Posted: January 12, 2016. Tags: Rails
Here's how to view the contents of your Rails cookie, updated for Rails 4.2. You now must give MessageEncryptor a serializer: NullSerializer option or it tries to deserialize using Marshal and dies.
cookie = "cookie-string-copied-from-browser--with_signature" cookie = CGI.unescape cookie config = Rails.application.config secrets = Rails.application.secrets encrypted_cookie_salt = config.action_dispatch.encrypted_cookie_salt # "encrypted cookie" by default encrypted_signed_cookie_salt = config.action_dispatch.encrypted_signed_cookie_salt # "signed encrypted cookie" by default key_generator = ActiveSupport::KeyGenerator.new secrets.secret_key_base, iterations: 1000 secret = key_generator.generate_key encrypted_cookie_salt sign_secret = key_generator.generate_key encrypted_signed_cookie_salt encryptor = ActiveSupport::MessageEncryptor.new secret, sign_secret, serializer: ActiveSupport::MessageEncryptor::NullSerializer encryptor.decrypt_and_verify cookie
A word of caution about coping cookies our of browsers: If you're coping your cookie out of the Google Chrome Resources -> Cookies page, make sure you triple-click the cookie value before copying it (right-click and select Copy or Cmd-C on Mac OS X). If you just right click it and select Copy, it will only select the first "word" and you'll miss some necessary junk at the end of the cookie. Safari's Resources -> Cookies page has a "Copy Row" that is immune to this problem.