CVE-2026-20796: Time-of-check time-of-use vulnerability in common teams API

Severity: low (CVSS 3.1 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: 2026-02-13 · updated 2026-06-17
Weakness: CWE-367

Description

Mattermost versions 10.11.x <= 10.11.9 fail to properly validate channel membership at the time of data retrieval which allows a deactivated user to learn team names they should not have access to via a race condition in the /common_teams API endpoint.. Mattermost Advisory ID: MMSA-2025-00549

Affected products

Mattermost: from 10.11.0 before 10.11.10 — fixed in 10.11.10

How to fix

Upgrade to a fixed version: Mattermost 10.11.10. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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