feat(http): add Origin header validation (#823)

Author: DaleSeo

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

@@ -64,6 +64,15 @@ pub struct StreamableHttpServerConfig {
     /// or with ports:
     ///     allowed_hosts = ["example.com", "example.com:8080"]
     pub allowed_hosts: Vec<String>,
+    /// Allowed browser origins for inbound `Origin` validation.
+    ///
+    /// Defaults to an empty list, which disables Origin validation. When
+    /// non-empty, requests carrying an `Origin` header must match per RFC 6454
+    /// `(scheme, host, port)`; missing-`Origin` requests still pass. Entries
+    /// must include a scheme; `"null"` matches the browser's `Origin: null`.
+    /// examples:
+    ///     allowed_origins = ["https://app.example.com", "http://localhost:8080"]
+    pub allowed_origins: Vec<String>,
     /// Optional external session store for cross-instance recovery.
     ///
     /// When set, [`SessionState`] (the client's `initialize` parameters) is
@@ -103,6 +112,7 @@ impl Default for StreamableHttpServerConfig {
             json_response: false,
             cancellation_token: CancellationToken::new(),
             allowed_hosts: vec!["localhost".into(), "127.0.0.1".into(), "::1".into()],
+            allowed_origins: vec![],
             session_store: None,
         }
     }
@@ -121,6 +131,18 @@ impl StreamableHttpServerConfig {
         self.allowed_hosts.clear();
         self
     }
+    pub fn with_allowed_origins(
+        mut self,
+        allowed_origins: impl IntoIterator<Item = impl Into<String>>,
+    ) -> Self {
+        self.allowed_origins = allowed_origins.into_iter().map(Into::into).collect();
+        self
+    }
+    /// Disable Origin validation, reverting to the default ignore-Origin behavior.
+    pub fn disable_allowed_origins(mut self) -> Self {
+        self.allowed_origins.clear();
+        self
+    }
     pub fn with_sse_keep_alive(mut self, duration: Option<Duration>) -> Self {
         self.sse_keep_alive = duration;
         self
@@ -243,6 +265,59 @@ fn host_is_allowed(host: &NormalizedAuthority, allowed_hosts: &[String]) -> bool
         })
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+enum NormalizedOrigin {
+    Null,
+    Tuple {
+        scheme: String,
+        host: String,
+        port: Option<u16>,
+    },
+}
+
+fn parse_origin_value(value: &str) -> Option<NormalizedOrigin> {
+    let value = value.trim();
+    if value.is_empty() {
+        return None;
+    }
+    if value.eq_ignore_ascii_case("null") {
+        return Some(NormalizedOrigin::Null);
+    }
+    let uri = http::Uri::try_from(value).ok()?;
+    let scheme = uri.scheme_str()?.to_ascii_lowercase();
+    let authority = uri.authority()?;
+    Some(NormalizedOrigin::Tuple {
+        scheme,
+        host: normalize_host(authority.host()),
+        port: authority.port_u16(),
+    })
+}
+
+fn origin_is_allowed(origin: &NormalizedOrigin, allowed_origins: &[String]) -> bool {
+    if allowed_origins.is_empty() {
+        return true;
+    }
+    allowed_origins
+        .iter()
+        .filter_map(|raw| parse_origin_value(raw))
+        .any(|allowed| match (&allowed, origin) {
+            (NormalizedOrigin::Null, NormalizedOrigin::Null) => true,
+            (
+                NormalizedOrigin::Tuple {
+                    scheme: a_scheme,
+                    host: a_host,
+                    port: a_port,
+                },
+                NormalizedOrigin::Tuple {
+                    scheme: o_scheme,
+                    host: o_host,
+                    port: o_port,
+                },
+            ) => a_scheme == o_scheme && a_host == o_host && (a_port.is_none() || a_port == o_port),
+            _ => false,
+        })
+}
+
 fn bad_request_response(message: &str) -> BoxResponse {
     let body = Full::from(message.to_string()).boxed();
 
