Fix: streamable-http server: a supplied Mcp-Method contradicting an initialize body is silently accepted

crates/rmcp/src/transport/streamable_http_server/tower.rs

@@ -39,8 +39,8 @@ use crate::{
         OneshotTransport, TransportAdapterIdentity,
         common::{
             http_header::{
-                EVENT_STREAM_MIME_TYPE, HEADER_LAST_EVENT_ID, HEADER_MCP_PROTOCOL_VERSION,
-                HEADER_SESSION_ID, JSON_MIME_TYPE,
+                EVENT_STREAM_MIME_TYPE, HEADER_LAST_EVENT_ID, HEADER_MCP_METHOD,
+                HEADER_MCP_PROTOCOL_VERSION, HEADER_SESSION_ID, JSON_MIME_TYPE,
             },
             mcp_headers,
             server_side_http::{
@@ -690,6 +690,69 @@ mod jsonrpc_http_status_tests {
     }
 }
 
+#[cfg(test)]
+mod standard_header_init_tests {
+    use super::*;
+
+    fn initialize_message() -> ClientJsonRpcMessage {
+        ClientJsonRpcMessage::request(
+            ClientRequest::InitializeRequest(InitializeRequest {
+                params: InitializeRequestParams {
+                    protocol_version: ProtocolVersion::STANDARD_HEADERS,
+                    ..Default::default()
+                },
+                ..Default::default()
+            }),
+            RequestId::Number(1),
+        )
+    }
+
+    fn headers_with(mcp_method: Option<&str>) -> HeaderMap {
+        let mut headers = HeaderMap::new();
+        headers.insert(
+            HEADER_MCP_PROTOCOL_VERSION,
+            http::HeaderValue::from_static(ProtocolVersion::STANDARD_HEADERS.as_str()),
+        );
+        if let Some(method) = mcp_method {
+            headers.insert(
+                HEADER_MCP_METHOD,
+                method.parse::<http::HeaderValue>().unwrap(),
+            );
+        }
+        headers
+    }
+
+    fn no_tool_schema(_: &str) -> Option<Arc<JsonObject>> {
+        None
+    }
+
+    /// A supplied Mcp-Method header contradicting an initialize body must be
+    /// rejected at >= STANDARD_HEADERS. Regression test for
+    /// https://github.com/modelcontextprotocol/rust-sdk/issues/1271
+    #[test]
+    fn initialize_rejects_contradicting_mcp_method_header() {
+        let headers = headers_with(Some("tools/list"));
+        assert!(
+            validate_standard_headers(&headers, &initialize_message(), no_tool_schema).is_err()
+        );
+    }
+
+    /// Absence of the Mcp-Method header on initialize stays accepted: clients
+    /// emit SEP-2243 headers only after the version has been negotiated.
+    #[test]
+    fn initialize_accepts_missing_mcp_method_header() {
+        let headers = headers_with(None);
+        assert!(validate_standard_headers(&headers, &initialize_message(), no_tool_schema).is_ok());
+    }
+
+    /// A supplied Mcp-Method header matching the initialize body is accepted.
+    #[test]
+    fn initialize_accepts_matching_mcp_method_header() {
+        let headers = headers_with(Some("initialize"));
+        assert!(validate_standard_headers(&headers, &initialize_message(), no_tool_schema).is_ok());
+    }
+}
+
 fn jsonrpc_message_response(
     message: ServerJsonRpcMessage,
     map_protocol_status: bool,
@@ -724,8 +787,11 @@ fn header_mismatch_jsonrpc_response(
 /// Validates SEP-2243 `Mcp-Method` / `Mcp-Name` / `Mcp-Param-*` headers against the body.
 ///
 /// Only enforced when the request declares a protocol version `>= STANDARD_HEADERS`.
-/// The `initialize` handshake is exempt: clients emit these headers only after the
-/// version has been negotiated. `tool_schema` supplies the called tool's input schema
+/// The `initialize` handshake is exempt from *requiring* them: clients emit these
+/// headers only after the version has been negotiated. But like
+/// `validate_header_matches_init_body`, a *supplied* `Mcp-Method` header that
+/// contradicts the body is rejected — middleboxes route on the header without
+/// parsing the body. `tool_schema` supplies the called tool's input schema
 /// so annotated `Mcp-Param-*` headers can be checked (no schema => those are skipped).
 fn validate_standard_headers(
     headers: &HeaderMap,
@@ -743,6 +809,22 @@ fn validate_standard_headers(
     let request_id = match message {
         ClientJsonRpcMessage::Request(req) => {
             if matches!(&req.request, ClientRequest::InitializeRequest(_)) {
+                // The handshake may omit SEP-2243 headers, but a supplied
+                // Mcp-Method header must still agree with the body: middleboxes
+                // route on the header without parsing the body.
+                if let Some(value) = headers
+                    .get(HEADER_MCP_METHOD)
+                    .and_then(|value| value.to_str().ok())
+                    && value != "initialize"
+                {
+                    return Err(header_mismatch_jsonrpc_response(
+                        Some(req.id.clone()),
+                        format!(
+                            "Mcp-Method header `{value}` does not match body method `initialize`"
+                        ),
+                    )
+                    .into());
+                }
                 return Ok(());
             }
             Some(req.id.clone())