Fake Captcha Kit Tricks Users into Executing Code via Windows Run Command

Security researchers have identified a sophisticated phishing campaign leveraging a fake CAPTCHA verification system dubbed “HuluCaptcha” that covertly executes malicious code through the Windows Run command.

The attack chain begins with seemingly legitimate CAPTCHA challenges that, upon interaction, trigger script execution without user awareness.

This technique bypasses traditional security measures by exploiting legitimate Windows functionality while maintaining a convincing user interface that mimics standard verification processes.

Attack Methodology

The HuluCaptcha toolkit operates through a multi-stage attack vector that begins with phishing emails containing links to fraudulent websites.

These sites present users with what appears to be standard CAPTCHA verification interfaces, complete with image recognition challenges and checkbox confirmations.

The malicious aspect lies in the hidden JavaScript code embedded within these interfaces, which captures user interactions.

When victims attempt to solve the CAPTCHA, the toolkit silently constructs and executes commands via the Windows Run dialog (accessible through Win+R), leveraging PowerShell or CMD to establish persistence and download additional payloads.

Technical analysis reveals sophisticated obfuscation techniques employed throughout the toolkit’s code.

The malware authors have implemented multiple layers of encoding, including Base64 and custom cipher algorithms, to evade detection by security solutions.

The JavaScript payload decodes at runtime and utilizes DOM manipulation to maintain the illusion of legitimate CAPTCHA functionality while simultaneously executing system commands.

Particularly concerning is the toolkit’s ability to detect virtual environments and security analysis tools, altering its behavior to appear benign during analysis.

The command execution mechanism exploits the shell: protocol handler in Windows, which allows direct execution of commands from browser contexts under certain conditions.

This approach circumvents many endpoint protection solutions that fail to monitor or restrict this specific execution pathway.

Once initial access is established, the malware deploys fileless components that reside primarily in registry keys and memory, further complicating detection and remediation efforts.

Distribution Networks

HuluCaptcha’s distribution infrastructure reveals a sophisticated operation leveraging compromised WordPress sites as first-stage distribution points.

Researchers discovered, these compromised sites host the initial phishing pages that lead victims to the fake CAPTCHA interfaces.

Traffic analysis indicates the campaign primarily targets financial institutions and enterprise environments with high-value data assets.

The attackers demonstrate advanced operational security, regularly rotating command and control servers and implementing certificate pinning to prevent traffic interception.

Forensic analysis of compromised systems shows that the attackers leverage legitimate Windows Management Instrumentation (WMI) functionality for persistence, creating scheduled tasks that periodically connect to command servers for instructions.

The malware’s modular architecture allows attackers to deploy specific payloads based on the victim’s environment, ranging from keyloggers and clipboard hijackers to more sophisticated data exfiltration tools that target specific enterprise applications.

Security teams should implement enhanced monitoring for suspicious PowerShell and WMI activity, particularly commands executed via the Run dialog that establish outbound connections.

Organizations are advised to deploy application allowlisting policies and restrict execution from temporary directories where the toolkit typically deploys its initial payloads.

Regular security awareness training should highlight the increasing sophistication of CAPTCHA-based phishing techniques.

Indicators of Compromise (IOC)

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