Validate Content-Type by parsed media type instead of substring match (v1.x)

.changeset/content-type-media-type-validation.md

@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/sdk': patch
+---
+
+Validate `Content-Type` by its parsed media type instead of a substring match. POSTs to the Streamable HTTP server transport whose media type is not `application/json` are now rejected with `415 Unsupported Media Type` (previously any value containing the substring passed, and
+valid case variants were wrongly rejected). Values with parameters (`application/json; charset=utf-8`, including malformed parameter sections like `application/json;`) continue to work, and the client's response dispatch uses the same parsed comparison.

eslint.config.mjs

@@ -23,6 +23,23 @@ export default tseslint.config(
     {
         ignores: ['src/spec.types.ts']
     },
+    {
+        files: ['src/**/*.ts'],
+        rules: {
+            'no-restricted-syntax': [
+                'error',
+                {
+                    selector:
+                        ":matches(CallExpression[callee.property.name='includes'], CallExpression[callee.property.name='indexOf'], " +
+                        "CallExpression[callee.property.name='startsWith'])[arguments.0.value='application/json']",
+                    message:
+                        "Substring-matching 'application/json' misclassifies Content-Type values whose media type is different " +
+                        "(e.g. 'text/plain; a=application/json') and mishandles parameters and case. " +
+                        'Parse the media type instead: isJsonContentType() from src/shared/mediaType.js.'
+                }
+            ]
+        }
+    },
     {
         files: ['src/client/**/*.ts', 'src/server/**/*.ts'],
         ignores: ['**/*.test.ts'],

src/client/streamableHttp.ts

@@ -1,3 +1,4 @@
+import { mediaTypeEssence } from '../shared/mediaType.js';
 import { Transport, FetchLike, createFetchWithInit, normalizeHeaders } from '../shared/transport.js';
 import { isInitializedNotification, isJSONRPCRequest, isJSONRPCResultResponse, JSONRPCMessage, JSONRPCMessageSchema } from '../types.js';
 import { auth, AuthResult, extractWWWAuthenticateParams, OAuthClientProvider, UnauthorizedError } from './auth.js';
@@ -572,16 +573,17 @@ export class StreamableHTTPClientTransport implements Transport {
 
             const hasRequests = messages.filter(msg => 'method' in msg && 'id' in msg && msg.id !== undefined).length > 0;
 
-            // Check the response type
+            // Check the response type (parsed media type — see mediaTypeEssence)
             const contentType = response.headers.get('content-type');
+            const responseMediaType = mediaTypeEssence(contentType);
 
             if (hasRequests) {
-                if (contentType?.includes('text/event-stream')) {
+                if (responseMediaType === 'text/event-stream') {
                     // Handle SSE stream responses for requests
                     // We use the same handler as standalone streams, which now supports
                     // reconnection with the last event ID
                     this._handleSseStream(response.body, { onresumptiontoken }, false);
-                } else if (contentType?.includes('application/json')) {
+                } else if (responseMediaType === 'application/json') {
                     // For non-streaming servers, we might get direct JSON responses
                     const data = await response.json();
                     const responseMessages = Array.isArray(data)

src/server/webStandardStreamableHttp.ts

@@ -7,6 +7,7 @@
  * For Node.js Express/HTTP compatibility, use `StreamableHTTPServerTransport` which wraps this transport.
  */
 
+import { isJsonContentType } from '../shared/mediaType.js';
 import { Transport } from '../shared/transport.js';
 import { AuthInfo } from './auth/types.js';
 import {
@@ -599,6 +600,8 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
             // Validate the Accept header
             const acceptHeader = req.headers.get('accept');
             // The client MUST include an Accept header, listing both application/json and text/event-stream as supported content types.
+            // Accept is a comma-separated list, so a substring check is the intended semantics here (unlike Content-Type below).
+            // eslint-disable-next-line no-restricted-syntax
             if (!acceptHeader?.includes('application/json') || !acceptHeader.includes('text/event-stream')) {
                 this.onerror?.(new Error('Not Acceptable: Client must accept both application/json and text/event-stream'));
                 return this.createJsonErrorResponse(
@@ -608,8 +611,9 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
                 );
             }
 
+            // Parsed media type, never a substring match — see isJsonContentType.
             const ct = req.headers.get('content-type');
-            if (!ct || !ct.includes('application/json')) {
+            if (!isJsonContentType(ct)) {
                 this.onerror?.(new Error('Unsupported Media Type: Content-Type must be application/json'));
                 return this.createJsonErrorResponse(415, -32000, 'Unsupported Media Type: Content-Type must be application/json');
             }

