steps/mail/sendmail.inc in Roundcube before 1.1.7 and 1.2.x before 1.2.3, when no SMTP server is configured and the sendmail program is enabled, does not properly restrict the use of custom envelope-from addresses on the sendmail command line, which allows remote authenticated users to execute arbitrary code via a modified HTTP request that sends a crafted e-mail message.
https://security.gentoo.org/glsa/201612-44
https://roundcube.net/news/2016/11/28/updates-1.2.3-and-1.1.7-released