Mail clients may leak plaintext messages while decrypting OpenPGP and S/MIME messages.
Email clients supporting the OpenPGP or S/MIME standards may be vulnerable to a CBC/CFB gadget attack which may allow an attacker to inject content into an encrypted email which would establish an exfiltration channel when decrypted by the victim's email client. For example, injecting an HTML image tag which, when rendered by the email client, sends the plaintext as part of an HTTP request.
CVE-2017-17688: OpenPGP CFB Attacks
A remote attack could recover plaintext from encrypted emails without access to the encryption keys.
The CERT/CC is currently unaware of a practical solution to this problem. However, there are some mitigations that may be taken:
Decrypt mail outside of mail client
Credit is attributed to Damian Poddebniak, Christian Dresen, Jens Muller, Fabian Ising, Sebastian Schinzel1, Simon Friedberger, Juraj Somorovsky, and Jorg Schwenk