Security Advisory

CVE-2026-4339

CVE vulnerability detail - eXtreme Datacenter Security Operations

Published 2026-06-26 14:44:58

Last updated 2026-06-26 15:40:33

Assigner Mattermost

CVSS score 6.5

State PUBLISHED

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

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