fix(http): fall back to :authority for HTTP/2

Author: DaleSeo

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

@@ -328,34 +328,43 @@ fn bad_request_response(message: &str) -> BoxResponse {
         .expect("failed to build bad request response")
 }
 
-fn parse_host_header(headers: &HeaderMap) -> Result<NormalizedAuthority, BoxResponse> {
-    let Some(host) = headers.get(http::header::HOST) else {
-        tracing::warn!("rejected request with missing Host header");
-        return Err(bad_request_response("Bad Request: missing Host header"));
-    };
-
-    let host_str = host
-        .to_str()
-        .inspect_err(|_| {
-            tracing::warn!(host = ?host, "rejected request with non-UTF-8 Host header");
-        })
-        .map_err(|_| bad_request_response("Bad Request: Invalid Host header encoding"))?;
-    let authority = http::uri::Authority::try_from(host_str)
-        .inspect_err(|_| {
-            tracing::warn!(
-                host = host_str,
-                "rejected request with malformed Host header"
-            );
-        })
-        .map_err(|_| bad_request_response("Bad Request: Invalid Host header"))?;
+fn parse_host_header(
+    uri: &http::Uri,
+    headers: &HeaderMap,
+) -> Result<NormalizedAuthority, BoxResponse> {
+    if let Some(host) = headers.get(http::header::HOST) {
+        let host_str = host
+            .to_str()
+            .inspect_err(|_| {
+                tracing::warn!(host = ?host, "rejected request with non-UTF-8 Host header");
+            })
+            .map_err(|_| bad_request_response("Bad Request: Invalid Host header encoding"))?;
+        let authority = http::uri::Authority::try_from(host_str)
+            .inspect_err(|_| {
+                tracing::warn!(
+                    host = host_str,
+                    "rejected request with malformed Host header"
+                );
+            })
+            .map_err(|_| bad_request_response("Bad Request: Invalid Host header"))?;
+        return Ok(normalize_authority(authority.host(), authority.port_u16()));
+    }
+    // HTTP/2 carries the host in the `:authority` pseudo-header; middleware
+    // such as `axum::Router::nest` can drop the `Host` header that hyper
+    // synthesizes from it. Fall back to the URI authority directly.
+    let authority = uri.authority().ok_or_else(|| {
+        tracing::warn!("rejected request with missing Host header and no :authority");
+        bad_request_response("Bad Request: missing Host header")
+    })?;
     Ok(normalize_authority(authority.host(), authority.port_u16()))
 }
 
 fn validate_dns_rebinding_headers(
+    uri: &http::Uri,
     headers: &HeaderMap,
     config: &StreamableHttpServerConfig,
 ) -> Result<(), BoxResponse> {
-    let host = parse_host_header(headers)?;
+    let host = parse_host_header(uri, headers)?;
     if !host_is_allowed(&host, &config.allowed_hosts) {
         tracing::warn!(
             host = ?host,
@@ -806,7 +815,9 @@ where
         B: Body + Send + 'static,
         B::Error: Display,
     {
-        if let Err(response) = validate_dns_rebinding_headers(request.headers(), &self.config) {
+        if let Err(response) =
+            validate_dns_rebinding_headers(request.uri(), request.headers(), &self.config)
+        {
             return response;
         }
         let method = request.method().clone();

crates/rmcp/tests/test_custom_headers.rs

@@ -1031,6 +1031,95 @@ async fn test_server_validates_host_header_port_for_dns_rebinding_protection() {
     assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
 }
 
+/// Integration test: Verify the validator falls back to the URI authority when
+/// the Host header is absent (HTTP/2 :authority pseudo-header scenario).
+#[tokio::test]
+#[cfg(all(feature = "transport-streamable-http-server", feature = "server",))]
+async fn test_server_falls_back_to_uri_authority_when_host_header_missing() {
+    use std::sync::Arc;
+
+    use bytes::Bytes;
+    use http::{Method, Request, header::CONTENT_TYPE};
+    use http_body_util::Full;
+    use rmcp::{
+        handler::server::ServerHandler,
+        model::{ServerCapabilities, ServerInfo},
+        transport::streamable_http_server::{
+            StreamableHttpServerConfig, StreamableHttpService, session::local::LocalSessionManager,
+        },
+    };
+    use serde_json::json;
+
+    #[derive(Clone)]
+    struct TestHandler;
+
+    impl ServerHandler for TestHandler {
+        fn get_info(&self) -> ServerInfo {
+            ServerInfo::new(ServerCapabilities::builder().build())
+        }
+    }
+
+    let service = StreamableHttpService::new(
+        || Ok(TestHandler),
+        Arc::new(LocalSessionManager::default()),
+        StreamableHttpServerConfig::default(),
+    );
+
+    let init_body = json!({
+        "jsonrpc": "2.0",
+        "id": 1,
+        "method": "initialize",
+        "params": {
+            "protocolVersion": "2025-03-26",
+            "capabilities": {},
+            "clientInfo": {
+                "name": "test-client",
+                "version": "1.0.0"
+            }
+        }
+    });
+
+    // Allowed authority via URI only — no Host header.
+    let allowed_request = Request::builder()
+        .method(Method::POST)
+        .uri("http://localhost:8080/")
+        .header("Accept", "application/json, text/event-stream")
+        .header(CONTENT_TYPE, "application/json")
+        .body(Full::new(Bytes::from(init_body.to_string())))
+        .unwrap();
+    assert!(allowed_request.headers().get("Host").is_none());
+
+    let response = service.handle(allowed_request).await;
+    assert_eq!(response.status(), http::StatusCode::OK);
+
+    // Disallowed authority via URI only — no Host header.
+    let bad_request = Request::builder()
+        .method(Method::POST)
+        .uri("http://attacker.example/")
+        .header("Accept", "application/json, text/event-stream")
+        .header(CONTENT_TYPE, "application/json")
+        .body(Full::new(Bytes::from(init_body.to_string())))
+        .unwrap();
+    assert!(bad_request.headers().get("Host").is_none());
+
+    let response = service.handle(bad_request).await;
+    assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
+
+    // Neither Host header nor URI authority — still a 400.
+    let missing_request = Request::builder()
+        .method(Method::POST)
+        .uri("/")
+        .header("Accept", "application/json, text/event-stream")
+        .header(CONTENT_TYPE, "application/json")
+        .body(Full::new(Bytes::from(init_body.to_string())))
+        .unwrap();
+    assert!(missing_request.headers().get("Host").is_none());
+    assert!(missing_request.uri().authority().is_none());
+
+    let response = service.handle(missing_request).await;
+    assert_eq!(response.status(), http::StatusCode::BAD_REQUEST);
+}
+
 #[cfg(all(feature = "transport-streamable-http-server", feature = "server"))]
 mod origin_validation {
     use std::sync::Arc;