It has been reported that malicious users can do e-mail enumeration on sign in via timing attacks despite paranoid mode being enabled.
Whenever you try to reset your password or confirm your account, Devise gives you precise information on how to proceed, if the e-mail given is valid, if the token has not expired and so on. This means that, by trying any given e-mail, a third-party person can know if a particular e-mail is registered in that website or not.
While this is not a problem for many applications, some applications would like to keep their user information completely private, even if it means loss of usability on features like account confirmation. For such use cases, Devise supports something called paranoid mode, which has been reported to still be vulnerable to enumeration on sign in.
Only applications using Devise paranoid mode need to update. New releases have been made for Devise branches 3.2 (3.2.1), 3.1 (3.1.2), 3.0 (3.0.4) and 2.2 (2.2.8).
Users running on those branches and cannot upgrade immediately can fix this issue by applying this patch. Users running on older versions are recommended to upgrade to a supported branch immediately.
We want to thank Tim Goddard, from YouDo Ltd for reporting the issue and working with us on a fix.