CVE-2026-4273: Mattermost Server

Medium severity, CVSS 4.3. EPSS: 0.2% chance of exploitation in the next 30 days.

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to validate that the RefreshedToken differs from the original invite token during remote cluster invite confirmation which allows an authenticated attacker to bypass token rotation and reuse the original invite token via sending a crafted invite confirmation with a RefreshedToken matching the original token. Mattermost Advisory ID: MMSA-2026-00575

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.14 (fixed in 10.11.14); from 11.5.0, before 11.5.2 (fixed in 11.5.2)

Published 2026-05-18. Last modified 2026-06-17.