Reject non application/json POST requests

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

@@ -23,6 +23,8 @@
  */
 final class HttpServletRequestUtils {
 
+	private static final String APPLICATION_JSON = "application/json";
+
 	private HttpServletRequestUtils() {
 	}
 
@@ -41,6 +43,27 @@ static Map<String, List<String>> extractHeaders(HttpServletRequest request) {
 		return headers;
 	}
 
+	/**
+	 * Checks whether a {@code Content-Type} header value denotes
+	 * {@code application/json}. Only the media type is compared, case-insensitively;
+	 * parameters such as {@code charset} are ignored. This is not a substring search, so
+	 * a value like {@code text/plain; a=application/json} is rejected.
+	 * <p>
+	 * Requiring {@code application/json} prevents browsers from sending cross-origin
+	 * JSON-RPC messages as CORS "simple requests" (e.g. with {@code text/plain}), which
+	 * would otherwise reach the server without a preflight.
+	 * @param contentType The {@code Content-Type} header value, may be {@code null}
+	 * @return {@code true} if the media type is {@code application/json}
+	 */
+	static boolean isJsonContentType(String contentType) {
+		if (contentType == null) {
+			return false;
+		}
+		int parametersStart = contentType.indexOf(';');
+		String mediaType = parametersStart == -1 ? contentType : contentType.substring(0, parametersStart);
+		return APPLICATION_JSON.equalsIgnoreCase(mediaType.trim());
+	}
+
 	/**
 	 * Reads the request body, decoded using the request's character encoding (or UTF-8 if
 	 * not specified), while bounding the number of bytes read.

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

@@ -354,6 +354,14 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 			return;
 		}
 
+		if (!HttpServletRequestUtils.isJsonContentType(request.getContentType())) {
+			this.responseError(response, HttpServletResponse.SC_UNSUPPORTED_MEDIA_TYPE,
+					McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+						.message("Unsupported Media Type: Content-Type must be application/json")
+						.build());
+			return;
+		}
+
 		// Get the session ID from the request parameter
 		String sessionId = request.getParameter("sessionId");
 		if (sessionId == null) {
@@ -452,6 +460,16 @@ private void sendEvent(PrintWriter writer, String eventType, String data) throws
 		}
 	}
 
+	private void responseError(HttpServletResponse response, int httpCode, McpError mcpError) throws IOException {
+		response.setContentType(APPLICATION_JSON);
+		response.setCharacterEncoding(UTF_8);
+		response.setStatus(httpCode);
+		String jsonError = jsonMapper.writeValueAsString(mcpError);
+		PrintWriter writer = response.getWriter();
+		writer.write(jsonError);
+		writer.flush();
+	}
+
 	/**
 	 * Cleans up resources when the servlet is being destroyed.
 	 * <p>

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

@@ -170,7 +170,13 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 			return;
 		}
 
-		McpTransportContext transportContext = this.contextExtractor.extract(request);
+		if (!HttpServletRequestUtils.isJsonContentType(request.getContentType())) {
+			this.responseError(response, HttpServletResponse.SC_UNSUPPORTED_MEDIA_TYPE,
+					McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+						.message("Unsupported Media Type: Content-Type must be application/json")
+						.build());
+			return;
+		}
 
 		String accept = request.getHeader(ACCEPT);
 		if (accept == null || !(accept.contains(APPLICATION_JSON) && accept.contains(TEXT_EVENT_STREAM))) {
@@ -179,6 +185,8 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 			return;
 		}
 
+		McpTransportContext transportContext = this.contextExtractor.extract(request);
+
 		try {
 			String body = HttpServletRequestUtils.readBody(request, this.requestMaxSize);
 

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

@@ -427,6 +427,14 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 			badRequestErrors.add("application/json required in Accept header");
 		}
 
+		if (!HttpServletRequestUtils.isJsonContentType(request.getContentType())) {
+			this.responseError(response, HttpServletResponse.SC_UNSUPPORTED_MEDIA_TYPE,
+					McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+						.message("Unsupported Media Type: Content-Type must be application/json")
+						.build());
+			return;
+		}
+
 		McpTransportContext transportContext = this.contextExtractor.extract(request);
 
 		try {

mcp-core/src/test/java/io/modelcontextprotocol/server/transport/HttpServletRequestUtilsTests.java

@@ -12,6 +12,9 @@
 import jakarta.servlet.ServletInputStream;
 import jakarta.servlet.http.HttpServletRequest;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.NullAndEmptySource;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
@@ -100,6 +103,22 @@ void honorsRequestCharacterEncoding() throws Exception {
 		assertThat(body).isEqualTo("café");
 	}
 
