Executive brief
Mattermost is a collaboration platform used for team communication and workflow management. A security flaw in the Mattermost Agents plugin allows an attacker to trick the server into making unauthorized requests to internal network services. This could lead to the exposure of sensitive data from private internal systems that are not normally accessible from the internet.
Technical details
A Server-Side Request Forgery (SSRF) vulnerability exists in the Mattermost Agents plugin MCP server. The component fails to properly validate attachment URLs against internal or private IP ranges when processing post creation requests. An attacker with access to the MCP server in stdio mode can supply malicious internal URLs as file attachments. This allows the attacker to bypass network boundaries and exfiltrate data from internal services. Users should update to the latest patched versions provided by Mattermost to mitigate this risk.
Affected products
- Mattermost Mattermost Server 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6
Timeline
- 2026-06-26: advisory: MMSA-2026-00635 published by Mattermost
- 2026-06-26: disclosed: CVE-2026-4339 published to NVD