Critical MCP Toolbox Vulnerability Exposes Enterprise Databases

Security researchers disclosed CVE-2026-9739 affecting the open-source MCP Toolbox: a hardcoded wildcard access-control header in the Server-Sent Events handler overrides CORS protections, allowing malicious websites to bypass security controls, hijack sessions, and proxy requests to exfiltrate data from linked databases (e.g., Postgres, BigQuery). Impact primarily affects deployments using the v2024-11-05 protocol; remediation is to remove the hardcoded header so global middleware correctly enforces origin restrictions.