+	@ParameterizedTest
+	@ValueSource(strings = { "application/json", "application/json; charset=utf-8", "application/json;charset=UTF-8",
+			"Application/JSON", " application/json ; charset=utf-8" })
+	void acceptsJsonContentType(String contentType) {
+		assertThat(HttpServletRequestUtils.isJsonContentType(contentType)).isTrue();
+	}
+
+	@ParameterizedTest
+	@NullAndEmptySource
+	@ValueSource(strings = { "text/plain", "text/plain;charset=UTF-8", "text/plain; a=application/json",
+			"application/x-www-form-urlencoded", "multipart/form-data", "application/json-seq", "application/jsonp",
+			"application/json, text/plain", "text/event-stream" })
+	void rejectsNonJsonContentType(String contentType) {
+		assertThat(HttpServletRequestUtils.isJsonContentType(contentType)).isFalse();
+	}
+
 	private static HttpServletRequest requestWithBody(String body, String characterEncoding) throws IOException {
 		HttpServletRequest request = mock(HttpServletRequest.class);
 		when(request.getInputStream()).thenReturn(servletInputStream(body.getBytes(StandardCharsets.UTF_8)));

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/server/transport/WebFluxServerRequestUtils.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2026-2026 the original author or authors.
+ */
+
+package io.modelcontextprotocol.server.transport;
+
+import org.springframework.http.InvalidMediaTypeException;
+import org.springframework.http.MediaType;
+import org.springframework.web.reactive.function.server.ServerRequest;
+
+/**
+ * Utility methods for working with {@link ServerRequest}. For internal use only.
+ *
+ * @author Daniel Garnier-Moiroux
+ */
+final class WebFluxServerRequestUtils {
+
+	private WebFluxServerRequestUtils() {
+	}
+
+	/**
+	 * Checks whether the request's {@code Content-Type} header denotes
+	 * {@code application/json}. Only the media type is compared, case-insensitively;
+	 * parameters such as {@code charset} are ignored. A missing or malformed header is
+	 * rejected.
+	 * <p>
+	 * Requiring {@code application/json} prevents browsers from sending cross-origin
+	 * JSON-RPC messages as CORS "simple requests" (e.g. with {@code text/plain}), which
+	 * would otherwise reach the server without a preflight.
+	 * @param request The incoming server request
+	 * @return {@code true} if the media type is {@code application/json}
+	 */
+	static boolean isJsonContentType(ServerRequest request) {
+		try {
+			return request.headers().contentType().map(MediaType.APPLICATION_JSON::equalsTypeAndSubtype).orElse(false);
+		}
+		catch (InvalidMediaTypeException ex) {
+			return false;
+		}
+	}
+
+}

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/server/transport/WebFluxSseServerTransportProvider.java

@@ -359,6 +359,13 @@ private Mono<ServerResponse> handleMessage(ServerRequest request) {
 			return ServerResponse.status(e.getStatusCode()).bodyValue(e.getMessage());
 		}
 
+		if (!WebFluxServerRequestUtils.isJsonContentType(request)) {
+			return ServerResponse.status(HttpStatus.UNSUPPORTED_MEDIA_TYPE)
+				.bodyValue(McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+					.message("Unsupported Media Type: Content-Type must be application/json")
+					.build());
+		}
+
 		if (request.queryParam("sessionId").isEmpty()) {
 			return ServerResponse.badRequest().bodyValue(new McpError("Session ID missing in message endpoint"));
 		}

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/server/transport/WebFluxStatelessServerTransport.java

@@ -114,6 +114,13 @@ private Mono<ServerResponse> handlePost(ServerRequest request) {
 			return ServerResponse.status(e.getStatusCode()).bodyValue(e.getMessage());
 		}
 
+		if (!WebFluxServerRequestUtils.isJsonContentType(request)) {
+			return ServerResponse.status(HttpStatus.UNSUPPORTED_MEDIA_TYPE)
+				.bodyValue(McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+					.message("Unsupported Media Type: Content-Type must be application/json")
+					.build());
+		}
+
 		McpTransportContext transportContext = this.contextExtractor.extract(request);
 
 		List<MediaType> acceptHeaders = request.headers().asHttpHeaders().getAccept();

mcp-spring/mcp-spring-webflux/src/main/java/io/modelcontextprotocol/server/transport/WebFluxStreamableServerTransportProvider.java

@@ -246,6 +246,13 @@ private Mono<ServerResponse> handlePost(ServerRequest request) {
 			return ServerResponse.status(e.getStatusCode()).bodyValue(e.getMessage());
 		}
 
+		if (!WebFluxServerRequestUtils.isJsonContentType(request)) {
+			return ServerResponse.status(HttpStatus.UNSUPPORTED_MEDIA_TYPE)
+				.bodyValue(McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST)
+					.message("Unsupported Media Type: Content-Type must be application/json")
+					.build());
+		}
+
 		McpTransportContext transportContext = this.contextExtractor.extract(request);
 
 		List<MediaType> acceptHeaders = request.headers().asHttpHeaders().getAccept();

mcp-spring/mcp-spring-webflux/src/test/java/io/modelcontextprotocol/WebFluxSseIntegrationTests.java

@@ -5,16 +5,25 @@
 package io.modelcontextprotocol;
 
 import java.time.Duration;
+import java.util.List;
 import java.util.Map;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.stream.Stream;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Timeout;
+import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.ValueSource;
 
