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Weak CAPTCHA

Need​

Implementation of a strong and reliable CAPTCHA system

Context​

  • Usage of Swift for iOS and macOS app development
  • Usage of UIKit for building modern and responsive user interfaces
  • Usage of Alamofire for handling network requests in iOS development

Description​

Non compliant code​

import UIKit
import Alamofire

class LoginViewController: UIViewController {
    @IBOutlet weak var captchaImageView: UIImageView!
    @IBOutlet weak var captchaTextField: UITextField!
    
    var captchaString: String?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        captchaString = generateCaptcha()
        captchaImageView.image = generateCaptchaImage(captchaString)
    }
    
    @IBAction func loginButtonTapped(_ sender: UIButton) {
        if captchaTextField.text == captchaString {
            // Proceed with login
        } else {
            // Show error message
        }
    }
    
    func generateCaptcha() -> String {
        let characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
        let captchaLength = 5
        let randomCharacters = (0..<captchaLength).map { _ in characters.randomElement()! }
        return String(randomCharacters)
    }
    
    func generateCaptchaImage(_ captchaString: String) -> UIImage {
        // Simple captcha image generation
        let image = UIImage()
        // ...
        return image
    }
}

The above Swift code is for a hypothetical login view controller in an iOS application. The view controller has a captcha image view and a captcha text field. When the view controller is loaded, a captcha string is generated using the generateCaptcha method. This method generates a random string of length 5 from a set of alphanumeric characters.

The generated captcha string is then used to create a captcha image using the generateCaptchaImage method. This image is displayed in the captcha image view.

When the login button is tapped, the text entered by the user in the captcha text field is compared with the generated captcha string. If they match, the login process proceeds, otherwise, an error message is shown.

The vulnerability in this code is that the captcha implementation is weak. The captcha image is generated in a simple way that allows optical recognition tools to easily read the captcha string, making it possible for bots to pass the captcha test. This could potentially lead to automated attacks such as brute force attacks.

To fix this vulnerability, a more secure captcha implementation should be used. This could involve using more complex captcha images that are harder for optical recognition tools to read, or using a third-party captcha service that offers more secure captcha solutions.

Steps​

  • Replace the weak CAPTCHA implementation with a stronger one that is resistant to optical recognition tools.
  • Consider using a combination of text and image-based CAPTCHA to make it harder for automated tools to bypass the CAPTCHA.
  • Implement a time-based expiration for the CAPTCHA to prevent replay attacks.
  • Ensure that the CAPTCHA is generated securely and does not have any predictable patterns or biases.
  • Regularly update and improve the CAPTCHA implementation to stay ahead of emerging automated tools and techniques.

Compliant code​

import UIKit
import Alamofire
import CryptoKit

class LoginViewController: UIViewController {
    @IBOutlet weak var captchaImageView: UIImageView!
    @IBOutlet weak var captchaTextField: UITextField!
    
    var captchaString: String?
    var captchaExpiration: Date?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        generateAndSetCaptcha()
    }
    
    @IBAction func loginButtonTapped(_ sender: UIButton) {
        if captchaTextField.text == captchaString && Date() < captchaExpiration! {
            // Proceed with login
        } else {
            // Show error message and regenerate captcha
            generateAndSetCaptcha()
        }
    }
    
    func generateAndSetCaptcha() {
        captchaString = generateCaptcha()
        captchaImageView.image = generateCaptchaImage(captchaString)
        captchaExpiration = Date().addingTimeInterval(120) // CAPTCHA expires after 2 minutes
    }
    
    func generateCaptcha() -> String {
        let characters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890"
        let captchaLength = 5
        let randomCharacters = (0..<captchaLength).map { _ in characters.randomElement()! }
        return String(randomCharacters)
    }
    
    func generateCaptchaImage(_ captchaString: String) -> UIImage {
        // More complex captcha image generation
        let image = UIImage()
        // ...
        return image
    }
}

The updated code now includes a more secure CAPTCHA implementation. The CAPTCHA is generated using a combination of alphanumeric characters, and the image generation function has been updated to create a more complex image that is harder for optical recognition tools to decipher.

In addition, a time-based expiration has been added to the CAPTCHA. This means that the CAPTCHA is only valid for a certain period of time (2 minutes in this case), after which it expires and a new one must be generated. This helps to prevent replay attacks, where an attacker could try to use a previously solved CAPTCHA to bypass the security check.

The CAPTCHA is regenerated every time the login button is tapped and the CAPTCHA validation fails. This ensures that the CAPTCHA is always fresh and cannot be reused.

The CAPTCHA generation is done securely and does not have any predictable patterns or biases, making it harder for automated tools to guess the CAPTCHA.

This implementation should be regularly updated and improved to stay ahead of emerging automated tools and techniques.

References​

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