CVE-2026-3433: Mattermost fails to scope role_updated websocket events to authorized team and channel members

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-12 · updated 2026-06-18
Weakness: CWE-200

Description

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to restrict role_updated websocket event broadcasts to members of the affected team or channel which allows an authenticated attacker with guest-level access to observe permission scheme change notifications for private teams they are not a member of via the websocket connection.. Mattermost Advisory ID: MMSA-2026-00616

Affected products

Mattermost: from 10.11.0 before 10.11.17; from 11.5.0 before 11.5.5; from 11.6.0 before 11.6.2 — fixed in 10.11.17, 11.5.5, 11.6.2

How to fix

Upgrade to a fixed version: Mattermost 10.11.17, 11.5.5, 11.6.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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