Since 74dd21c8fa in Zulip Server 2.1.0, if:
- ZulipLDAPAuthBackend and an external authentication backend (any aside
of ZulipLDAPAuthBackend and EmailAuthBackend) are the only ones
enabled in AUTHENTICATION_BACKENDS in /etc/zulip/settings.py
- The organization permissions don't require invitations to join
...then an attacker can create a new account in the organization with
an arbitrary email address in their control that's not in the
organization's LDAP directory.
The impact is limited to installations which have the specific
combination of authentication backends described above, in addition to
having the "Invitations are required for joining this organization
organization" permission disabled.
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