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Juice-Shop Write-up: CAPTCHA Bypass

Challenge Overview

Title: CAPTCHA Bypass
Category: Broken Anti-Automation
Difficulty: ⭐⭐⭐ (3/6)

The "CAPTCHA Bypass" challenge requires to circumvent CAPTCHA protections to submit multiple customer feedback entries within a short period.

Tools Used

  • Web Browser: To interact with the feedback submission form.
  • HTTP Interception Tool (e.g., Burp Suite): To capture and manipulate HTTP requests.

Methodology and Solution

Initial Feedback Submission

  1. Submitting a Feedback:

    • Navigate to the feedback submission form and enter a comment, rating, and solve the CAPTCHA challenge as normal. Click submit to send the feedback.
    dummy feedback

Interception and Analysis

  1. Capturing the Submission Request:
    • Use Burp Suite to intercept the HTTP POST request made when submitting feedback. Analyze the request payload and headers.

Identifying CAPTCHA Mechanism Flaws

  1. Analyzing CAPTCHA Implementation:

    • Notice that the CAPTCHA validation in the request relies solely on the captchaId and the provided captcha answer.
    • Determine that the CAPTCHA system might not track or validate previous submissions effectively, possibly allowing the reuse of a single CAPTCHA solution multiple times.
    request for captcha

Automating CAPTCHA Reuse

  1. Replaying Requests:

    • Modify the intercepted POST request to reuse the same captchaId and captcha answer. Change other fields like comment to simulate different feedback entries.
    • Rapidly replay the modified request multiple times (more than 10 times) within 20 seconds to challenge the anti-automation controls.
    modified request

Verifying Challenge Completion

  1. Confirming Multiple Submissions:
    • After submitting multiple feedback entries quickly, check the application’s response or dashboard to confirm that all entries were accepted without requiring new CAPTCHA solutions.

Solution Explanation

The challenge was successfully completed by exploiting a weakness in the CAPTCHA mechanism, where a single validated CAPTCHA solution could be reused multiple times. This indicates a failure in the application’s anti-automation logic, as it should ideally track and validate each CAPTCHA attempt individually to prevent abuse.

Remediation

To prevent such vulnerabilities in real-world applications:

  • CAPTCHA Robustness: Implement CAPTCHA systems that track attempts and ensure each CAPTCHA challenge is only valid for one submission.
  • Enhance CAPTCHA Logic: Consider using more sophisticated CAPTCHA solutions like reCAPTCHA, which includes advanced risk analysis.
  • Rate Limiting: Introduce rate limiting for form submissions to reduce the risk of automated attacks.
  • Session-Based Validation: Ensure that CAPTCHA validations are tied to user sessions and that each session can only use a CAPTCHA solution once.