An issue was discovered in cyrus-imapd in Cyrus IMAP through 3.12.2. The vacation "fcc" feature skips the destination-mailbox ACL. A user whose vacation Sieve script used :fcc (to save a copy of the sent message) could deliver vacation auto-reply copies into any mailbox the script could name, regardless of whether the script owner had insert permissions on the destination mailbox.
https://www.cyrusimap.org/imap/download/release-notes/index.html
https://www.cyrusimap.org/3.12/imap/download/release-notes/3.12/x/3.12.3.html