Description

CVE-2026-9739 affects MCP Toolbox deployments that rely on Server-Sent Events for database communication. The issue comes from an unsafe cross-origin configuration in the SSE handler, where a wildcard CORS setting allows any external website to interact with internal endpoints. This breaks normal browser security boundaries and is classified under CWE-942. In practical terms, it means that requests meant to be restricted to trusted origins can instead be triggered from untrusted web pages, potentially exposing internal database connectors and service responses. The risk increases when combined with DNS rebinding attacks. In this scenario, an attacker lures a user into visiting a malicious site that initially resolves to an external address but later switches to an internal network target. Because the MCP Toolbox SSE endpoint does not properly restrict origins, the victim’s browser may still be allowed to communicate with internal services. This can effectively turn a normal browser session into a bridge for reaching protected infrastructure, especially if the user is already authenticated within the enterprise environment. Organizations are advised to upgrade immediately, restrict SSE exposure, and carefully define trusted origins. Additional protections include monitoring unusual internal traffic patterns, disabling unused streaming endpoints, and strengthening network segmentation between browser-accessible services and backend systems. Regular configuration audits and strict CORS enforcement can significantly reduce the risk of DNS rebinding exploitation and help prevent unauthorized access to sensitive enterprise resources.