CVE-2026-42559

@@ -7,6 +7,7 @@ pub const JSON_MIME_TYPE: &str = "application/json";
 /// Reserved headers that must not be overridden by user-supplied custom headers.
 /// `MCP-Protocol-Version` is in this list but is allowed through because the worker
 /// injects it after initialization.
+#[allow(dead_code)]
 pub(crate) const RESERVED_HEADERS: &[&str] = &[
     "accept",
     HEADER_SESSION_ID,
@@ -36,6 +37,7 @@ pub(crate) fn validate_custom_header(name: &http::HeaderName) -> Result<(), Stri
 
 /// Extracts the `scope=` parameter from a `WWW-Authenticate` header value.
 /// Handles both quoted (`scope="files:read files:write"`) and unquoted (`scope=read:data`) forms.
+#[cfg(feature = "client-side-sse")]
 pub(crate) fn extract_scope_from_header(header: &str) -> Option<String> {
     let header_lowercase = header.to_ascii_lowercase();
     let scope_key = "scope=";

@@ -2,7 +2,7 @@ use std::{convert::Infallible, fmt::Display, sync::Arc, time::Duration};
 
 use bytes::Bytes;
 use futures::{StreamExt, future::BoxFuture};
-use http::{Method, Request, Response, header::ALLOW};
+use http::{HeaderMap, Method, Request, Response, header::ALLOW};
 use http_body::Body;
 use http_body_util::{BodyExt, Full, combinators::BoxBody};
 use tokio_stream::wrappers::ReceiverStream;
@@ -29,8 +29,8 @@ use crate::{
     },
 };
 
-#[derive(Debug, Clone)]
 #[non_exhaustive]
+#[derive(Debug, Clone)]
 pub struct StreamableHttpServerConfig {
     /// The ping message duration for SSE connections.
     pub sse_keep_alive: Option<Duration>,
@@ -49,6 +49,16 @@ pub struct StreamableHttpServerConfig {
     /// When this token is cancelled, all active sessions are terminated and
     /// the server stops accepting new requests.
     pub cancellation_token: CancellationToken,
+    /// Allowed hostnames or `host:port` authorities for inbound `Host` validation.
+    ///
+    /// By default, Streamable HTTP servers only accept loopback hosts to
+    /// prevent DNS rebinding attacks against locally running servers. Public
+    /// deployments should override this list with their own hostnames.
+    /// examples:
+    ///     allowed_hosts = ["localhost", "127.0.0.1", "0.0.0.0"]
+    /// or with ports:
+    ///     allowed_hosts = ["example.com", "example.com:8080"]
+    pub allowed_hosts: Vec<String>,
 }
 
 impl Default for StreamableHttpServerConfig {
@@ -59,11 +69,24 @@ impl Default for StreamableHttpServerConfig {
             stateful_mode: true,
             json_response: false,
             cancellation_token: CancellationToken::new(),
+            allowed_hosts: vec!["localhost".into(), "127.0.0.1".into(), "::1".into()],
         }
     }
 }
 
 impl StreamableHttpServerConfig {
+    pub fn with_allowed_hosts(
+        mut self,
+        allowed_hosts: impl IntoIterator<Item = impl Into<String>>,
+    ) -> Self {
+        self.allowed_hosts = allowed_hosts.into_iter().map(Into::into).collect();
+        self
+    }
+    /// Disable allowed hosts. This will allow requests with any `Host` header, which is NOT recommended for public deployments.
+    pub fn disable_allowed_hosts(mut self) -> Self {
+        self.allowed_hosts.clear();
+        self
+    }
     pub fn with_sse_keep_alive(mut self, duration: Option<Duration>) -> Self {
         self.sse_keep_alive = duration;
         self
@@ -130,6 +153,97 @@ fn validate_protocol_version_header(headers: &http::HeaderMap) -> Result<(), Box
     Ok(())
 }
 