@@ -274,7 +349,30 @@ fn validate_dns_rebinding_headers(
     if !host_is_allowed(&host, &config.allowed_hosts) {
         return Err(forbidden_response("Forbidden: Host header is not allowed"));
     }
+    validate_origin_header(headers, &config.allowed_origins)?;
+    Ok(())
+}
 
+fn validate_origin_header(
+    headers: &HeaderMap,
+    allowed_origins: &[String],
+) -> Result<(), BoxResponse> {
+    if allowed_origins.is_empty() {
+        return Ok(());
+    }
+    let Some(origin_header) = headers.get(http::header::ORIGIN) else {
+        return Ok(());
+    };
+    let origin_str = origin_header
+        .to_str()
+        .map_err(|_| bad_request_response("Bad Request: Invalid Origin header encoding"))?;
+    let origin = parse_origin_value(origin_str)
+        .ok_or_else(|| bad_request_response("Bad Request: Invalid Origin header"))?;
+    if !origin_is_allowed(&origin, allowed_origins) {
+        return Err(forbidden_response(
+            "Forbidden: Origin header is not allowed",
+        ));
+    }
     Ok(())
 }
 

crates/rmcp/tests/test_custom_headers.rs

@@ -1030,3 +1030,111 @@ async fn test_server_validates_host_header_port_for_dns_rebinding_protection() {
     let response = service.handle(wrong_port_request).await;
     assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
 }
+
+#[cfg(all(feature = "transport-streamable-http-server", feature = "server"))]
+mod origin_validation {
+    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())
+        }
+    }
+
+    fn service_with_allowed_origins(
+        origins: &[&str],
+    ) -> StreamableHttpService<TestHandler, LocalSessionManager> {
+        StreamableHttpService::new(
+            || Ok(TestHandler),
+            Arc::new(LocalSessionManager::default()),
+            StreamableHttpServerConfig::default().with_allowed_origins(origins.iter().copied()),
+        )
+    }
+
+    fn init_request(origin: Option<&str>) -> Request<Full<Bytes>> {
+        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 mut builder = Request::builder()
+            .method(Method::POST)
+            .header("Accept", "application/json, text/event-stream")
+            .header(CONTENT_TYPE, "application/json")
+            .header("Host", "localhost:8080");
+        if let Some(origin) = origin {
+            builder = builder.header("Origin", origin);
+        }
+        builder
+            .body(Full::new(Bytes::from(init_body.to_string())))
+            .unwrap()
+    }
+
+    #[tokio::test]
+    async fn allowlisted_origin_is_allowed() {
+        let service = service_with_allowed_origins(&["http://localhost:8080"]);
+        let response = service
+            .handle(init_request(Some("http://localhost:8080")))
+            .await;
+        assert_eq!(response.status(), http::StatusCode::OK);
+    }
+
+    #[tokio::test]
+    async fn non_allowlisted_origin_is_forbidden() {
+        let service = service_with_allowed_origins(&["http://localhost:8080"]);
+        let response = service
+            .handle(init_request(Some("http://attacker.example")))
+            .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"]);
+        let response = service.handle(init_request(None)).await;
+        assert_eq!(response.status(), http::StatusCode::OK);
+    }
+
+    #[tokio::test]
+    async fn scheme_mismatch_is_forbidden() {
+        let service = service_with_allowed_origins(&["http://localhost:8080"]);
+        let response = service
+            .handle(init_request(Some("https://localhost:8080")))
+            .await;
+        assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
+    }
+
+    #[tokio::test]
+    async fn null_origin_is_allowed_when_allowlisted() {
+        let service = service_with_allowed_origins(&["null"]);
+        let response = service.handle(init_request(Some("null"))).await;
+        assert_eq!(response.status(), http::StatusCode::OK);
+    }
+
+    #[tokio::test]
+    async fn null_origin_is_forbidden_when_not_allowlisted() {
+        let service = service_with_allowed_origins(&["http://localhost:8080"]);
+        let response = service.handle(init_request(Some("null"))).await;
+        assert_eq!(response.status(), http::StatusCode::FORBIDDEN);
+    }
+}