Zulip vulnerable to insufficient authorization check for edition/deletion of messages and topics in private streams by former subscribers

Vulnerability Description

Zulip is an open-source team collaboration tool with topic-based threading that combines email and chat. Users who used to be subscribed to a private stream and have been removed from it since retain the ability to edit messages/topics, move messages to other streams, and delete messages that they used to have access to, if other relevant organization permissions allow these actions. For example, a user may be able to edit or delete their old messages they posted in such a private stream. An administrator will be able to delete old messages (that they had access to) from the private stream. This issue was fixed in Zulip Server version 7.3.

References

• https://github.com/zulip/zulip/security/advisories/GHSA-q3wg-jm9p-35fj
• https://zulip.readthedocs.io/en/latest/overview/changelog.html#zulip-server-7-3