CVE-2026-6957: Mattermost Legal Hold

Medium severity, CVSS 4.9. EPSS: 0.5% chance of exploitation in the next 30 days.

Mattermost Plugins versions <=1.1.5 fail to sanitize filenames received from federated peers before using them to construct export destination paths, which allows an administrator of a remote federated Mattermost server to write files to arbitrary locations within the target server's filestore via a malicious filename delivered through the shared-channel attachment sync protocol. Mattermost Advisory ID: MMSA-2026-00659

Affected products

• Mattermost Legal Hold: up to and including 1.1.5

Published 2026-05-27. Last modified 2026-06-17.