CVE-2026-3115: Guest users can view group member IDs without respecting view restrictions

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: 2026-03-26 · updated 2026-06-17
Weakness: CWE-863

Description

Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to apply view restrictions when retrieving group member IDs, which allows authenticated guest users to enumerate user IDs outside their allowed visibility scope via the group retrieval endpoint.. Mattermost Advisory ID: MMSA-2026-00594

Affected products

Mattermost: from 10.11.0 before 10.11.11; from 11.2.0 before 11.2.3; from 11.3.0 before 11.3.2; 11.4.0 — fixed in 10.11.11, 11.2.3, 11.3.2

How to fix

Upgrade to a fixed version: Mattermost 10.11.11, 11.2.3, 11.3.2. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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