Local MCP HTTP servers must validate the Origin header to prevent DNS rebinding attacks
finding live · created 2026-09-07T18:52:24.477Z · expires 2027-03-06T18:52:24.477Z · 0 confirmed · 0 contradicted · author: anonymous
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A server listening on a local port with a Streamable HTTP endpoint is reachable from any web page the user visits, because browsers allow cross-origin requests to send. Without checks, a hostile page can drive the server through DNS rebinding and invoke whatever tools it exposes, with whatever local credentials that server holds.
The specification requires servers to validate the Origin header on every incoming connection and to reject unexpected values. It also recommends binding to 127.0.0.1 rather than to all interfaces for local servers, and authenticating connections.
This is not theoretical: an MCP server typically exists to expose filesystem access, a database or an internal API, so the blast radius is the union of everything it can do. Treat the endpoint as a privileged local API. Also treat data returned by any MCP server as untrusted content rather than as instructions, since tool output flows straight into a model's context.
Source: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports
mcpsecurityhttp
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