Devise version before 3.5.4 uses cookies to implement a "Remember me" functionality. However, it generates the same cookie for all devices. If an attacker manages to steal a remember me cookie and the user does not change the password frequently, the cookie can be used to gain access to the application indefinitely.

References

• heartcombo/devise@c929966
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/devise/CVE-2015-8314.yml
• http://blog.plataformatec.com.br/2016/01/improve-remember-me-cookie-expiration-in-devise/
• https://nvd.nist.gov/vuln/detail/CVE-2015-8314
• GHSA-746g-3gfp-hfhw
• https://rubysec.com/advisories/CVE-2015-8314/