CVE-2026-9597: Deactivated guest accounts can authenticate via magic-link token in Mattermost REST API login endpoint

Severity: medium (CVSS 5.4 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: 2026-07-13 · updated 2026-07-13
Weakness: CWE-305

Description

Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4 fail to verify whether a guest account is deactivated before creating a session in the magic-link token login path, which allows a deactivated guest user to obtain a fully functional session via a magic-link token issued prior to deactivation.. Mattermost Advisory ID: MMSA-2026-00681

Affected products

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

How to fix

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

References

https://mattermost.com/security-updates

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