A10 Unvalidated Redirects and Forwards (redirect_to)

Description

Applications frequently redirect users to other pages, or use internal forwards in a similar manner. Sometimes the target page is specified in an unvalidated parameter, allowing attackers to choose the destination page. Detecting unchecked redirects is easy. Look for redirects where you can set the full URL. Unchecked forwards are harder, because they target internal pages.

Railsgoat allows the redirection to the paths previously requested but for which the user did not have access. Following authentication, the user is redirected.

Bug

The application performs zero validation of the path for which they will redirect users, following authentication. The URL parameter is used to determine where to redirect the user, if the url parameter is not present, the user will be redirect to their home page.

app/controllers/sessions_controller.rb

def create
  path = params[:url].present? ? params[:url] : home_dashboard_index_path
  begin
    # Normalize the email address, why not
    user = User.authenticate(params[:email].to_s.downcase, params[:password])
   # @url = params[:url]
  rescue Exception => e
  end
  
  if user
    session[:user_id] = user.user_id if User.where(:user_id => user.user_id).exists?
    redirect_to path
  else
    # Removed this code, just doesn't seem specific enough!
    #  flash[:error] = "Either your username and password is incorrect"
    flash[:error] = e.message
    render "new"
  end
end

Solution

Unvalidated Redirects and Forwards - ATTACK

Ensure you are logged out of the application. When requesting the login page, ensure you append a url=. Then, authenticate to the application. Once authenticated, you should be redirected to your test url.

Unvalidated Redirects and Forwards - SOLUTION

To fix this vulnerability, validate the path. In our case, we really only want to redirect users to our site so the TLD is not important. In this case, leveraging URI.parse() can be incredibly helpful. We can change the code to something like:

path = home_dashboard_index_path
begin
 if params[:url].present?
  path = URI.parse(params[:url]).path
 end
rescue
end

Further validation can occur with regular expression. If you must redirect to another application, remember to use URI.parse() and the host, path, and scheme (ssl or not) options FIRST, prior to performing regular expression validation. Additionally, always open and close your validation regexp using Ruby anchor tags \A and \z.

Hint

Read the description section, fairly big hint there.