+fn forbidden_response(message: impl Into<String>) -> BoxResponse {
+    Response::builder()
+        .status(http::StatusCode::FORBIDDEN)
+        .body(Full::new(Bytes::from(message.into())).boxed())
+        .expect("valid response")
+}
+
+fn normalize_host(host: &str) -> String {
+    host.trim_matches('[')
+        .trim_matches(']')
+        .to_ascii_lowercase()
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct NormalizedAuthority {
+    host: String,
+    port: Option<u16>,
+}
+
+fn normalize_authority(host: &str, port: Option<u16>) -> NormalizedAuthority {
+    NormalizedAuthority {
+        host: normalize_host(host),
+        port,
+    }
+}
+
+fn parse_allowed_authority(allowed: &str) -> Option<NormalizedAuthority> {
+    let allowed = allowed.trim();
+    if allowed.is_empty() {
+        return None;
+    }
+
+    if let Ok(authority) = http::uri::Authority::try_from(allowed) {
+        return Some(normalize_authority(authority.host(), authority.port_u16()));
+    }
+
+    Some(normalize_authority(allowed, None))
+}
+
+fn host_is_allowed(host: &NormalizedAuthority, allowed_hosts: &[String]) -> bool {
+    if allowed_hosts.is_empty() {
+        // If the allowed hosts list is empty, allow all hosts (not recommended).
+        return true;
+    }
+    allowed_hosts
+        .iter()
+        .filter_map(|allowed| parse_allowed_authority(allowed))
+        .any(|allowed| {
+            allowed.host == host.host
+                && match allowed.port {
+                    Some(port) => host.port == Some(port),
+                    None => true,
+                }
+        })
+}
+
+fn bad_request_response(message: &str) -> BoxResponse {
+    let body = Full::from(message.to_string()).boxed();
+
+    http::Response::builder()
+        .status(http::StatusCode::BAD_REQUEST)
+        .header(http::header::CONTENT_TYPE, "text/plain; charset=utf-8")
+        .body(body)
+        .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 {
+        return Err(bad_request_response("Bad Request: missing Host header"));
+    };
+
+    let host = host
+        .to_str()
+        .map_err(|_| bad_request_response("Bad Request: Invalid Host header encoding"))?;
+    let authority = http::uri::Authority::try_from(host)
+        .map_err(|_| bad_request_response("Bad Request: Invalid Host header"))?;
+    Ok(normalize_authority(authority.host(), authority.port_u16()))
+}
+
+fn validate_dns_rebinding_headers(
+    headers: &HeaderMap,
+    config: &StreamableHttpServerConfig,
+) -> Result<(), BoxResponse> {
+    let host = parse_host_header(headers)?;
+    if !host_is_allowed(&host, &config.allowed_hosts) {
+        return Err(forbidden_response("Forbidden: Host header is not allowed"));
+    }
+
+    Ok(())
+}
+
 /// # Streamable HTTP server
 ///
 /// An HTTP service that implements the
@@ -279,6 +393,9 @@ where
         B: Body + Send + 'static,
         B::Error: Display,
     {
+        if let Err(response) = validate_dns_rebinding_headers(request.headers(), &self.config) {
+            return response;
+        }
         let method = request.method().clone();
         let allowed_methods = match self.config.stateful_mode {
             true => "GET, POST, DELETE",

@@ -761,6 +761,7 @@ async fn test_server_rejects_unsupported_protocol_version() {
         .method(Method::POST)
         .header("Accept", "application/json, text/event-stream")
         .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:8080")
         .body(Full::new(Bytes::from(init_body.to_string())))
         .unwrap();
 
