Delivering Trojans Via ClickFix Captcha

Overview

Recently, a new social engineering technique has emerged, rapidly gaining traction as a highly effective method for delivering various types of malware, including Infostealers, Ransomware, and Quakbot. This technique exploits a seemingly harmless element — a ClickFix Captcha — to deceive users and bypass traditional security measures.

Quakbot (Qbot) — The Evolving Banking Trojan

First discovered in 2008, Quakbot (Qbot) is a sophisticated banking trojan that has undergone continuous development, with periods of dormancy followed by renewed activity. Despite occasional gaps in operational use, it remains one of the most effective and versatile malware families, regularly updated with new capabilities and techniques to evade detection and enhance persistence.

 DarkAtlas Research Squad is set to present a real-world scenario demonstrating how QakBot is being delivered through the ClickFix Captcha technique. This presentation will provide an in-depth analysis of how attackers leverage seemingly legitimate captchas to bypass user suspicion and execute malicious payloads.

protectedsound[.]digital redirects to a ClickFix captcha at cfcaptcha[.]com

Quakbot being delivered through ClickFix Captcha, where the captcha tricks the user into performing an unsuspecting activity on their local machine by using the Windows key + R and executing a malicious command preloaded into the clipboard.

The command attempts to download a text file from a specified URL and use its contents as input for the next command. The iex at the end of the URL stands for Invoke-Expression, a PowerShell command that executes the retrieved string as a PowerShell command, enabling the execution of malicious code directly from the downloaded file.

While the contents of the file at cfcaptcha[.]com/flwssetp.txt are mostly encrypted, certain parts are seen in clear text and give insight into what the encrypted contents might be doing.

This PowerShell command displays a graphical message box with the text “Verification Complete!” and an “OK” button. The intent is to deceive the user into believing that the verification process was successful, encouraging them to proceed on the website without suspicion.

Another section of the obfuscated content appeared to involve XOR Decryption of a long hex string, suggesting that the payload was encoded to evade detection and analysis. This technique is commonly used by malware to obfuscate malicious code, making it harder for security tools to identify and analyze the threat.

decode the hex To get more Value.

1- The decoded payload reveals a PowerShell script designed to:

2- Download a ZIP file from https://duolingos.com/flswunwa.zip.

3- Save the ZIP to the $env:AppData directory as flswunwa.zip.

4- Extract the ZIP using Expand-Archive to the same directory.

5- Load the .NET assembly System.IO.Compression.FileSystem to handle the ZIP file.

6- Open the ZIP file and likely execute its contents (though the full payload isn’t visible yet).

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We going to download “hxxps://duolingos[.]com/flswunwa[.]zip” Manully to Check it.

Fake Zip Dowloader

This technique allows Qbot operators to create the appearance of multiple files or subdirectories, such as:

hxxps://duolingos[.]com/xxxxx[.]zip

hxxps://duolingos[.]com/yyyyy[.]zip

hxxps://duolingos[.]com/files/zzzzz[.]zip

Purpose and Impact:
Endless Unique URLs
Attackers can generate an unlimited number of unique URLs for malware distribution, making it difficult for security solutions to blacklist or detect malicious activity effectively.

Obfuscation of Malicious Code
The actual location of the malicious payload remains concealed, complicating efforts by security researchers and website owners to trace, analyze, and mitigate the threat.

This explains why executing hxxps://duolingos[.]com/flswunwa[.]zip in a sandbox environment failed to reveal meaningful details about the file.

VirusTotal analysis for hxxps://duolingos[.]com/flswunwa.zip — Detected by 1/96 security vendors as malicious. Status: 404 (Not Found), Content type: text/html, indicating possible obfuscation or removal.

URLScan results for hxxps://duolingos[.]com/flswunwa.zip — 3 HTTP transactions observed, all returning 404 errors. The site is hosted on Cloudflare (188.114.96.3), indicating possible obfuscation or temporary removal of malicious content

Hybrid Analysis report for hxxps://duolingos[.]com/flswunwa.zip — Marked as suspicious with a threat score of 50/100. Detected phishing activity, network behavior involving 3 domains and 4 hosts, and 11 indicators mapped to 8 MITRE ATT&CK techniques and 5 tactics

In reality, all these requests are intercepted and redirected by a rewrite rule to a malicious PHP Script hosted on the compromised server. This PHP Script functions as a Proxy, dynamically fetching the actual malicious content from a secondary tier of attacker-controlled servers. By operating as an intermediary, the PHP Script conceals the true origin of the Malicious Payload, adding a layer of obfuscation and making it harder for defenders to trace the source of the attack.

In this particular case, the malicious PHP Script is embedded directly within the body of the page, further complicating detection and analysis. This tactic enables attackers to dynamically generate and serve malicious content, bypassing conventional security measures and increasing the resilience of the Attack Infrastructure.

The file in question is a PHP-based QakBot dropper, designed to download and execute QakBot or other malware, such as stealer malware and ransomware, from a remote server onto the victim’s system. The dropper facilitates the execution of the malicious payload by either exploiting system vulnerabilities or using Social Engineering techniques to deceive the victim into allowing execution.

As of the time of publishing this article, the PHP dropper file has been removed, and the associated domains — hxxps://duolingos[.]com and hxxp://cfcaptcha[.]com — are in the process of being taken down. This action aims to disrupt the malware delivery infrastructure and limit further infections.

Summary:

The success of ClickFix stems from its ability to exploit user trust in captchas, software updates, and websites that appear legitimate. This psychological leverage increases the likelihood of users unknowingly executing malicious commands on their local systems, thereby enabling attackers to gain a foothold and deploy additional payloads without raising suspicion.