rmcp: return forbidden for invalid origin

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

@@ -893,13 +893,13 @@ fn validate_origin_header(
         .inspect_err(|_| {
             tracing::warn!(origin = ?origin_header, "rejected request with non-UTF-8 Origin header");
         })
-        .map_err(|_| bad_request_response("Bad Request: Invalid Origin header encoding"))?;
+        .map_err(|_| forbidden_response("Forbidden: Invalid Origin header encoding"))?;
     let origin = parse_origin_value(origin_str).ok_or_else(|| {
         tracing::warn!(
             origin = origin_str,
             "rejected request with malformed Origin header",
         );
-        bad_request_response("Bad Request: Invalid Origin header")
+        forbidden_response("Forbidden: Invalid Origin header")
     })?;
     if !origin_is_allowed(&origin, allowed_origins) {
         tracing::warn!(

crates/rmcp/tests/test_custom_headers.rs

@@ -1127,7 +1127,10 @@ mod origin_validation {
     use std::sync::Arc;
 
     use bytes::Bytes;
-    use http::{Method, Request, header::CONTENT_TYPE};
+    use http::{
+        Method, Request,
+        header::{CONTENT_TYPE, HeaderValue, ORIGIN},
+    };
     use http_body_util::Full;
     use rmcp::{
         handler::server::ServerHandler,
@@ -1199,6 +1202,24 @@ mod origin_validation {
         assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
     }
 
+    #[tokio::test]
+    async fn malformed_origin_is_forbidden() {
+        let service = service_with_allowed_origins(&["http://localhost:8080"]);
+        let response = service.handle(init_request(Some("not-an-origin"))).await;
+        assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
+    }
+
+    #[tokio::test]
+    async fn non_utf8_origin_is_forbidden() {
+        let service = service_with_allowed_origins(&["http://localhost:8080"]);
+        let mut request = init_request(None);
+        request
+            .headers_mut()
+            .insert(ORIGIN, HeaderValue::from_bytes(&[0xff]).unwrap());
+        let response = service.handle(request).await;
+        assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
+    }
+
     #[tokio::test]
     async fn missing_origin_passes_through() {
         let service = service_with_allowed_origins(&["http://localhost:8080"]);