop fileless malware attacks in their tracks. A simple RAM flush can temporarily remove the malware from the device. However, many fileless malware remain persistent and need more than a reboot to permanently remove. 

To protect yourself, use PIA to secure all your internet traffic when you browse the web. When you use a VPN, all data travels within a shielded tunnel away from prying eyes. PIA also runs its own DNS servers, protecting you from fileless malware attacks, man-in-the-middle attacks (MitM), and distributed denial of service attacks (DDoS).


What is fileless malware and how does it work?

Fileless malware “lives off the land”, which means it doesn’t leave any trace on the system disk. A fileless virus will inject itself into the memory where it executes malicious code and spreads itself across the network. 

Fileless malware attacks exploit vulnerabilities in native apps and software installed on your devices. This means you don’t need to download any infected file to compromise your system as the attack happens in a ‘fileless’ manner instead.

Protect yourself from cyber attacks with PIA VPN! We secure your online experience end-to-end with military-grade encryption. PIA also runs its own DNS servers to secure you from man-in-the-middle attacks, and has an effective antivirus add-on.

How is fileless malware delivered?

An attacker can deliver fileless malware to your system in a variety of ways. If you click on a link in a phishing email, you might unknowingly inject malicious code into your device memory. You might also download an app with a known vulnerability left unpatched. Or, you could simply click on a dodgy ad and all hell breaks loose on your device.

PIA VPN includes PIA MACE, an advanced ad and malware blocker to protect you against malware and ads, as well as an antivirus add-on that keeps you safe from local threats.

Is fileless malware really fileless?

Yes, fileless viruses are not linked to any physical file entities stored on a disk. They conceal themselves in the memory and execute programmed instructions without leaving traditional virus-like signatures. 

Attackers often use fileless malware to gain an entry point into a system. Once captured, the fileless virus then downloads a payload file. The latter takes the attack a step further with a conventional file-based infection.  

Thankfully, you can protect yourself from fileless malware attacks using a VPN. Use PIA to mask your traffic with 256-bit AES encryption, which stops attackers from tracking you online. They can’t see you online, so they can’t target you with attacks.

How do you get rid of fileless malware?

Rebooting the device is one way of removing a memory-only fileless virus from a system. When you restart a device, the RAM, which is volatile memory, loses all its contents. This wipes out any fileless malware residing in the memory. However, if a fileless virus remains in the RAM for a long time, it can become persistent.

Remove any unnecessary apps from your device to reduce the attack surface. We recommend always browsing the web using a VPN like PIA, which provides open source apps for all your devices. Best of all, you can try us risk-free for 30 days with our 30-day money-back guarantee.