## Motivation and Context
MCP 2025-11-25 (modelcontextprotocol/modelcontextprotocol#1439) requires servers to validate
the `Origin` header on incoming Streamable HTTP connections and recommends binding only to localhost,
to prevent DNS rebinding attacks against locally bound MCP servers.
`StreamableHTTPTransport` previously validated neither `Host` nor `Origin`,
so a malicious web page could rebind its own hostname to 127.0.0.1 and reach a local server cross-origin,
invoking tools and exfiltrating their output. This is disclosed as GHSA-rjr6-rcgv-9m7m.
The TypeScript SDK (`enableDnsRebindingProtection` defaults to `false`, and is deprecated in favor of
external middleware) and the Python SDK (`transport_security.py` disables protection by default)
both ship this off for backwards compatibility, a constraint they cannot escape after 1.0.
Because this gem is still pre-1.0, it adopts the secure default now so protection is baked into the stable baseline:
- `Host` is validated against the loopback defaults (`127.0.0.1`, `::1`, `localhost`) plus any `allowed_hosts:`,
compared case-insensitively; each entry matches the bare host name (any port) or a full `host:port`.
This check is the primary anti-rebinding control, because a rebound request carries a foreign `Host`.
A request without a `Host` header (e.g. HTTP/1.0) is allowed because the rebinding vector always sends one.
- `Origin`, when present, must be same-origin or in `allowed_origins:`. Same-origin compares authority (host:port)
case-insensitively and uses the `Origin`'s own scheme to drop a redundant default port, so `http://example.com` matches
`Host: example.com:80`. Non-browser MCP clients that send no `Origin` header are unaffected.
- `dns_rebinding_protection: false` disables the check when an upstream proxy or middleware already validates these headers.
A rejected request returns HTTP 403 with a JSON-RPC error response carrying no `id`, as the spec permits.
## How Has This Been Tested?
New tests in `test/mcp/server/transports/streamable_http_transport_test.rb`:
- a cross-origin `Origin` is rejected by default, and a same-origin browser request is allowed
- a rebound foreign `Host` is rejected with the `Invalid Host header` body, while loopback values
(`127.0.0.1`, `localhost`, `[::1]`) are allowed
- an upper-cased loopback `Host` is accepted (case-insensitive matching)
- a non-loopback `Host` listed in `allowed_hosts:` is allowed
- a request with no `Host` header, and one with no `Origin` header, are allowed
- a default-port `Origin` (`http://localhost`) matches `Host: localhost:80`
- opaque and malformed `Origin`s (`null`, `file://`, a path suffix) are rejected
- a `host:port` `allowed_hosts:` entry matches that port only
- `Host` validation also applies to GET requests
- `dns_rebinding_protection: false` lets a forged `Host`/`Origin` through
## Breaking Changes
`StreamableHTTPTransport` now rejects non-loopback `Host` and cross-origin `Origin` requests by default.
A deployment behind a reverse proxy or bound to a non-loopback interface must set `allowed_hosts:` / `allowed_origins:`,
or pass `dns_rebinding_protection: false` when validation is handled upstream. Loopback servers and non-browser clients
are unaffected.
Co-authored-by: tonghuaroot <tonghuaroot@gmail.com>
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