We found two vulnerable implementations:

1. Apple’s Mail on current versions of iOS is vulnerable to what we call an empty line oracle — they expect an empty line between the mail's headers and the body. This is exploitable by an attacker that can sniff (TLS encrypted) IMAP traffic while sending mails to the victim and Mail decrypting them in the background, no user interaction required.

2. Google Workspaces (Google’s hosted S/MIME solution) is vulnerable to a classic Vaudenay padding oracle attack on CBC padding. The oracle is an error message via SMTP. This error message ironically was introduced to prevent the EFAIL attacks.