Drupal core 7.x versions before 7.57 has an external link injection vulnerability when the language switcher block is used. A similar vulnerability exists in various custom and contributed modules. This vulnerability could allow an attacker to trick users into unwillingly navigating to an external site.
https://www.drupal.org/sa-core-2018-001
https://www.debian.org/security/2018/dsa-4123
https://lists.debian.org/debian-lts-announce/2018/02/msg00030.html