Bound HTTP server reads

- Tests are not in the abstract base class because the spring transports
  are not vulnerable

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

Author: Kehrlann

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

@@ -601,6 +601,10 @@ else if (statusCode == BAD_REQUEST) {
 					return Flux.<McpSchema.JSONRPCMessage>error(new McpTransportException(
 							"Bad Request. Status code:" + statusCode + ", response-event:" + responseEvent));
 				}
+				else if (statusCode >= 400 && statusCode < 500) {
+					return Flux.<McpSchema.JSONRPCMessage>error(
+							new McpTransportException("Invalid request. Status code: " + statusCode));
+				}
 
 				return Flux.<McpSchema.JSONRPCMessage>error(
 						new RuntimeException("Failed to send message: " + responseEvent));

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

@@ -4,6 +4,10 @@
 
 package io.modelcontextprotocol.server.transport;
 
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.nio.charset.StandardCharsets;
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.HashMap;
@@ -37,4 +41,32 @@ static Map<String, List<String>> extractHeaders(HttpServletRequest request) {
 		return headers;
 	}
 
+	/**
+	 * Reads the request body, decoded using the request's character encoding (or UTF-8 if
+	 * not specified), while bounding the number of bytes read.
+	 * @param request The HTTP servlet request
+	 * @param maxSize The maximum number of bytes to read from the request body
+	 * @return The decoded request body
+	 * @throws MaxSizeExceededException If the body exceeds {@code maxSize}
+	 * @throws IOException If an I/O error occurs while reading the request body
+	 */
+	static String readBody(HttpServletRequest request, int maxSize) throws MaxSizeExceededException, IOException {
+		InputStream inputStream = request.getInputStream();
+		ByteArrayOutputStream bodyBytes = new ByteArrayOutputStream();
+		byte[] buf = new byte[8192];
+		int totalBytes = 0;
+		int readBytes;
+		while ((readBytes = inputStream.read(buf, 0, buf.length)) != -1) {
+			totalBytes += readBytes;
+			if (totalBytes > maxSize) {
+				throw new MaxSizeExceededException(
+						"Request body exceeds the maximum allowed size of " + maxSize + " bytes");
+			}
+			bodyBytes.write(buf, 0, readBytes);
+		}
+		String charset = request.getCharacterEncoding() != null ? request.getCharacterEncoding()
+				: StandardCharsets.UTF_8.name();
+		return bodyBytes.toString(charset);
+	}
+
 }

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

@@ -4,7 +4,6 @@
 
 package io.modelcontextprotocol.server.transport;
 
-import java.io.BufferedReader;
 import java.io.IOException;
 import java.io.PrintWriter;
 import java.time.Duration;
@@ -70,6 +69,11 @@
 @WebServlet(asyncSupported = true)
 public class HttpServletSseServerTransportProvider extends HttpServlet implements McpServerTransportProvider {
 
+	/**
+	 * Default maximum size of a single request body: 16 MiB (16 * 1024 * 1024 bytes).
+	 */
+	private static final int DEFAULT_REQUEST_MAX_SIZE = 16 * 1024 * 1024;
+
 	/**
 	 * Logger for this class
 	 */
@@ -105,6 +109,11 @@ public class HttpServletSseServerTransportProvider extends HttpServlet implement
 	 */
 	private final McpJsonMapper jsonMapper;
 
+	/**
+	 * Maximum size, in bytes, of a single request body accepted by this transport.
+	 */
+	private final int requestMaxSize;
+
 	/**
 	 * Base URL for the server transport
 	 */
@@ -160,24 +169,28 @@ public class HttpServletSseServerTransportProvider extends HttpServlet implement
 	 * keep-alive functionality
 	 * @param contextExtractor The extractor for transport context from the request.
 	 * @param securityValidator The security validator for validating HTTP requests.
+	 * @param requestMaxSize The maximum size, in bytes, of a single request body. Must be
+	 * positive.
 	 */
 	private HttpServletSseServerTransportProvider(McpJsonMapper jsonMapper, String baseUrl, String messageEndpoint,
 			String sseEndpoint, Duration keepAliveInterval,
 			McpTransportContextExtractor<HttpServletRequest> contextExtractor,
-			ServerTransportSecurityValidator securityValidator) {
+			ServerTransportSecurityValidator securityValidator, int requestMaxSize) {
 
 		Assert.notNull(jsonMapper, "JsonMapper must not be null");
 		Assert.notNull(messageEndpoint, "messageEndpoint must not be null");
 		Assert.notNull(sseEndpoint, "sseEndpoint must not be null");
 		Assert.notNull(contextExtractor, "Context extractor must not be null");
 		Assert.notNull(securityValidator, "Security validator must not be null");
+		Assert.isTrue(requestMaxSize > 0, "requestMaxSize must be positive");
 
 		this.jsonMapper = jsonMapper;
 		this.baseUrl = baseUrl;
 		this.messageEndpoint = messageEndpoint;
 		this.sseEndpoint = sseEndpoint;
 		this.contextExtractor = contextExtractor;
 		this.securityValidator = securityValidator;
+		this.requestMaxSize = requestMaxSize;
 
 		if (keepAliveInterval != null) {
 
@@ -321,6 +334,11 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 			return;
 		}
 
