Basecamp's Google Sign-In for Rails allowed redirects to protocol-relative URI

Basecamp's Google Sign-In adds Google sign-in to Rails applications. Prior to version 1.3.1, it is possible to redirect a user to another origin if the "proceed_to" value in the session store is set to a protocol-relative URL. Normally the value of this URL is only written and read by the library or the calling application. However, it may be possible to set this session value from a malicious site with a form submission. Any Rails applications using the google_sign_in gem may be vulnerable, if this vector can be chained with another attack that is able to modify the OAuth2 request parameters. This issue has been patched in version 1.3.1. There are no workarounds.