DefaultSseMessageEndpointValidator allows same-origin message endpoints

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

Author: Kehrlann

mcp-core/src/main/java/io/modelcontextprotocol/client/transport/DefaultSseMessageEndpointValidator.java

@@ -10,8 +10,8 @@
 
 /**
  * Default {@link SseMessageEndpointValidator} that validates the {@code message} endpoint
- * advertised by an SSE server. Message endpoints must be a relative URI, without path
- * traversal or authority.
+ * advertised by an SSE server. Message endpoints must either have the same origin as the
+ * SSE uri, or be a relative uri.
  *
  * @author Daniel Garnier-Moiroux
  */
@@ -30,16 +30,19 @@ public void validate(URI sseUri, String messageEndpoint) throws InvalidSseMessag
 					messageEndpoint);
 		}
 
-		if (endpointUri.isAbsolute()) {
-			// Exclude absolute URIs e.g. https://example.com/mcp
-			throw new InvalidSseMessageEndpointException("messageEndpoint must be a relative path, not an absolute URI",
-					messageEndpoint);
-		}
+		if (endpointUri.isAbsolute() || endpointUri.getRawAuthority() != null) {
+			String scheme = endpointUri.getScheme();
+			String host = endpointUri.getHost();
+			int port = endpointUri.getPort();
 
-		if (endpointUri.getRawAuthority() != null) {
-			// Exclude network paths e.g. //example.com/mcp
-			throw new InvalidSseMessageEndpointException(
-					"messageEndpoint must be a relative path and must not contain an authority", messageEndpoint);
+			boolean sameScheme = scheme != null && scheme.equalsIgnoreCase(sseUri.getScheme());
+			boolean sameHost = host != null && host.equalsIgnoreCase(sseUri.getHost());
+			boolean samePort = port == sseUri.getPort();
+
+			if (!sameScheme || !sameHost || !samePort) {
+				throw new InvalidSseMessageEndpointException(
+						"messageEndpoint must be a relative path or a same-origin URI", messageEndpoint);
+			}
 		}
 
 		// Exclude path-traversal

mcp-core/src/test/java/io/modelcontextprotocol/client/transport/DefaultSseMessageEndpointValidatorTests.java

@@ -25,7 +25,7 @@ class DefaultSseMessageEndpointValidatorTests {
 	private final DefaultSseMessageEndpointValidator validator = new DefaultSseMessageEndpointValidator();
 
 	@ParameterizedTest
-	@ValueSource(strings = { "/messages", "messages?session=abc", "/" })
+	@ValueSource(strings = { "/messages", "messages?session=abc", "/", "https://mcp.example.com/messages" })
 	void valid(String endpoint) {
 		assertThatCode(() -> validator.validate(SSE_URI, endpoint)).doesNotThrowAnyException();
 	}
@@ -41,32 +41,33 @@ void invalidEmpty(String endpoint) {
 	@ParameterizedTest
 	@ValueSource(strings = { "/foo/../bar", "/foo/./bar", "../bar", "./bar", "/foo/%2E%2E/bar", "/foo/%2e/bar" })
 	void invalidPathTraversal(String endpoint) {
-		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint)).hasMessageContaining("path-traversal")
+		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint))
+			.hasMessageContaining("must not contain path-traversal segments")
 			.asInstanceOf(type(InvalidSseMessageEndpointException.class))
 			.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
 			.isEqualTo(endpoint);
 	}
 
 	@ParameterizedTest
-	@ValueSource(strings = { "https://mcp.example.com/messages", "https://127.0.0.1/messages",
-			"https://mcp.example.com:8443/messages", "http://localhost:1234/messages", "file:///etc/passwd",
-			"gopher://mcp.example.com/_test" })
+	@ValueSource(strings = { "https://127.0.0.1/messages", "https://mcp.example.com:8443/messages",
+			"http://localhost:1234/messages", "file:///etc/passwd", "gopher://mcp.example.com/_test" })
 	void invalidAbsoluteUris(String endpoint) {
-		// Even an absolute URI on the same origin must be rejected: the contract
-		// is that the messageEndpoint is a path-only relative reference.
-		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint)).hasMessageContaining("must be a relative path")
+		// Absolute URIs must be same-origin.
+		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint))
+			.hasMessageContaining("must be a relative path or a same-origin URI")
 			.asInstanceOf(type(InvalidSseMessageEndpointException.class))
 			.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
 			.isEqualTo(endpoint);
 
 	}
 
 	@ParameterizedTest
-	@ValueSource(strings = { "//example/messages", "//user:secret@example/messages" })
+	@ValueSource(strings = { "//example/messages", "//user:secret@example/messages", "//mcp.example.com/messages" })
 	void invalidNetworkReference(String endpoint) {
 		// `//host/...` introduces an authority and is therefore not a pure path.
+		// It is missing a scheme, so it fails same-origin check.
 		assertThatThrownBy(() -> validator.validate(SSE_URI, endpoint))
-			.hasMessageContaining("must not contain an authority")
+			.hasMessageContaining("must be a relative path or a same-origin URI")
 			.asInstanceOf(type(InvalidSseMessageEndpointException.class))
 			.extracting(InvalidSseMessageEndpointException::getMessageEndpoint)
 			.isEqualTo(endpoint);