How Lumma Stealer sneaks into organizations

Introduction

The evolution of Malware-as-a-Service (MaaS) has significantly lowered the barriers to entry for cybercriminals, with information stealers becoming one of the most commercially successful categories in this underground economy. Among these threats, Lumma Stealer has emerged as a particularly sophisticated player since its introduction in 2022 by the threat actor known as Lumma. Initially marketed as LummaC2, this information stealer quickly gained traction in underground forums, with prices starting at $250. As of March 2025, its presence on dark web marketplaces and Telegram channels continues to grow, with over a thousand active subscribers.

LummaC2 seller’s official website

Lumma delivery usually involves human interaction, such as clicking a link, running malicious commands, etc. Recently, while investigating an incident as part of our incident response services, our Global Emergency Response Team (GERT) encountered Lumma on a customer’s system. The analysis revealed that the incident was triggered by human interaction, namely the user was tricked into executing a malicious command by a fake CAPTCHA page. In this article, we will review in detail how the fake CAPTCHA campaign works and share a list of IoCs that we discovered during our analysis and investigation of the campaign. Although we already described this distribution method in an earlier article, more details about this campaign have been discovered since then.

Lumma Stealer’s distribution vectors

Lumma Stealer’s distribution methods are diverse, using common techniques typically seen in information-stealing malware campaigns. Primary infection vectors include phishing emails with malicious attachments or links, as well as trojanized legitimate applications. These deceptive tactics trick users into executing the malware, which runs silently in the background harvesting valuable data. Lumma has also been observed using exploit kits, social engineering, and compromised websites to extend its reach and evade detection by security solutions. In this article, we’ll focus mainly on the fake CAPTCHA distribution vector.

This vector involves fake verification pages that resemble legitimate services, often hosted on platforms that use Content Delivery Networks (CDNs). These pages typically masquerade as frequently used CAPTCHAs, such as Google reCAPTCHA or Cloudflare CAPTCHA, to trick users into believing they are interacting with a trusted service.

Fake CAPTCHA distribution vectors

Fake CAPTCHA distribution scheme

There are two types of resources used to promote fake CAPTCHA pages:

• Pirated media, adult content, and cracked software sites. The attackers clone these websites and inject malicious advertisements into the cloned page that redirect users to a malicious CAPTCHA.
• Fake Telegram channels for pirated content and cryptocurrencies. The attackers create Telegram channels with names containing keywords related to cryptocurrencies or pirated content, such as software, movies, etc. When a user searches for such content, the fraudulent channels appear at the top of the search. The attackers also use social media posts to lure victims to these channels. When a user joins such a channel, they are prompted to complete an identity verification via a fraudulent “Safeguard Captcha” bot.
  

  Safeguard Captcha bot

  Once the user clicks the Verify button, the bot opens a pop-up page with a fake CAPTCHA.

Fake CAPTCHA page

Users are presented with a pop-up page that looks like a standard CAPTCHA verification, prompting them to click I’m not a robot/Verify/Copy or some similar button. However, this is where the deception begins.

Fake CAPTCHA page examples

Fake page malicious content

When the I’m not a robot/Verify/Copy button is clicked, the user is instructed to perform an unusual sequence:

• Open the Run dialog(Win+R)
• Press Ctrl+V
• Hit Enter

Without the user’s knowledge, clicking the button automatically copies a PowerShell command to the clipboard. Once the user pastes the command into the Run dialog and presses Enter, the system executes the command.

Examples of scripts copied to the clipboard and executed via the Run dialog

The command may vary slightly from site to site and changes every few days, but it is typically used to download Lumma Stealer from a remote server, which is usually a known CDN with a free trial period or a legitimate code hosting and collaboration platform such as GitHub, and begin the malware installation process. Let’s take a closer look at this infection chain using the following command that was executed in our customer’s incident as an example:

Command triggering Lumma’s infection chain

The command is rather simple. It decodes and runs the contents from the remote win15.txt file hosted at https[:]//win15.b-cdn[.]net/win15.txt. The win15.txt file contains a Base64-encoded PowerShell script that then downloads and runs the Lumma Stealer. When decoded, the malicious PowerShell script looks like this:

Contents of win15.txt

The script performs the following actions:

1. Downloads the malware. It downloads the win15.zip file from https[:]//win15.b-cdn[.]net/win15.zip to [User Profile]\AppData\Roaming\bFylC6zX.zip.
2. Extracts the malware. The downloaded ZIP file is extracted to C:\Users\[User]\AppData\Roaming\7oCDTWYu, a hidden folder under the user’s AppData directory.
3. Executes the malware. The script runs the Set-up.exe file from the unpacked archive, which is now located at C:\Users\[User]\AppData\Roaming\7oCDTWYu\Set-up.exe.
4. Establishes persistence mechanism. The script creates an entry in the Windows Registry for persistency, ensuring that the malware runs every time the system starts. The registry key is added under HKCU:\SOFTWARE\Microsoft\Windows\CurrentVersion\Run. The key name is 5TQjtTuo, with the value pointing to Set-up.exe.

However, in some cases, the malware delivery mechanism can be more complex. In the following example, the delivery script is a JavaScript code hidden in what looks like an .mp3 file (other file formats such as .mp4 and .png have also been used). In fact, in addition to the JavaScript, the file may contain a corrupt .mp3/.mp4 file, legitimate software code, or just random data.

The script is executed using the Microsoft HTML Application engine mshta.exe by prompting the user to paste the following command into the Run dialog box:

Command triggering JS-based infection chain

The mshta command parses the file as an HTA file (Microsoft HTML Application) and executes any JavaScript code within the 

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