@@ -785,6 +786,7 @@ async fn test_server_rejects_unsupported_protocol_version() {
         .method(Method::POST)
         .header("Accept", "application/json, text/event-stream")
         .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:8080")
         .header("mcp-session-id", &session_id)
         .header("mcp-protocol-version", "2025-03-26")
         .body(Full::new(Bytes::from(initialized_body.to_string())))
@@ -802,6 +804,7 @@ async fn test_server_rejects_unsupported_protocol_version() {
         .method(Method::POST)
         .header("Accept", "application/json, text/event-stream")
         .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:8080")
         .header("mcp-session-id", &session_id)
         .header("mcp-protocol-version", "2025-03-26")
         .body(Full::new(Bytes::from(valid_body.to_string())))
@@ -823,6 +826,7 @@ async fn test_server_rejects_unsupported_protocol_version() {
         .method(Method::POST)
         .header("Accept", "application/json, text/event-stream")
         .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:8080")
         .header("mcp-session-id", &session_id)
         .header("mcp-protocol-version", "9999-01-01")
         .body(Full::new(Bytes::from(invalid_body.to_string())))
@@ -844,6 +848,7 @@ async fn test_server_rejects_unsupported_protocol_version() {
         .method(Method::POST)
         .header("Accept", "application/json, text/event-stream")
         .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:8080")
         .header("mcp-session-id", &session_id)
         .body(Full::new(Bytes::from(no_version_body.to_string())))
         .unwrap();
@@ -870,3 +875,156 @@ fn test_protocol_version_utilities() {
     assert!(ProtocolVersion::KNOWN_VERSIONS.contains(&ProtocolVersion::V_2025_03_26));
     assert!(ProtocolVersion::KNOWN_VERSIONS.contains(&ProtocolVersion::V_2025_06_18));
 }
+
+/// Integration test: Verify server validates only the Host header for DNS rebinding protection
+#[tokio::test]
+#[cfg(all(feature = "transport-streamable-http-server", feature = "server",))]
+async fn test_server_validates_host_header_for_dns_rebinding_protection() {
+    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"
+            }
+        }
+    });
+
+    let allowed_request = Request::builder()
+        .method(Method::POST)
+        .header("Accept", "application/json, text/event-stream")
+        .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:8080")
+        .header("Origin", "http://localhost:8080")
+        .body(Full::new(Bytes::from(init_body.to_string())))
+        .unwrap();
+
+    let response = service.handle(allowed_request).await;
+    assert_eq!(response.status(), http::StatusCode::OK);
+
+    let bad_host_request = Request::builder()
+        .method(Method::POST)
+        .header("Accept", "application/json, text/event-stream")
+        .header(CONTENT_TYPE, "application/json")
+        .header("Host", "attacker.example")
+        .body(Full::new(Bytes::from(init_body.to_string())))
+        .unwrap();
+
+    let response = service.handle(bad_host_request).await;
+    assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
+
+    let ignored_origin_request = Request::builder()
+        .method(Method::POST)
+        .header("Accept", "application/json, text/event-stream")
+        .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:8080")
+        .header("Origin", "http://attacker.example")
+        .body(Full::new(Bytes::from(init_body.to_string())))
+        .unwrap();
+
+    let response = service.handle(ignored_origin_request).await;
+    assert_eq!(response.status(), http::StatusCode::OK);
+}
+
+/// Integration test: Verify server can enforce an allowed Host port when configured
+#[tokio::test]
+#[cfg(all(feature = "transport-streamable-http-server", feature = "server",))]
+async fn test_server_validates_host_header_port_for_dns_rebinding_protection() {
+    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().with_allowed_hosts(["localhost:8080"]),
+    );
+
+    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"
+            }
+        }
+    });
+
+    let allowed_request = Request::builder()
+        .method(Method::POST)
+        .header("Accept", "application/json, text/event-stream")
+        .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:8080")
+        .body(Full::new(Bytes::from(init_body.to_string())))
+        .unwrap();
+
+    let response = service.handle(allowed_request).await;
+    assert_eq!(response.status(), http::StatusCode::OK);
+
+    let wrong_port_request = Request::builder()
+        .method(Method::POST)
+        .header("Accept", "application/json, text/event-stream")
+        .header(CONTENT_TYPE, "application/json")
+        .header("Host", "localhost:3000")
+        .body(Full::new(Bytes::from(init_body.to_string())))
+        .unwrap();
+
+    let response = service.handle(wrong_port_request).await;
+    assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
+}