The user gets an e-mail with password recovery link, which includes reset password token. The user clicks this link and is expected to enter a new password twice. Before entering the password the user clicks a link to a picture (https://xkcd.com/936/). When this happens, cross-domain referer leakage takes place.
GET /936/ HTTP/1.1 Host: xkcd.com User-Agent: Mozilla/5.0 (Windows NT 6.2; WOW64; rv:25.0) Gecko/20100101 Firefox/25.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8 Accept-Language: pl,en-us;q=0.7,en;q=0.3 Accept-Encoding: gzip, deflate Referer: https://hackerone.com/users/password/edit?reset_password_token=HERE_IS_THE_VALUE_OF_RESET_PASSWORD_TOKEN Connection: keep-alive
It allows the person who has control of xkcd.com to change the user's password (CSRF attack), because this person knows reset password token of the user, uses a new user's password of his choice and authenticity_token is not needed to make it happen.