An issue was discovered in KDE KMail before 19.12.3. By using the proprietary (non-RFC6068) "mailto?attach=..." parameter, a website (or other source of mailto links) can make KMail attach local files to a composed email message without showing a warning to the user, as demonstrated by an attach=.bash_history value.
The S/MIME specification allows a Cipher Block Chaining (CBC) malleability-gadget attack that can indirectly lead to plaintext exfiltration, aka EFAIL.