Reject Mcp-* headers with more than one value

Fixes #1321

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

Author: Kehrlann

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

@@ -391,14 +391,23 @@ private void writeJson(HttpServletResponse response, int status, JSONRPCResponse
 	/**
 	 * Validates {@code Mcp-Method}, {@code MCP-Protocol-Version} and, for the three named
 	 * methods, {@code Mcp-Name} against the request body.
+	 * <p>
+	 * Each of these headers must carry exactly one value. Intermediaries disagree on
+	 * which copy of a repeated header counts (the first, the last, or all of them joined
+	 * with {@code ", "}), so accepting a repeated header would let a request be routed or
+	 * authorized on one value while the server validates another.
 	 * @return a human-readable mismatch description, or {@code null} if the headers are
 	 * consistent with the body
 	 */
 	private String validateHeaders(HttpServletRequest request, JSONRPCRequest jsonRpcRequest) {
-		String methodHeader = request.getHeader("Mcp-Method");
-		if (methodHeader == null) {
+		List<String> methodHeaders = Collections.list(request.getHeaders("Mcp-Method"));
+		if (methodHeaders.isEmpty()) {
 			return "Missing required header: Mcp-Method";
 		}
+		if (methodHeaders.size() > 1) {
+			return "Mcp-Method header has more than one value";
+		}
+		String methodHeader = methodHeaders.get(0);
 		if (!methodHeader.equals(jsonRpcRequest.method())) {
 			return "Mcp-Method header does not match request method";
 		}
@@ -407,10 +416,14 @@ private String validateHeaders(HttpServletRequest request, JSONRPCRequest jsonRp
 		// which answers it with -32602.
 		String bodyVersion = protocolVersionFromMeta(jsonRpcRequest.params());
 		if (bodyVersion != null) {
-			String versionHeader = request.getHeader("MCP-Protocol-Version");
-			if (versionHeader == null) {
+			List<String> versionHeaders = Collections.list(request.getHeaders("MCP-Protocol-Version"));
+			if (versionHeaders.isEmpty()) {
 				return "Missing required header: MCP-Protocol-Version";
 			}
+			if (versionHeaders.size() > 1) {
+				return "MCP-Protocol-Version header has more than one value";
+			}
+			String versionHeader = versionHeaders.get(0);
 			if (!versionHeader.equals(bodyVersion)) {
 				return "MCP-Protocol-Version header does not match _meta protocol version";
 			}
@@ -427,10 +440,14 @@ private String validateHeaders(HttpServletRequest request, JSONRPCRequest jsonRp
 		if (!(paramsMap.get(nameKey) instanceof String expectedName)) {
 			return null;
 		}
-		String nameHeader = request.getHeader("Mcp-Name");
-		if (nameHeader == null) {
+		List<String> nameHeaders = Collections.list(request.getHeaders("Mcp-Name"));
+		if (nameHeaders.isEmpty()) {
 			return "Missing required header: Mcp-Name";
 		}
+		if (nameHeaders.size() > 1) {
+			return "Mcp-Name header has more than one value";
+		}
+		String nameHeader = nameHeaders.get(0);
 		String decoded = McpHeaderDecoder.decodeValue(nameHeader);
 		if (decoded == null) {
 			return "Mcp-Name header has a malformed value";

mcp-test/src/test/java/io/modelcontextprotocol/modern/server/transport/HttpServletMcpTransportIntegrationTests.java

@@ -55,6 +55,7 @@
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 import org.junit.jupiter.params.provider.NullAndEmptySource;
 import org.junit.jupiter.params.provider.ValueSource;
 import reactor.core.publisher.Flux;
@@ -403,6 +404,39 @@ void headerMismatchIsRejected() throws Exception {
 		assertThat(((Number) error.get("code")).intValue()).isEqualTo(ErrorCodes.HEADER_MISMATCH);
 	}
 
+	@ParameterizedTest
+	@CsvSource({ "Mcp-Method, tools/call", "Mcp-Method, tools/list", "Mcp-Name, echo", "Mcp-Name, other",
+			"MCP-Protocol-Version, " + McpSchema.LATEST_PROTOCOL_VERSION })
+	void repeatedStandardHeaderIsRejected(String name, String extraValue) throws Exception {
+		Map<String, Object> params = new HashMap<>();
+		params.put("_meta", meta());
+		params.put("name", "echo");
+		// The request already carries a valid value for every standard header; this adds
+		// a second line for one of them.
+		HttpRequest request = post("tools/call", params).header("Mcp-Name", "echo").header(name, extraValue).build();
+		HttpResponse<String> response = HttpClient.newHttpClient().send(request, HttpResponse.BodyHandlers.ofString());
+
+		assertThat(response.statusCode()).isEqualTo(400);
+		assertThatJson(response.body()).inPath("$.error.code").isEqualTo(ErrorCodes.HEADER_MISMATCH);
+		assertThatJson(response.body()).inPath("$.error.message").isEqualTo(name + " header has more than one value");
+	}
+
+	@Test
+	void commaJoinedMcpMethodIsRejected() throws Exception {
+		Map<String, Object> params = new HashMap<>();
+		params.put("_meta", meta());
+		params.put("name", "echo");
+		HttpRequest request = post("tools/call", params).header("Mcp-Name", "echo")
+			.setHeader("Mcp-Method", "tools/call, tools/list")
+			.build();
+		HttpResponse<String> response = HttpClient.newHttpClient().send(request, HttpResponse.BodyHandlers.ofString());
+
+		assertThat(response.statusCode()).isEqualTo(400);
+		assertThatJson(response.body()).inPath("$.error.code").isEqualTo(ErrorCodes.HEADER_MISMATCH);
+		assertThatJson(response.body()).inPath("$.error.message")
+			.isEqualTo("Mcp-Method header does not match request method");
+	}
+
 	@Test
 	void protocolVersionHeaderMismatchIsRejected() throws Exception {
 		Map<String, Object> meta = meta();