Zulip is an open-source team collaboration tool with topic-based threading. Zulip Server version 2.0.0 and above are vulnerable to insufficient access control with multi-use invitations. A Zulip Server deployment which hosts multiple organizations is vulnerable to an attack where an invitation created in one organization (potentially as a role with elevated permissions) can be used to join any other organization. This bypasses any restrictions on required domains on users' email addresses, may be used to gain access to organizations which are only accessible by invitation, and may be used to gain access with elevated privileges. This issue has been patched in release 4.10. There are no known workarounds for this issue. ### Patches Has the problem been patched? What versions should users upgrade to? ### Workarounds Is there a way for users to fix or remediate the vulnerability without upgrading? ### References Are there any links users can visit to find out more? ### For more information If you have any questions or comments about this advisory, you can discuss them on the "developer community Zulip server" (https://zulip.com/developer-community/), or email the "Zulip security team" (mailto:security@zulip.com).