In Redmine before 3.2.7 and 3.3.x before 3.3.4, the reminders function in app/models/mailer.rb does not check whether an issue is visible, which allows remote authenticated users to obtain sensitive information by reading e-mail reminder messages.
https://github.com/redmine/redmine/commit/0f09f161f64f4190a52166675ff380a15b72a8bc