Rocket.Chat 6.12.0, 6.11.2, 6.10.5, 6.9.6, 6.8.6, 6.7.8, and before is vulnerable to a message forgery / impersonation issue. Attackers can abuse the UpdateOTRAck method to send ephemeral messages as if they were any other user they choose.
https://github.com/RocketChat/Rocket.Chat/pull/33246
https://docs.rocket.chat/docs/rocketchat-security-fixes-updates-and-advisories