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User enumeration

Need​

Prevention of user enumeration

Context​

  • Usage of Swift for iOS and macOS app development
  • Usage of UIKit for building user interfaces with a modern design and enhanced functionality
  • Usage of FirebaseAuth for user authentication and authorization in Firebase

Description​

Non compliant code​

import UIKit
import FirebaseAuth

class ViewController: UIViewController {

    @IBOutlet weak var emailField: UITextField!
    @IBOutlet weak var passwordField: UITextField!
    
    // ...

    @IBAction func loginButtonTapped(_ sender: Any) {
        let email = emailField.text
        let password = passwordField.text
        
        Auth.auth().signIn(withEmail: email!, password: password!) { (user, error) in
            if let error = error {
                print("Error: \\(error.localizedDescription)")
                return
            }
            
            if let user = user {
                print("User: \\(user.user.email!) logged in successfully.")
            } else {
                print("Error: User not found.")
            }
        }
    }
    
    // ...
}

In the above Swift code, the application is using Firebase Authentication to handle user login. The vulnerability lies in the loginButtonTapped function, which is triggered when the user taps the login button.

The function takes the email and password entered by the user and attempts to sign in using Firebase's signIn(withEmail:password:completion:) method. If the sign-in process encounters an error (such as the user not being found), it prints an error message to the console.

The problem here is that the error message is different depending on whether the user exists or not. If the user exists but the password is incorrect, the error message will indicate that. If the user does not exist, the error message will say "User not found." This difference in error messages can allow an attacker to determine if a user exists in the system, leading to user enumeration.

Furthermore, the fact that the user's email is printed to the console when login is successful is also a potential privacy concern, as it exposes user data in the logs.

Steps​

  • Implement a consistent error handling mechanism that does not reveal sensitive information.
  • Remove or modify the error messages that disclose whether a user exists or not.
  • Use a generic error message for all authentication failures to avoid providing hints to attackers.
  • Implement a randomized response time for authentication requests to prevent timing attacks.
  • Avoid using different response times or frames count that could be used to enumerate valid users.
  • Consider implementing rate limiting or account lockout mechanisms to prevent brute-force attacks.
  • Regularly review and update the authentication mechanism to address any new vulnerabilities.

Compliant code​

import UIKit
import FirebaseAuth

class ViewController: UIViewController {

    @IBOutlet weak var emailField: UITextField!
    @IBOutlet weak var passwordField: UITextField!
    
    // ...

    @IBAction func loginButtonTapped(_ sender: Any) {
        let email = emailField.text
        let password = passwordField.text
        
        Auth.auth().signIn(withEmail: email!, password: password!) { (user, error) in
            if let _ = error {
                print("Error: Authentication failed.")
                return
            }
            
            if let user = user {
                print("User logged in successfully.")
            } else {
                print("Error: Authentication failed.")
            }
        }
    }
    
    // ...
}

The updated code now implements a consistent error handling mechanism that does not reveal whether a user exists or not. This is achieved by using a generic error message "Error: Authentication failed." for all authentication failures, regardless of whether the failure was due to a non-existent user or incorrect password. This prevents attackers from enumerating valid users via error messages.

The code also removes the disclosure of the user's email upon successful login. This is a good practice as it minimizes the amount of sensitive user information that is outputted by the system.

In addition to these changes, it is recommended to implement a randomized response time for authentication requests and consider implementing rate limiting or account lockout mechanisms to further enhance the security of the system. Regular reviews and updates of the authentication mechanism should also be carried out to address any new vulnerabilities.

References​

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