CVE-2026-9571: Deactivated user accounts can continue to obtain valid OAuth access tokens via refresh token grant in Mattermost

Severity: medium (CVSS 6.5 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: 2026-07-13 · updated 2026-07-15
Weakness: CWE-305

Description

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x <= 10.11.19 fail to invalidate OAuth refresh tokens upon user account deactivation, which allows a deactivated user or an attacker in possession of a valid refresh token to obtain new functional access tokens via the OAuth refresh token grant endpoint.. Mattermost Advisory ID: MMSA-2026-00680

Affected products

Mattermost: from 10.11.0 before 10.11.20; from 11.6.0 before 11.6.5; from 11.7.0 before 11.7.3 — fixed in 10.11.20, 11.6.5, 11.7.3

How to fix

Upgrade to a fixed version: Mattermost 10.11.20, 11.6.5, 11.7.3. Check the vendor advisory for mitigations.

References

https://mattermost.com/security-updates

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