Improper access control in Zulip Server - CVE-2026-40300
Published: April 27, 2026
Vulnerability identifier: #VU127959
CSH Severity: Medium
CVSS v4: 6 [CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-40300
CWE-ID: CWE-284
Exploitation vector: Remote access
Exploit availability:
Public exploit is available
Vulnerability details
The vulnerability allows a remote attacker to gain unauthorized access to otherwise restricted functionality.
The vulnerability exists due to improper access restrictions in "moves only" policy within /api/v1/messages/{id}/history. A remote user can bypass implemented security restrictions and gain unauthorized access to message edit history.
Affected software
Zulip Server
How to mitigate CVE-2026-40300
Install updates from vendor's website.
Zulip Server - update to 12.0