| Package | Affected versions | Patched versions |
|---|---|---|
| io.modelcontextprotocol.sdk:mcp-core | ≥ 1.0.0, < 1.0.1≥ 1.1.0, < 1.1.1< 0.18.3 | 1.0.11.1.10.18.3 |
| mcp java sdk | = 1.1.0< 1.0.1 | 1.0.1 |
Hardcoded Wildcard CORS (Access-Control-Allow-Origin: * )
An attacker-controlled web page instructs the victim's browser to open GET https://internal-mcp-server/sse. Because Access-Control-Allow-Origin: * allows cross-origin SSE reads, the attacker's page receives the endpoint event — which contains the session ID. The attacker can then POST to that endpoint from their page using the victim's browser as a relay.
No Access-Control-Allow-Origin header is emitted by either Python transport. The browser's default same-origin policy remains in full effect. https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/sse.py https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py
In the SDK, the transport layer should not own CORS policy. Server implementors who need cross-origin access can add a CORS filter at the servlet filter or Spring Security layer.
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