CVE-2026-4339

Name of the Vulnerable Software and Affected Versions Mattermost versions 10.11.0 through 10.11.18 Mattermost versions 11.5.0 through 11.5.6 Mattermost versions 11.6.0 through 11.6.3

Description The Mattermost Agents plugin MCP server fails to validate attachment URLs against internal or private IP ranges. This allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF), a technique where the server is tricked into making requests to an unintended location, and exfiltrate data from internal network services by providing internal URLs as file attachments in post creation requests.

Recommendations Update Mattermost versions 10.11.0 through 10.11.18 to a version newer than 10.11.18. Update Mattermost versions 11.5.0 through 11.5.6 to a version newer than 11.5.6. Update Mattermost versions 11.6.0 through 11.6.3 to a version newer than 11.6.3.