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User enumeration

Need​

Prevention of user enumeration

Context​

  • Requirement of Ruby 2.0 or higher for running the application
  • Usage of Rails framework for building web applications
  • Usage of ActiveRecord for Object-Relational Mapping (ORM) in database operations

Description​

Non compliant code​

class UsersController < ApplicationController
  def show
    @user = User.find(params[:id])
    if @user
      render json: { user: @user }
    else
      render json: { error: 'User not found' }, status: :not_found
    end
  end
end

The above code is a show action in a UsersController in a Ruby on Rails application. This action is used to fetch a user's details based on the user ID passed in the request parameters.

The vulnerability lies in the way the application handles non-existent users. If a user exists, the application returns the user's details in the response. If the user does not exist, the application returns a different response - an error message stating 'User not found' and a status of :not_found.

This difference in responses can be exploited by an attacker to enumerate valid users. By sending a series of requests with different user IDs, an attacker can distinguish between existent and non-existent users based on the response they receive. For example, if they receive a user's details, they know the user ID is valid. If they receive the 'User not found' error, they know the user ID is not valid.

This is a User Enumeration vulnerability because it allows an attacker to identify valid users in the system. This information can be used for further attacks, such as password guessing attacks, targeted phishing attacks, and more.

Steps​

  • Implement a consistent error response for both existent and non-existent users.
  • Avoid providing specific error messages that disclose user existence.
  • Use a constant response time for all user requests to avoid timing-based enumeration.
  • Implement rate limiting or throttling mechanisms to prevent enumeration through excessive requests.
  • Consider implementing a CAPTCHA or other user verification mechanisms to protect against automated enumeration techniques.
  • Regularly monitor and analyze server logs for any suspicious patterns or unusual activity related to user enumeration.
  • Educate developers about the risks of user enumeration and provide guidelines on secure coding practices.

Compliant code​

class UsersController < ApplicationController
  def show
    @user = User.find_by(id: params[:id])
    if @user
      render json: { user: @user }
    else
      render json: { error: 'An error occurred' }, status: :not_found
    end
  end
end

The updated code implements a consistent error response for both existent and non-existent users. This is achieved by using the find_by method instead of find. The find_by method returns nil if no record is found, unlike find which raises an exception. This allows us to handle both cases in the same way and return a consistent error message.

The error message has been changed from 'User not found' to 'An error occurred'. This is a more generic message that does not disclose whether a user exists or not, preventing user enumeration.

Please note that this code fix is just one part of the solution. To fully protect against user enumeration, you should also consider implementing additional measures such as response time padding, rate limiting, CAPTCHA, and regular log analysis, as mentioned in the steps to fix the vulnerability.

References​

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