Description
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
CVSS breakdown
CVSS 3.1
Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Changed
Confidentiality
High
Integrity
None
Availability
None
Affected products
- Mattermost / Mattermost10.11.0 – 10.11.18
- Mattermost / Mattermost11.6.0 – 11.6.3
- Mattermost / Mattermost11.5.0 – 11.5.6
- Mattermost / Mattermost11.7.0 – 11.7.0
- Mattermost / Mattermost10.11.19 – 10.11.19
- Mattermost / Mattermost11.6.4 – 11.6.4
- Mattermost / Mattermost11.5.7 – 11.5.7