Need

To prevent denial of service by repeatedly changing user's password.

Context

• Usage of Elixir (1.12.0 and above) for building scalable and fault-tolerant applications
• Usage of Phoenix Framework for building web applications
• Usage of Ecto for data persistence

Description

Non compliant code

        defmodule MyAppWeb.AccountController do
  use MyAppWeb, :controller

  def change_password(conn, %{"password" => password, "user_id" => id}) do
    user = Repo.get!(User, id)
    changeset = User.changeset(user, %{password: password})
    Repo.update!(changeset)
    render(conn, "password_changed.html")
  end
end
        
        

This code is vulnerable because it allows a user's password to be changed without any limit on the number of requests. If an attacker is able to authenticate as a user, they can lock the user out of their account by repeatedly changing the password.

Steps

• Implement a rate limit on password change requests.
• Consider using a library or built-in framework feature to enforce rate limiting.
• Monitor account activity for suspicious behavior such as repeated password change requests.

Compliant code

        defmodule MyAppWeb.AccountController do
  use MyAppWeb, :controller

  def change_password(conn, %{"password" => password, "user_id" => id}) do
    user = Repo.get!(User, id)
    if can_change_password?(user) do
      changeset = User.changeset(user, %{password: password})
      Repo.update!(changeset)
      update_last_password_change(user)
      render(conn, "password_changed.html")
    else
      render(conn, "password_change_limit.html")
    end
  end

  defp can_change_password?(user) do
    # check if user can change password
  end

  defp update_last_password_change(user) do
    # update the time of last password change
  end
end
        
        

In this secure code example, the can_change_password? function checks if the user has already changed their password within the rate limit period. If they have, the password change request is rejected and a different view is rendered.

References

• 296. Weak credential policy - Password Change Limit