Zulip Server has access control bypass for restrictions on creation of specific channel types

Vulnerability Description

Zulip is an open-source team chat application. Starting in version 10.0 and prior to version 10.3, the "Who can create public channels" access control mechanism can be circumvented by creating a private or web-public channel, and then changing the channel privacy to public. A similar technique works for creating private channels without permission, though such a process requires either the API or modifying the HTML, as we do mark the "private" radio button as disabled in such cases. Version 10.3 contains a patch.

References

• https://github.com/zulip/zulip/security/advisories/GHSA-rqg7-xfqg-v7q5
• https://github.com/zulip/zulip/commit/d2ff4bda4c3efa30fc3ab1f151255cfdbf370f78
• https://zulip.com/help/configure-who-can-create-channels
• https://zulip.readthedocs.io/en/latest/overview/changelog.html#zulip-server-10-3