CVE-2026-20796: 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…

PriorityP413low3.1CVSS 3.1

AVNACHPRLUINSUCLINAN

EPSS

0.20%

9.9th percentile

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

4 ranges

Vendor	Product	Version range	Fixed in
github.com	mattermost_mattermost-server	>= 10.11.0 < 10.11.10	10.11.10
github.com	mattermost_mattermost-server	>= 10.11.0+incompatible < 10.11.10+incompatible	10.11.10+incompatible
mattermost	mattermost	10.11.0 – 10.11.9	—
mattermost	mattermost_server	>= 10.11.0 < 10.11.10	10.11.10

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