CVE-2026-4339 in Agents Plugin
Summary
by MITRE • 06/26/2026
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
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Analysis
by VulDB Data Team • 06/26/2026
The vulnerability in Mattermost versions 10.11.x through 10.11.18, 11.6.x through 11.6.3, and 11.5.x through 11.5.6 represents a critical server-side request forgery flaw within the Mattermost Agents plugin MCP server implementation. This security weakness specifically affects systems operating in stdio mode where the MCP server processes file attachment requests from users. The root cause stems from inadequate validation of attachment URLs, which fails to properly filter or reject connections to internal or private IP address ranges that should typically be restricted from external access.
The technical flaw manifests when an attacker with access to the MCP server in stdio mode submits malicious post creation requests containing internal URLs as file attachments. The system's failure to validate these URLs against established private IP range boundaries creates an attack vector where the server will attempt to resolve and fetch content from internal network services. This bypass of normal network security controls allows for unauthorized access to services that should remain isolated from external exposure, effectively enabling attackers to probe internal network infrastructure and potentially exfiltrate sensitive data from otherwise protected systems.
This vulnerability directly maps to CWE-918, Server-Side Request Forgery, which is categorized under the broader weakness of insecure direct object references. The operational impact extends beyond simple information disclosure as it provides attackers with the capability to enumerate internal services, potentially leading to further exploitation opportunities within the network perimeter. The attack surface is particularly concerning because it leverages legitimate system functionality to bypass standard network security controls, making detection more difficult and the attack more effective.
The security implications are compounded by the fact that this vulnerability requires minimal privileges to exploit, as attackers only need access to the MCP server in stdio mode rather than elevated system permissions. This makes the attack vector particularly dangerous in environments where multiple users have access to various system components. The potential for data exfiltration increases significantly when internal services such as databases, configuration management systems, or other sensitive back-end services are accessible through the affected network paths.
Organizations should implement immediate mitigations including upgrading to patched versions of Mattermost that address this vulnerability, configuring proper network segmentation to restrict access to MCP server functionality, and implementing strict URL validation policies for all attachment handling processes. Network monitoring solutions should be enhanced to detect unusual outbound requests from the MCP server to internal IP ranges, while also ensuring that proper access controls are in place to limit who can interact with the stdio mode MCP server. The ATT&CK framework categorizes this type of vulnerability under T1566 - Phishing and T1071.1 - Application Layer Protocol: Web Protocols, highlighting the need for comprehensive network security monitoring and application-level protections.