+		if (request.getContentLengthLong() > this.requestMaxSize) {
+			response.sendError(HttpServletResponse.SC_REQUEST_ENTITY_TOO_LARGE);
+			return;
+		}
+
 		String requestURI = request.getRequestURI();
 		if (!requestURI.endsWith(messageEndpoint)) {
 			response.sendError(HttpServletResponse.SC_NOT_FOUND);
@@ -363,22 +381,20 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 		}
 
 		try {
-			BufferedReader reader = request.getReader();
-			StringBuilder body = new StringBuilder();
-			String line;
-			while ((line = reader.readLine()) != null) {
-				body.append(line);
-			}
+			String body = HttpServletRequestUtils.readBody(request, this.requestMaxSize);
 
 			final McpTransportContext transportContext = this.contextExtractor.extract(request);
-			McpSchema.JSONRPCMessage message = McpSchema.deserializeJsonRpcMessage(jsonMapper, body.toString());
+			McpSchema.JSONRPCMessage message = McpSchema.deserializeJsonRpcMessage(jsonMapper, body);
 
 			// Process the message through the session's handle method
 			// Block for Servlet compatibility
 			session.handle(message).contextWrite(ctx -> ctx.put(McpTransportContext.KEY, transportContext)).block();
 
 			response.setStatus(HttpServletResponse.SC_OK);
 		}
+		catch (MaxSizeExceededException e) {
+			response.sendError(HttpServletResponse.SC_REQUEST_ENTITY_TOO_LARGE);
+		}
 		catch (Exception e) {
 			logger.error("Error processing message: {}", e.getMessage());
 			try {
@@ -574,6 +590,8 @@ public static class Builder {
 
 		private ServerTransportSecurityValidator securityValidator = ServerTransportSecurityValidator.NOOP;
 
+		private int requestMaxSize = DEFAULT_REQUEST_MAX_SIZE;
+
 		/**
 		 * Sets the JsonMapper implementation to use for serialization/deserialization. If
 		 * not specified, a JacksonJsonMapper will be created from the configured
@@ -660,6 +678,19 @@ public Builder securityValidator(ServerTransportSecurityValidator securityValida
 			return this;
 		}
 
+		/**
+		 * Sets the maximum size, in bytes, of a single request body accepted by this
+		 * transport. Requests whose body exceeds this size are rejected with a 413
+		 * (Payload Too Large) response. Defaults to 16 MiB if not set.
+		 * @param requestMaxSize The maximum request body size, in bytes. Must be
+		 * positive.
+		 * @return This builder instance
+		 */
+		public Builder maxRequestSize(int requestMaxSize) {
+			this.requestMaxSize = requestMaxSize;
+			return this;
+		}
+
 		/**
 		 * Builds a new instance of HttpServletSseServerTransportProvider with the
 		 * configured settings.
@@ -672,7 +703,7 @@ public HttpServletSseServerTransportProvider build() {
 			}
 			return new HttpServletSseServerTransportProvider(
 					jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, baseUrl, messageEndpoint,
-					sseEndpoint, keepAliveInterval, contextExtractor, securityValidator);
+					sseEndpoint, keepAliveInterval, contextExtractor, securityValidator, requestMaxSize);
 		}
 
 	}

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

@@ -4,7 +4,6 @@
 
 package io.modelcontextprotocol.server.transport;
 
-import java.io.BufferedReader;
 import java.io.IOException;
 import java.io.PrintWriter;
 import java.util.List;
@@ -39,6 +38,11 @@
 @WebServlet(asyncSupported = true)
 public class HttpServletStatelessServerTransport extends HttpServlet implements McpStatelessServerTransport {
 
+	/**
+	 * Default maximum size of a single request body: 16 MiB (16 * 1024 * 1024 bytes).
+	 */
+	private static final int DEFAULT_REQUEST_MAX_SIZE = 16 * 1024 * 1024;
+
 	private static final Logger logger = LoggerFactory.getLogger(HttpServletStatelessServerTransport.class);
 
 	public static final String UTF_8 = "UTF-8";
@@ -66,18 +70,37 @@ public class HttpServletStatelessServerTransport extends HttpServlet implements
 	 */
 	private final ServerTransportSecurityValidator securityValidator;
 
