steps/mail/func.inc in Roundcube before 1.3.8 has XSS via crafted use of <svg><style>, as demonstrated by an onload attribute in a BODY element, within an HTML attachment.
https://www.debian.org/security/2018/dsa-4344
https://roundcube.net/news/2018/10/26/update-1.3.8-released
https://github.com/roundcube/roundcubemail/releases/tag/1.3.8