CVE-2026-9693: Mattermost thread memberships persist after team removal, exposing private channel thread metadata on re-invite

Severity: low (CVSS 3.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-17 · updated 2026-08-19
Weakness: CWE-459

Description

Mattermost versions 10.11.x <= 10.11.20, 11.7.x <= 11.7.5 Mattermost fails to remove thread membership records when a user is removed from or leaves a team, which allows a previously removed user who is later re-invited to the team to view private channel thread root post content and metadata via the team threads API.. Mattermost Advisory ID: MMSA-2026-00682

Affected products

Mattermost: from 10.11.0 before 10.11.21; from 11.7.0 before 11.7.6 — fixed in 10.11.21, 11.7.6

How to fix

Upgrade to a fixed version: Mattermost 10.11.21, 11.7.6. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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