CVE-2026-4339

Mattermost CPT: CVE-2026-4339 affects Mattermost versions 10.11.x up to 10.11.18, 11.6.x up to 11.6.3, and 11.5.x up to 11.5.6. The vulnerability arises from the Agents plugin MCP server failing to validate attachment URLs against internal/private IP ranges, enabling an attacker with MCP stdio ac...