We love responsible reports of (potential) security issues in Zulip, whether in the latest release or our development branch.
Our security contact is security@zulip.com. Reporters should expect a response within 24 hours.
Please include details on the issue and how you'd like to be credited in our release notes when we publish the fix.
Our security model document may be a helpful resource.
We send security announcements to our announcement mailing list. If you are running Zulip in production, you should subscribe, by clicking "Join group" at the top of that page.
Zulip provides security support for the latest major release, in the form of minor security/maintenance releases.
We work hard to make upgrades reliable, so that there's no reason to run older major releases.
See also our documentation on the Zulip release lifecycle.