NetWalker (fka Mailto) Overview

This threat report is about the NetWalker ransomware previously known as Mailto. Since it was first detected in August 2019, NetWalker has quickly evolved and is now considered highly dangerous due to its implementation of advanced anti-detection and persistence techniques.

This report gives a technical analysis of its recently-observed behavior, as well as preventative measures that can be implemented to protect against NetWalker’s highly effective extortion efforts.

Related Reading: Ragnar Locker Ransomware New Invasion Technique

Netwalker Ransomware Tactics, Techniques, and Procedures

Although variants of NetWalker have been observed since August 2019, there has been a significant increase in prevalence since March 2020.

As the threat has evolved over the past few months, its establishment as a robust ransomware-as-a-service (RaaS) model has been evident. This enables even novice cybercriminals to launch a successful ransomware attack against targets.

NetWalker has been observed primarily infiltrating large-scale corporate networks. Researchers have linked large sums of bitcoin to ransomware, indicating its extortion efforts have likely been effective.

The following shows the global prevalence of these attacks:

Netwalker Ransomware heat map
Source: McAfee

As the evolution of this malware has progressed, researchers have observed the implementation of more sophisticated defense evasion techniques along with other advanced features making these attacks more difficult to investigate by security analysts.

In one recently analyzed case, the payload began with a PowerShell script that was executed directly in memory. No ransomware binary was stored on the disk, this enabled the malware to maintain persistence and evade detection by performing the attack through tools that were already present in the system.

One of the techniques leveraged with this process is referred to as "reflective DLL injection". This allows a DLL injection from memory rather than from a disk. This method does not require an actual DLL file on a disk or a Windows loader for injection, allowing evasion from DLL load monitoring tools.

In this case, the ransomware DLL was injected into the memory of the legitimate process explorer.exe. The diagram below shows a graphical representation of the PowerShell script observed in this variant of NetWalker:

Source: Trend Micro

The malware’s first action in the executable process is to combine all required functions into one, combining the Windows modules with additional DLLs.

The malware also uses Process Environment Block (PEB) to inhibit analysis. If the malware fails to obtain its configuration file, it will terminate itself.

After successful completion of all encryption actions, the malware shows a ransom note, displayed in Notepad, informing the user of what has happened and threatening permanent data loss if the ransom is not paid. After all ransom actions have been completed, the malware attempts to remove itself to avoid being detected or analyzed.

Further technical information can be found linked in the Sources section below.

Related Reading: Ransomware Prevention to Incident Response

What Netwalker Vulnerability Means to You

  • The infection will lead to encryption or loss of data.
  • Critical company data could be leaked to the public.
  • Could result in major production downtime costs.

What You Can Do About Netwalker Ransomware

Employing adequate preventative measures greatly minimizes the risk of being infected. One of the most effective methods is having good security policies and practices in place before an incident occurs:

  • We highly recommend you block or set up detection for the existing IoCs associated with NetWalker linked in the Sources section below.
  • Implement regular vulnerability scanning to identify and address security weaknesses in your environment.
  • Consider implementing a security awareness training program for your organization to encourage best security practices when handling email content.
  • Make sure you are using the latest version of the OS your systems are running. Each OS is coded for the best security practices of the time. Because these threats are constantly evolving it is important to implement these upgrades as they become available to you.
  • Make sure all software on your devices is fully updated. Malicious actors and programs can utilize little-known vulnerabilities in outdated software to gain initial access. Perform regular software audits to identify unused applications and uninstall them as these could also present a risk.
  • Recognize the risk with USB drives. If a USB drive has been infected or loaded with malware, it can automatically execute the malicious software on a device by simply being plugged in. Cloud storage services offer many conveniences in relation to file sharing and may be an option if your company would like to limit the use of USB drives.
  • Utilize user security permissions on devices appropriately as an additional layer of protection in the case that a host is infected. Security permissions can limit an executable’s privileges when running at a locked-down user level. Accounts with administrative access should only be used when privileged access is necessary. This method of locking down security on individual devices can help quarantine infections from spreading rampantly through a network.
  • Consider using the internal software firewalls on systems to block the use of unnecessary ports. In most modern-day network topologies, it is not necessary for individual computers on a network to communicate directly. Enabling this will aid in preventing the spread of infection through discovered open ports.
  • Turn off protocols that you do not need on individual PCs. The more information a bad actor or piece of software can obtain about a network, the more likely it is to be able to infect it. Turning off unnecessary sharing will also aid in shielding a PC from being attack by an infected device on that network.

Using Backups to Protect Against Breaches

Although the complications of a ransomware attack can be detrimental, it is only one of many ways critical data can be lost. Because of this, it is imperative that companies have a regular backup routine in place. Data backed up should also be moved to an isolated location off-site for protection in the case that ransomware or any other location-specific event were to occur.

Related Reading: 5 Ways to Prevent Ransomware

Risks of Paying the Ransom

In the unfortunate event that ransomware is active in your environment, keep in mind that paying the ransom does not guarantee you will receive the decryption key to get your data back.

It is also not guaranteed that you will not be infected again shortly after, since the vulnerabilities in your infrastructure that caused the infection, to begin with, are still exposed and susceptible to attack.

Finally, the more victims who pay the ransom, the more profitable it is for cybercriminals, so this response encourages the continuation of these attacks.

Sources and MITRE ATT&CK Techniques

IBM X-Force Exchange (IOC list):


Trend Micro:

MITRE ATT&CK Techniques:

Note: The Avertium Threat Report analyzes one current threat that has been shared by threat intelligence networks across the globe. Used internally by the Avertium CyberOps Team, this report will outline a “top-of-mind” threat and how it ought to be addressed accordingly.

This informed analysis is based on the latest data available.

Contact us for more information about Avertium’s managed security service capabilities. 

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