+import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.MediaType;
+import org.springframework.http.codec.ServerSentEvent;
 import org.springframework.http.server.reactive.HttpHandler;
 import org.springframework.http.server.reactive.ReactorHttpHandlerAdapter;
+import org.springframework.web.reactive.function.client.ClientResponse;
 import org.springframework.web.reactive.function.client.WebClient;
 import org.springframework.web.reactive.function.server.RouterFunctions;
 import org.springframework.web.reactive.function.server.ServerRequest;
@@ -26,12 +35,20 @@
 import io.modelcontextprotocol.server.McpServer;
 import io.modelcontextprotocol.server.McpServer.AsyncSpecification;
 import io.modelcontextprotocol.server.McpServer.SingleSessionSyncSpecification;
+import io.modelcontextprotocol.server.McpServerFeatures;
 import io.modelcontextprotocol.server.McpTransportContextExtractor;
 import io.modelcontextprotocol.server.TestUtil;
 import io.modelcontextprotocol.server.transport.WebFluxSseServerTransportProvider;
+import io.modelcontextprotocol.spec.McpSchema;
+import reactor.core.Disposable;
+import reactor.core.publisher.Mono;
+import reactor.core.publisher.Sinks;
 import reactor.netty.DisposableServer;
 import reactor.netty.http.server.HttpServer;
 
+import static io.modelcontextprotocol.util.ToolsUtils.EMPTY_JSON_SCHEMA;
+import static org.assertj.core.api.Assertions.assertThat;
+
 @Timeout(15)
 class WebFluxSseIntegrationTests extends AbstractMcpClientServerIntegrationTests {
 
@@ -103,4 +120,69 @@ public void after() {
 		}
 	}
 
+	@ParameterizedTest
+	@ValueSource(strings = { "text/plain;charset=UTF-8", "application/x-www-form-urlencoded", "multipart/form-data" })
+	void rejectsNonJsonContentType(String contentType) {
+		var webClient = WebClient.create("http://localhost:" + PORT);
+		var toolCalled = new AtomicBoolean();
+		prepareAsyncServerBuilder().capabilities(McpSchema.ServerCapabilities.builder().tools(true).build())
+			.tools(McpServerFeatures.AsyncToolSpecification.builder()
+				.tool(McpSchema.Tool.builder().name("tool1").inputSchema(EMPTY_JSON_SCHEMA).build())
+				.callHandler((exchange, request) -> {
+					toolCalled.set(true);
+					return Mono.just(McpSchema.CallToolResult.builder().build());
+				})
+				.build())
+			.build();
+
+		// Establish an SSE session to obtain the session-scoped message endpoint. The
+		// SSE stream must stay open for the session to remain active.
+		var messageEndpoint = Sinks.<String>one();
+		Disposable sseSubscription = webClient.get()
+			.uri(CUSTOM_SSE_ENDPOINT)
+			.accept(MediaType.TEXT_EVENT_STREAM)
+			.retrieve()
+			.bodyToFlux(new ParameterizedTypeReference<ServerSentEvent<String>>() {
+			})
+			.filter(event -> WebFluxSseServerTransportProvider.ENDPOINT_EVENT_TYPE.equals(event.event()))
+			.subscribe(event -> messageEndpoint.tryEmitValue(event.data()));
+		try {
+			String endpoint = messageEndpoint.asMono().block(Duration.ofSeconds(5));
+
+			// Initialize the session, so that the tool call below would be handled if it
+			// were accepted
+			for (String message : List.of("""
+					{"jsonrpc":"2.0","id":"init","method":"initialize","params":{
+					"protocolVersion":"2024-11-05","capabilities":{},
+					"clientInfo":{"name":"test-client","version":"1.0.0"}}}""", """
+					{"jsonrpc":"2.0","method":"notifications/initialized"}""")) {
+				var initResponse = webClient.post()
+					.uri(endpoint)
+					.contentType(MediaType.APPLICATION_JSON)
+					.bodyValue(message)
+					.exchangeToMono(ClientResponse::toBodilessEntity)
+					.block();
+				assertThat(initResponse.getStatusCode()).isEqualTo(HttpStatus.OK);
+			}
+
+			// CORS-safelisted content types can be sent cross-origin by a browser without
+			// a preflight, so they must be rejected before the message is handled
+			var response = webClient.post()
+				.uri(endpoint)
+				.contentType(MediaType.parseMediaType(contentType))
+				.bodyValue(
+						"""
+								{"jsonrpc":"2.0","id":"call-1","method":"tools/call","params":{"name":"tool1","arguments":{}}}""")
+				.exchangeToMono(clientResponse -> clientResponse.toEntity(String.class))
+				.block();
+
+			assertThat(response.getStatusCode()).isEqualTo(HttpStatus.UNSUPPORTED_MEDIA_TYPE);
+			assertThat(response.getBody()).contains("Unsupported Media Type: Content-Type must be application/json");
+			assertThat(toolCalled).isFalse();
+		}
+		finally {
+			sseSubscription.dispose();
+		}
+	}
+
 }