CVE-2026-4339: Mattermost Server

Medium severity, CVSS 6.5. EPSS: 0.1% chance of exploitation in the next 30 days.

Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisory ID: MMSA-2026-00635

Affected products

• Mattermost Mattermost Server: from 10.11.0, before 10.11.19 (fixed in 10.11.19); from 11.5.0, before 11.5.7 (fixed in 11.5.7); from 11.6.0, before 11.6.4 (fixed in 11.6.4)

Published 2026-06-26. Last modified 2026-06-29.