Auth bypass in Zulip

CVE-2026-100404

Zulip is an open-source team collaboration tool. In Zulip Server versions through 12.1, guest users can receive new message events for public channels outside their subscriptions. A guest can trigger the issue by registering an event queue with parameters that request messages from an unsubscribed public channel, or an existing event queue can remain affected after the guest's access to the channel is removed. The flaw discloses new public-channel messages to the guest. This issue is fixed in version 12.2.

Vulnerability class: Broken Access Control

Published .

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N.

EPSS exploit prediction

No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.

Affected products

  • Zulip — versions < 12.2

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-100404?
CVE-2026-100404 is a medium-severity vulnerability in Zulip, classified under Missing Authorization. CVSS score: 6.5/10. Published 2026-10-14.
How severe is CVE-2026-100404?
Medium severity. CVSS v3 base score is 6.5 out of 10.