CWE-804: Guessable CAPTCHA

What is Guessable CAPTCHA?

• Overview: Guessable CAPTCHA is a security vulnerability where a CAPTCHA challenge intended to differentiate between human users and automated bots can be easily guessed or bypassed by non-human actors. This undermines its purpose as a security measure.

• Exploitation Methods:

• Attackers can design scripts or bots to automatically recognize and solve CAPTCHA challenges.
• Common attack patterns include using machine learning to process and solve image CAPTCHAs, exploiting predictable patterns in question-based CAPTCHAs, or utilizing databases for trivia-based questions.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized access to systems and automated execution of tasks meant to be human-restricted, such as account creation or posting spam.
• Potential cascading effects include an increase in spam, abuse of system resources, and overloading services.
• Business impact could involve reputational damage, loss of user trust, and increased operational costs due to the mitigation of attacks.

• Prevention Guidelines:

• Specific code-level fixes include increasing the complexity and variability of CAPTCHA challenges and ensuring that audio and visual distortions are sufficient to resist automated recognition.
• Security best practices involve regularly updating CAPTCHA mechanisms and incorporating adaptive challenges that adjust difficulty based on user behavior.
• Recommended tools and frameworks include using advanced CAPTCHA services like reCAPTCHA, which leverage behavioral analysis and machine learning to improve security.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Web Server

Vulnerable Code Example

Certainly! Let's improve the examples to ensure they clearly demonstrate the vulnerability and the fix while adhering to best practices for Python. We will also address formatting issues and ensure comments are thorough and informative.

# captcha_service.py {15-19}
import random

def generate_captcha():
    # Vulnerable CAPTCHA generation using predictable patterns
    # This generates a 4-digit numeric CAPTCHA which is easily guessable
    captcha = str(random.randint(1000, 9999))  # Only 9000 possible combinations
    return captcha

def verify_captcha(user_input, actual_captcha):
    # Simple comparison for CAPTCHA verification
    return user_input == actual_captcha

Explanation

• Issue: The generate_captcha function creates a CAPTCHA using a 4-digit numeric code. This is highly predictable and can be brute-forced by bots due to the small number of possible combinations (only 9000).
• Impact: Automated bots can easily guess the CAPTCHA, bypassing any security measures that rely on it.

How to fix Guessable CAPTCHA?

To fix this vulnerability, we should use a more complex and less predictable CAPTCHA. Enhancements can include:

1. Alphanumeric CAPTCHA: Use a mix of letters and numbers to increase complexity.
2. Image-based CAPTCHA: Implement CAPTCHAs that require user interaction with images.
3. Use of CAPTCHA Libraries: Utilize established libraries or services that provide more secure CAPTCHA solutions.
4. Rate Limiting: Implement rate limiting to prevent bots from making numerous guesses in a short period.

Fixed Code Example

# captcha_service.py {14-21}
import string
import random
from captcha.image import ImageCaptcha  # Using an external library for better CAPTCHA

def generate_secure_captcha():
    # More secure CAPTCHA generation using alphanumeric characters
    # This increases the complexity significantly
    characters = string.ascii_letters + string.digits  # Mix of letters and numbers
    captcha_text = ''.join(random.choice(characters) for _ in range(6))  # 6-character CAPTCHA
    image = ImageCaptcha()  # Generate image-based CAPTCHA
    image_data = image.generate_image(captcha_text)  # Create the CAPTCHA image
    image_data.save(f"{captcha_text}.png")  # Save the image with the CAPTCHA text as filename
    return captcha_text

def verify_captcha(user_input, actual_captcha):
    # Secure verification by comparing user input with the actual CAPTCHA
    return user_input == actual_captcha

Explanation

• Secure CAPTCHA Generation: The fixed code generates a 6-character alphanumeric CAPTCHA, significantly increasing the complexity and making brute force attacks much harder.
• Use of Libraries: The captcha library provides a mechanism to generate CAPTCHAs as images, adding an additional layer of difficulty for automated bots.
• Increased Complexity: By including both letters and numbers and using a longer string, the CAPTCHA becomes far less predictable.

By implementing these changes, the CAPTCHA is much more resistant to guessing and automated attacks.