Zulip 8.3 is vulnerable to Cross Site Scripting (XSS) via the construct_copy_div function in copy_and_paste.js.
https://github.com/zulip/zulip/commit/e1029b59ede0c4f314c367ffa1ba2904ffaf6768
https://github.com/zulip/zulip/blob/8.3/web/src/copy_and_paste.js#L90
https://gist.github.com/1047524396/64720d2aa5afd943eb7e5a1ed4808ad6