fix: explain transport host rejections

README.v2.md

@@ -1371,6 +1371,28 @@ This configuration is necessary because:
 - Browsers restrict access to response headers unless explicitly exposed via CORS
 - Without this configuration, browser-based clients won't be able to read the session ID from initialization responses
 
+#### Reverse Proxy Host Headers
+
+DNS rebinding protection checks the incoming `Host` header when transport security is enabled. If your server is behind
+nginx, Cloudflare, or another reverse proxy, include the public hostname in `TransportSecuritySettings.allowed_hosts`.
+Some proxies preserve the port, so include both forms when needed:
+
+```python
+from mcp.server.transport_security import TransportSecuritySettings
+
+transport_security = TransportSecuritySettings(
+    allowed_hosts=[
+        "mcp.example.com",
+        "mcp.example.com:443",
+    ],
+)
+
+mcp_app = server.streamable_http_app(transport_security=transport_security)
+```
+
+If a request is rejected by this check, the server returns HTTP 421 with `host_not_allowed`, the received host, and the
+setting to configure.
+
 ### Mounting to an Existing ASGI Server
 
 By default, SSE servers are mounted at `/sse` and Streamable HTTP servers are mounted at `/mcp`. You can customize these paths using the methods described below.

src/mcp/server/transport_security.py

@@ -4,7 +4,7 @@
 
 from pydantic import BaseModel, Field
 from starlette.requests import Request
-from starlette.responses import Response
+from starlette.responses import JSONResponse, Response
 
 logger = logging.getLogger(__name__)
 
@@ -106,7 +106,14 @@ async def validate_request(self, request: Request, is_post: bool = False) -> Res
         # Validate Host header
         host = request.headers.get("host")
         if not self._validate_host(host):
-            return Response("Invalid Host header", status_code=421)
+            return JSONResponse(
+                {
+                    "error": "host_not_allowed",
+                    "received_host": host,
+                    "configure": "TransportSecuritySettings.allowed_hosts",
+                },
+                status_code=421,
+            )
 
         # Validate Origin header
         origin = request.headers.get("origin")

tests/interaction/transports/test_hosting_http.py

@@ -330,7 +330,16 @@ async def test_origin_validation_rejects_disallowed_origins_when_enabled() -> No
             assert [event async for event in ok.aiter_sse()]
 
     assert (bad_origin.status_code, bad_origin.text) == snapshot((403, "Invalid Origin header"))
-    assert (bad_host.status_code, bad_host.text) == snapshot((421, "Invalid Host header"))
+    assert (bad_host.status_code, bad_host.json()) == snapshot(
+        (
+            421,
+            {
+                "error": "host_not_allowed",
+                "received_host": "evil.example",
+                "configure": "TransportSecuritySettings.allowed_hosts",
+            },
+        )
+    )
 
     async with mounted_app(
         Server("unguarded"), transport_security=TransportSecuritySettings(enable_dns_rebinding_protection=False)

tests/server/test_sse_security.py

@@ -122,7 +122,11 @@ async def test_sse_security_invalid_host_header(server_port: int):
         async with httpx.AsyncClient() as client:
             response = await client.get(f"http://127.0.0.1:{server_port}/sse", headers=headers)
             assert response.status_code == 421
-            assert response.text == "Invalid Host header"
+            assert response.json() == {
+                "error": "host_not_allowed",
+                "received_host": "evil.com",
+                "configure": "TransportSecuritySettings.allowed_hosts",
+            }
 
     finally:
         process.terminate()
@@ -232,7 +236,11 @@ async def test_sse_security_custom_allowed_hosts(server_port: int):
         async with httpx.AsyncClient() as client:
             response = await client.get(f"http://127.0.0.1:{server_port}/sse", headers=headers)
             assert response.status_code == 421
-            assert response.text == "Invalid Host header"
+            assert response.json() == {
+                "error": "host_not_allowed",
+                "received_host": "evil.com",
+                "configure": "TransportSecuritySettings.allowed_hosts",
+            }
 
     finally:
         process.terminate()

tests/server/test_streamable_http_security.py

@@ -126,7 +126,11 @@ async def test_streamable_http_security_invalid_host_header(server_port: int):
                 headers=headers,
             )
             assert response.status_code == 421
-            assert response.text == "Invalid Host header"
+            assert response.json() == {
+                "error": "host_not_allowed",
+                "received_host": "evil.com",
+                "configure": "TransportSecuritySettings.allowed_hosts",
+            }
 
     finally:
         process.terminate()
@@ -269,7 +273,11 @@ async def test_streamable_http_security_get_request(server_port: int):
         async with httpx.AsyncClient(timeout=5.0) as client:
             response = await client.get(f"http://127.0.0.1:{server_port}/", headers=headers)
             assert response.status_code == 421
-            assert response.text == "Invalid Host header"
+            assert response.json() == {
+                "error": "host_not_allowed",
+                "received_host": "evil.com",
+                "configure": "TransportSecuritySettings.allowed_hosts",
+            }
 
         # Test GET request with valid host header
         headers = {

tests/server/test_transport_security.py

@@ -48,6 +48,20 @@ async def test_validate_request_checks_host_then_origin(
     assert (None if response is None else response.status_code) == expected
 
 
+@pytest.mark.anyio
+async def test_validate_request_explains_host_rejection() -> None:
+    middleware = TransportSecurityMiddleware(SETTINGS)
+    response = await middleware.validate_request(_request("evil.example", None))
+
+    assert response is not None
+    assert response.status_code == 421
+    assert response.media_type == "application/json"
+    assert response.body == (
+        b'{"error":"host_not_allowed","received_host":"evil.example",'
+        b'"configure":"TransportSecuritySettings.allowed_hosts"}'
+    )
+
+
 @pytest.mark.anyio
 async def test_validate_request_skips_host_and_origin_when_protection_is_disabled() -> None:
     """With DNS-rebinding protection off, any Host/Origin is accepted."""