fix: reject unsupported MCP protocol version headers

Author: zxyasfas

mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java

@@ -279,6 +279,10 @@ protected void doGet(HttpServletRequest request, HttpServletResponse response)
 			return;
 		}
 
+		if (!validateProtocolVersion(request, response)) {
+			return;
+		}
+
 		List<String> badRequestErrors = new ArrayList<>();
 
 		String accept = request.getHeader(ACCEPT);
@@ -480,6 +484,10 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 				}
 			}
 
+			if (!validateProtocolVersion(request, response)) {
+				return;
+			}
+
 			String sessionId = request.getHeader(HttpHeaders.MCP_SESSION_ID);
 
 			if (sessionId == null || sessionId.isBlank()) {
@@ -596,6 +604,10 @@ protected void doDelete(HttpServletRequest request, HttpServletResponse response
 			return;
 		}
 
+		if (!validateProtocolVersion(request, response)) {
+			return;
+		}
+
 		if (this.disallowDelete) {
 			response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED);
 			return;
@@ -648,6 +660,22 @@ public void responseError(HttpServletResponse response, int httpCode, McpError m
 		return;
 	}
 
+	private boolean validateProtocolVersion(HttpServletRequest request, HttpServletResponse response)
+			throws IOException {
+		String protocolVersion = request.getHeader(HttpHeaders.PROTOCOL_VERSION);
+		if (protocolVersion == null || protocolVersion.isBlank()) {
+			return true;
+		}
+		if (protocolVersions().contains(protocolVersion)) {
+			return true;
+		}
+		this.responseError(response, HttpServletResponse.SC_BAD_REQUEST,
+				McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+					.message("Unsupported or invalid MCP protocol version: " + protocolVersion)
+					.build());
+		return false;
+	}
+
 	/**
 	 * Sends an SSE event to a client with a specific ID.
 	 * @param writer The writer to send the event through

mcp-test/src/test/java/io/modelcontextprotocol/common/HttpClientStreamableHttpVersionNegotiationIntegrationTests.java

@@ -4,6 +4,10 @@
 
 package io.modelcontextprotocol.common;
 
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
 import java.util.List;
 import java.util.Map;
 import java.util.function.BiFunction;
@@ -24,6 +28,8 @@
 import org.apache.catalina.startup.Tomcat;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -122,6 +128,55 @@ void usesServerSupportedVersion() {
 		mcpServer.close();
 	}
 
+	@ParameterizedTest
+	@ValueSource(strings = { "1900-01-01", "not-a-version" })
+	void rejectsUnsupportedProtocolVersionHeaders(String protocolVersion) throws Exception {
+		startTomcat();
+
+		HttpClient httpClient = HttpClient.newHttpClient();
+		String endpoint = "http://localhost:" + PORT + "/mcp";
+
+		HttpResponse<String> initializeResponse = httpClient.send(HttpRequest.newBuilder()
+			.uri(URI.create(endpoint))
+			.header("Content-Type", "application/json")
+			.header("Accept", "application/json, text/event-stream")
+			.header("MCP-Protocol-Version", ProtocolVersions.MCP_2025_11_25)
+			.POST(HttpRequest.BodyPublishers.ofString(
+					"""
+							{"jsonrpc":"2.0","id":"init-1","method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"test-client","version":"1.0.0"}}}
+							"""))
+			.build(), HttpResponse.BodyHandlers.ofString());
+
+		assertThat(initializeResponse.statusCode()).isEqualTo(200);
+		String sessionId = initializeResponse.headers().firstValue("Mcp-Session-Id").orElseThrow();
+
+		HttpResponse<String> initializedResponse = httpClient.send(HttpRequest.newBuilder()
+			.uri(URI.create(endpoint))
+			.header("Content-Type", "application/json")
+			.header("Accept", "application/json, text/event-stream")
+			.header("MCP-Protocol-Version", ProtocolVersions.MCP_2025_11_25)
+			.header("Mcp-Session-Id", sessionId)
+			.POST(HttpRequest.BodyPublishers
+				.ofString("{\"jsonrpc\":\"2.0\",\"method\":\"notifications/initialized\",\"params\":{}}"))
+			.build(), HttpResponse.BodyHandlers.ofString());
+
+		assertThat(initializedResponse.statusCode()).isEqualTo(202);
+
+		HttpResponse<String> badVersionResponse = httpClient.send(HttpRequest.newBuilder()
+			.uri(URI.create(endpoint))
+			.header("Content-Type", "application/json")
+			.header("Accept", "application/json, text/event-stream")
+			.header("MCP-Protocol-Version", protocolVersion)
+			.header("Mcp-Session-Id", sessionId)
+			.POST(HttpRequest.BodyPublishers
+				.ofString("{\"jsonrpc\":\"2.0\",\"id\":\"bad-version-1\",\"method\":\"tools/list\",\"params\":{}}"))
+			.build(), HttpResponse.BodyHandlers.ofString());
+
+		assertThat(badVersionResponse.statusCode()).isEqualTo(400);
+		assertThat(badVersionResponse.body()).contains("Unsupported or invalid MCP protocol version");
+		mcpServer.close();
+	}
+
 	private void startTomcat() {
 		tomcat = TomcatTestUtil.createTomcatServer("", PORT, transport, requestRecordingFilter);
 		try {