CVE-2026-9699: Mattermost

Medium severity, CVSS 6.8. EPSS: 0.5% chance of exploitation in the next 30 days.

Mattermost Plugins versions <=11.6 10.18.11 11.3.6 11.6.5.0 fail to sanitize error responses from the OpenAI API before logging, which allows a user with access to server logs or support packets to obtain a valid or partially reconstructable OpenAI API key via inspection of mattermost.log entries generated during authentication failures. Mattermost Advisory ID: MMSA-2026-00609

Affected products

• Mattermost Mattermost: up to and including 10.18.11; up to and including 11.3.6; up to and including 11.6.5

Published 2026-06-26. Last modified 2026-06-26.