I simply added a <input value=USER_ID name=public_key[user_id]> field to Public key update form, where USER_ID = 4223 (from https://api.github.com/users/rails).
@key = PublicKey.find(params[:id])
@key.update_attributes(params[:public_key]) #Oh no! We passed public_key[user_id] of our victim!
Now our victim (Rails) has our public key associated with their account. You can read/write in any public/private repo on github.
Thoughts on this from 2014:
it was one of my first hacks and I didn't know how to behave. I was angry because nobody wanted to take me and mass-assignment issue seriously. After I did the commit this vulnerability was fixed on github within 1 hour and in rails within 5 hours. This was really effective, many people learned about the bug and fixed it in their apps, but I still regret about this irresponsible disclosure.