An XSS issue was discovered in packages/rocketchat-mentions/Mentions.js in Rocket.Chat before 0.65. The real name of a username is displayed unescaped when the user is mentioned (using the @ symbol) in a channel or private chat. Consequently, it is possible to exfiltrate the secret token of every user and also admins in the channel.
A reflected XSS issue was discovered in the registration form in Rocket.Chat before 0.66. When one creates an account, the next step will ask for a username. This field will not save HTML control characters but an error will be displayed that shows the attempted username unescaped via packages/rocketchat-ui-login/client/username/username.js in packages/rocketchat-ui-login/client/username/username.html.