Blog | G5 Cyber Security

JavaScript CAPTCHA Bypass

G5 Cyber Security

TL;DR

Bypassing a 100% client-side JavaScript CAPTCHA is often possible by inspecting the code, understanding its logic, and manipulating it directly in your browser’s developer tools. This guide shows you how to do that. However, remember this is generally unethical unless you have permission from the website owner.

How to Bypass a JavaScript CAPTCHA

  1. Open Developer Tools: Press F12 (or right-click and select ‘Inspect’) in your browser. Go to the ‘Sources’ or ‘Debugger’ tab.
  2. Find the CAPTCHA Script: Look for JavaScript files related to CAPTCHA, often named something like captcha.js, security.js, or similar. Use the search box (Ctrl+F) within the Sources tab to find keywords like ‘captcha’, ‘challenge’, ‘validate’, etc.
  3. Understand the Logic: Open the CAPTCHA script and read through it. Look for how the CAPTCHA generates challenges, validates responses, and submits the form.
    • Challenge Generation: How is the CAPTCHA question or image created? Is it random numbers, text, images, or something else?
    • Validation Function: What function checks if your answer is correct? This is the key to bypassing the CAPTCHA.
    • Form Submission: How does the script prevent form submission without a valid CAPTCHA response?
  4. Manipulate the Code (Method 1 – Override Validation): The most common approach is to modify the validation function.
    1. Find the validation function. It might look something like this: 
      function validateCaptcha(response) {
  if (response === expectedAnswer) {
    return true;
  } else {
    return false;
  }
}
    2. Change the function to always return true: 
      function validateCaptcha(response) {
  return true;
}
    3. Save the changes (usually by pressing Ctrl+S). The browser should automatically reload the modified script.
  5. Manipulate the Code (Method 2 – Pre-fill Response): If the CAPTCHA requires a specific input field, you can pre-fill it with the correct answer.
    1. Find where the CAPTCHA response is handled. Look for code that reads the value from an input field and uses it in validation.
    2. Set the input field’s value to the expected answer directly in the JavaScript code: 
      document.getElementById('captcha-response').value = 'correctAnswer';
    3. Save the changes (Ctrl+S).
  6. Manipulate the Code (Method 3 – Disable Submission Prevention): Some CAPTCHAs prevent form submission if validation fails. Find and remove or comment out this prevention code.
    1. Look for code that prevents the form from submitting, often using something like event.preventDefault() or setting a flag to disable the submit button. 
      form.submitButton.disabled = true;
    2. Comment out or remove this line: 
      // form.submitButton.disabled = true;
    3. Save the changes (Ctrl+S).
  7. Refresh and Test: Refresh the page and try submitting the form again. If you’ve successfully modified the code, it should now submit without requiring a valid CAPTCHA response.

Important Considerations

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