Zulip events can leak private channel names

Vulnerability Description

Zulip is an open source team chat application. A weekly cron job (added in 50256f48314250978f521ef439cafa704e056539) demotes channels to being "inactive" after they have not received traffic for 180 days. However, upon doing so, an event was sent to all users in the organization, not just users in the channel. This event contained the name of the private channel. Similarly, the same commit (50256f48314250978f521ef439cafa704e056539) added functionality to notify clients when channels stopped being "inactive." The first message sent to a private channel which had not previously had any messages for over 180 days (and were thus already marked "inactive") would leak an event to all users in the organization; this event also contained the name of the private channel. Commits 75be449d456d29fef27e9d1828bafa30174284b4 and a2a1a7f8d152296c8966f1380872c0ac69e5c87e fixed the issue. This vulnerability only existed in `main`, and was not part of any published versions.

References

• https://github.com/zulip/zulip/security/advisories/GHSA-x8cx-3hq5-4wj9
• https://github.com/zulip/zulip/commit/50256f48314250978f521ef439cafa704e056539
• https://github.com/zulip/zulip/commit/75be449d456d29fef27e9d1828bafa30174284b4
• https://github.com/zulip/zulip/commit/a2a1a7f8d152296c8966f1380872c0ac69e5c87e