+	/**
+	 * Maximum size, in bytes, of a single request body accepted by this transport.
+	 */
+	private final int requestMaxSize;
+
+	/**
+	 * Constructs a new HttpServletStatelessServerTransport instance.
+	 * @param jsonMapper The JsonMapper to use for JSON serialization/deserialization of
+	 * messages.
+	 * @param mcpEndpoint The endpoint URI where clients should send their JSON-RPC
+	 * messages.
+	 * @param contextExtractor The extractor for transport context from the request.
+	 * @param securityValidator The security validator for validating HTTP requests.
+	 * @param requestMaxSize The maximum size, in bytes, of a single request body. Must be
+	 * positive.
+	 * @throws IllegalArgumentException if any parameter is null
+	 */
 	private HttpServletStatelessServerTransport(McpJsonMapper jsonMapper, String mcpEndpoint,
 			McpTransportContextExtractor<HttpServletRequest> contextExtractor,
-			ServerTransportSecurityValidator securityValidator) {
+			ServerTransportSecurityValidator securityValidator, int requestMaxSize) {
 		Assert.notNull(jsonMapper, "jsonMapper must not be null");
 		Assert.notNull(mcpEndpoint, "mcpEndpoint must not be null");
 		Assert.notNull(contextExtractor, "contextExtractor must not be null");
 		Assert.notNull(securityValidator, "Security validator must not be null");
+		Assert.isTrue(requestMaxSize > 0, "requestMaxSize must be positive");
 
 		this.jsonMapper = jsonMapper;
 		this.mcpEndpoint = mcpEndpoint;
 		this.contextExtractor = contextExtractor;
 		this.securityValidator = securityValidator;
+		this.requestMaxSize = requestMaxSize;
 	}
 
 	@Override
@@ -133,6 +156,11 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 			return;
 		}
 
+		if (request.getContentLengthLong() > this.requestMaxSize) {
+			response.sendError(HttpServletResponse.SC_REQUEST_ENTITY_TOO_LARGE);
+			return;
+		}
+
 		try {
 			Map<String, List<String>> headers = HttpServletRequestUtils.extractHeaders(request);
 			this.securityValidator.validateHeaders(headers);
@@ -152,14 +180,9 @@ protected void doPost(HttpServletRequest request, HttpServletResponse response)
 		}
 
 		try {
-			BufferedReader reader = request.getReader();
-			StringBuilder body = new StringBuilder();
-			String line;
-			while ((line = reader.readLine()) != null) {
-				body.append(line);
-			}
+			String body = HttpServletRequestUtils.readBody(request, this.requestMaxSize);
 
-			McpSchema.JSONRPCMessage message = McpSchema.deserializeJsonRpcMessage(jsonMapper, body.toString());
+			McpSchema.JSONRPCMessage message = McpSchema.deserializeJsonRpcMessage(jsonMapper, body);
 
 			if (message instanceof McpSchema.JSONRPCRequest jsonrpcRequest) {
 				try {
@@ -201,6 +224,9 @@ else if (message instanceof McpSchema.JSONRPCNotification jsonrpcNotification) {
 						new McpError("The server accepts either requests or notifications"));
 			}
 		}
+		catch (MaxSizeExceededException e) {
+			response.sendError(HttpServletResponse.SC_REQUEST_ENTITY_TOO_LARGE);
+		}
 		catch (IllegalArgumentException | IOException e) {
 			logger.error("Failed to deserialize message: {}", e.getMessage());
 			this.responseError(response, HttpServletResponse.SC_BAD_REQUEST, new McpError("Invalid message format"));
@@ -265,6 +291,8 @@ public static class Builder {
 
 		private ServerTransportSecurityValidator securityValidator = ServerTransportSecurityValidator.NOOP;
 
+		private int requestMaxSize = DEFAULT_REQUEST_MAX_SIZE;
+
 		private Builder() {
 			// used by a static method
 		}
@@ -322,6 +350,19 @@ public Builder securityValidator(ServerTransportSecurityValidator securityValida
 			return this;
 		}
 
+		/**
+		 * Sets the maximum size, in bytes, of a single request body accepted by this
+		 * transport. Requests whose body exceeds this size are rejected with a 413
+		 * (Payload Too Large) response. Defaults to 16 MiB if not set.
+		 * @param requestMaxSize The maximum request body size, in bytes. Must be
+		 * positive.
+		 * @return this builder instance
+		 */
+		public Builder maxRequestSize(int requestMaxSize) {
+			this.requestMaxSize = requestMaxSize;
+			return this;
+		}
+
 		/**
 		 * Builds a new instance of {@link HttpServletStatelessServerTransport} with the
 		 * configured settings.
@@ -332,7 +373,7 @@ public HttpServletStatelessServerTransport build() {
 			Assert.notNull(mcpEndpoint, "Message endpoint must be set");
 			return new HttpServletStatelessServerTransport(
 					jsonMapper == null ? McpJsonDefaults.getMapper() : jsonMapper, mcpEndpoint, contextExtractor,
-					securityValidator);
+					securityValidator, requestMaxSize);
 		}
 
 	}