Hace un par de días que sacaron parche para Thunderbird y se recomienda que si lo usas cifrando el correo actualices.

https://www.debian.org/security/2019/dsa-4523

https://security-tracker.debian.org/tracker/source-package/thunderbird

"Multiple security issues have been found in Thunderbird which could potentially result in the execution of arbitrary code, cross-site scripting, information disclosure and a covert content attack on S/MIME encryption using a crafted multipart/alternative message."