A Fake CAPTCHA Appeared in Our Admin Portal: What I Learned About ClickFix Attacks and Third-Party…

Introduction

As developers, we regularly use third-party JavaScript libraries to save time and avoid reinventing the wheel. We install npm packages, integrate chatbots, use CDN-hosted libraries, and depend on external services to build applications faster.

Most of the time, everything works as expected. But what happens when something suspicious suddenly appears inside an application that we believe is secure?

Recently, while working on an admin portal for a property management project, I encountered an unexpected security issue. A page that should have displayed the normal application interface suddenly showed a fake "I'm not a robot" verification screen.

At first, it looked like an ordinary CAPTCHA. However, the next screen revealed something much more concerning: it instructed the user to open Windows PowerShell or Terminal, paste a command, and press Enter to complete verification.

That was the moment I realized this was not a normal CAPTCHA. It was a social engineering technique known as ClickFix.

1. What Happened in Our Project?

The incident occurred in an admin portal built with a JavaScript-based application stack including Node.js. A fake verification screen unexpectedly appeared over the normal application interface.

The user was asked to perform the following steps:

1. Press Windows + X to open the Windows Power User menu.
2. Open PowerShell or Terminal.
3. Press Ctrl + V to paste a command.
4. Press Enter to finish verification.

What Is ClickFix?

ClickFix is a social engineering technique in which attackers trick users into executing malicious commands themselves. Instead of relying entirely on automated exploits, attackers manipulate users into believing that running a command is necessary to fix an error or prove they are human.

2. Understanding the Attack Mechanism & PowerShell Misuse

During the investigation, we analyzed the malicious payload pattern. Attackers often utilize common PowerShell commands like Invoke-RestMethod (or alias IRM) paired with Invoke-Expression (or alias IEX) to fetch and immediately execute remote scripts.

Attackers also use string obfuscation techniques (such as inserting quotes within command strings) to bypass basic antivirus heuristics.

The Golden Rule: A legitimate CAPTCHA will never ask you to open a terminal or run system commands. If a website asks you to do this, immediately close the tab.

3. How Did This Vulnerability Enter the Portal?

When investigating third-party integrations (such as recently added chatbot widgets or frontend helper scripts), it's important to understand how script delivery can become a risk vector.

Instead of pointing fingers at any specific CDN or open-source package, we must evaluate the broader possibilities:

• Subresource Tampering: A script fetched dynamically at runtime from an external server might change without your knowledge.
• Cross-Site Scripting (XSS): Unsanitized inputs allowing unauthorized script execution.
• Compromised Build/Deployment Artifacts: Tampered pipeline assets.
• Browser Extensions: Malicious extensions modifying the DOM locally.

4. How to Prevent and Overcome Such Attacks

To protect Node.js and web applications from script manipulation and ClickFix vectors, teams should implement the following defense-in-depth measures:

Step 1: Host Critical Third-Party Scripts Locally

Instead of referencing dynamic external scripts directly via CDN URLs, bundle verified dependencies into your application via npm or host static, audited copies locally in your build process.

# Prefer installing verified packages via npm/yarn
npm install example-library

# Prefer installing verified packages via npm/yarn
npm install example-library

Step 2: Use Subresource Integrity (SRI)

If you must load a static library from an external host, enforce Subresource Integrity (SRI). SRI ensures that the browser will refuse to execute the file if its hash does not match your expected value.

HTML

<script
  src="https://cdn.example.com/library.min.js"
  integrity="sha384-EXPECTED_HASH_HERE"
  crossorigin="anonymous">
</script>

<script
  src="https://cdn.example.com/library.min.js"
  integrity="sha384-EXPECTED_HASH_HERE"
  crossorigin="anonymous">
</script>

Step 3: Implement a Strict Content Security Policy (CSP)

Use security headers (for example, via helmet in Node.js/Express) to restrict which external domains are permitted to execute scripts or fetch data.

JavaScript

import express from 'express';
import helmet from 'helmet';
const app = express();
app.use(
  helmet({
    contentSecurityPolicy: {
      directives: {
        defaultSrc: ["'self'"],
        scriptSrc: ["'self'"], // Blocks untrusted external script execution
        objectSrc: ["'none'"],
        baseUri: ["'self'"],
      },
    },
  })
);

import express from 'express';
import helmet from 'helmet';
const app = express();
app.use(
  helmet({
    contentSecurityPolicy: {
      directives: {
        defaultSrc: ["'self'"],
        scriptSrc: ["'self'"], // Blocks untrusted external script execution
        objectSrc: ["'none'"],
        baseUri: ["'self'"],
      },
    },
  })
);

Step 4: Sanitize Dynamic Content & Chatbot Outputs

Ensure all dynamic outputs (especially Markdown parsed into HTML) are strictly sanitized before being rendered into the DOM to prevent DOM-based XSS attacks.

Summary Security Checklist

Defensive LayerPrimary BenefitLocal BundlingPrevents unexpected updates to runtime code.SRI CheckingDetects unauthorized modifications on external assets.CSP HeadersRestricts unauthorized script sources and limits XSS impact.Input SanitizationPrevents malicious HTML/JS injection from untrusted inputs.

Conclusion

Modern software development relies on open-source tools and external libraries to remain fast and efficient. However, convenience should never compromise application security.

By shifting from unverified dynamic script loading to controlled bundling, implementing strict CSP headers, and educating users about social engineering tactics like ClickFix, developers can build more resilient applications.

Over to You!

Have you ever encountered a suspicious script injection, fake CAPTCHA, or unexpected third-party JavaScript behavior in your projects? What helped you identify the root cause?

Drop a comment below and share your experience — let's learn from each other!