src/shared/mediaType.ts

@@ -0,0 +1,57 @@
+import contentType from 'content-type';
+
+/**
+ * Extracts the media type (the lowercased `type/subtype` pair, without
+ * parameters) from a raw `Content-Type` header value, or `undefined` when the
+ * header is missing or empty.
+ *
+ * Content-Type comparisons must use the parsed media type, never a substring
+ * search of the raw header: a value like `text/plain; a=application/json`
+ * contains the substring `application/json` but its media type is
+ * `text/plain`, and case variants or parameters make naive string comparison
+ * wrong in both directions.
+ *
+ * "Essence" is the WHATWG MIME Sniffing standard's term for the bare
+ * `type/subtype` pair (https://mimesniff.spec.whatwg.org/#mime-type-essence);
+ * the Fetch standard's request classification is defined against it
+ * (https://fetch.spec.whatwg.org/#cors-safelisted-request-header).
+ *
+ * Parsing is RFC 9110 (`content-type` package) first. When the parameter
+ * section is malformed (`application/json;`, `application/json; charset=`),
+ * browsers and most HTTP stacks still derive the media type from the segment
+ * before the first `;` — the fallback matches that widely-implemented
+ * behavior, so a header whose media type is unambiguous is not rejected for
+ * a sloppy parameter section.
+ */
+export function mediaTypeEssence(header: string | null | undefined): string | undefined {
+    if (!header) {
+        return undefined;
+    }
+    try {
+        return contentType.parse(header).type;
+    } catch {
+        const essence = (header.split(';', 1)[0] ?? '').trim().toLowerCase();
+        // A comma in the parameter tail of an unparseable value indicates
+        // joined duplicate headers — ambiguous, so no essence at all (keeps
+        // duplicate-header handling uniform whether or not the first copy
+        // carries parameters).
+        if (essence === '' || header.slice(essence.length).includes(',')) {
+            return undefined;
+        }
+        return essence;
+    }
+}
+
+/**
+ * Whether a raw `Content-Type` header value denotes `application/json`.
+ * Parameters (for example `charset=utf-8`) are allowed and ignored; malformed
+ * parameter sections do not reject a header whose media type is unambiguously
+ * `application/json` (see `mediaTypeEssence` for the exact grammar).
+ */
+export function isJsonContentType(header: string | null | undefined): boolean {
+    // Fast path: the exact literal is what SDK clients send on every POST.
+    if (header === 'application/json') {
+        return true;
+    }
+    return mediaTypeEssence(header) === 'application/json';
+}

test/server/contentTypeValidation.test.ts

@@ -0,0 +1,115 @@
+/**
+ * Content-Type validation at the Streamable HTTP entry — the parsed media
+ * type decides, never a substring search of the raw header.
+ *
+ * The shape pinned here: `Content-Type: text/plain; a=application/json`
+ * contains the substring `application/json`, but its media type is
+ * `text/plain` — the transport must answer it (and any other non-JSON media
+ * type) with 415 before the body is dispatched, while values whose media type
+ * is `application/json` keep working regardless of parameters or case.
+ */
+import { z } from 'zod';
+import { McpServer } from '../../src/server/mcp.js';
+import { WebStandardStreamableHTTPServerTransport } from '../../src/server/webStandardStreamableHttp.js';
+import { CallToolResult } from '../../src/types.js';
+
+const executed: string[] = [];
+
+function factory(): McpServer {
+    const mcpServer = new McpServer({ name: 'ct-fixture', version: '1.0.0' });
+    mcpServer.tool('run', 'records each dispatch', { cmd: z.string() }, async ({ cmd }): Promise<CallToolResult> => {
+        executed.push(cmd);
+        return { content: [{ type: 'text', text: `ran: ${cmd}` }] };
+    });
+    return mcpServer;
+}
+
+const TOOL_CALL = {
+    jsonrpc: '2.0',
+    id: 1,
+    method: 'tools/call',
+    params: { name: 'run', arguments: { cmd: 'hello' } }
+};
+
+function postRequest(body: unknown, headers: Record<string, string>): Request {
+    return new Request('http://127.0.0.1/mcp', {
+        method: 'POST',
+        headers: {
+            Accept: 'application/json, text/event-stream',
+            ...headers
+        },
+        body: JSON.stringify(body)
+    });
+}
+
+async function sendToTransport(request: Request): Promise<Response> {
+    const server = factory();
+    const transport = new WebStandardStreamableHTTPServerTransport({
+        sessionIdGenerator: undefined,
+        enableJsonResponse: true
+    });
+    await server.connect(transport);
+    const response = await transport.handleRequest(request);
+    await server.close();
+    return response;
+}
+
+async function postToTransport(headers: Record<string, string>): Promise<Response> {
+    return sendToTransport(postRequest(TOOL_CALL, headers));
+}
+
+beforeEach(() => {
+    executed.length = 0;
+});
+
+describe('WebStandardStreamableHTTPServerTransport Content-Type validation', () => {
+    it('serves an application/json POST (control)', async () => {
+        const response = await postToTransport({ 'Content-Type': 'application/json' });
+        expect(response.status).toBe(200);
+        expect(executed).toEqual(['hello']);
+    });
+
+    it('accepts application/json with parameters', async () => {
+        const response = await postToTransport({ 'Content-Type': 'application/json; charset=utf-8' });
+        expect(response.status).toBe(200);
+        expect(executed).toEqual(['hello']);
+    });
+
+    it('rejects text/plain with 415', async () => {
+        const response = await postToTransport({ 'Content-Type': 'text/plain' });
+        expect(response.status).toBe(415);
+        expect(executed).toEqual([]);
+    });
+
+    it('rejects a non-JSON media type whose parameters contain `application/json` and does not dispatch', async () => {
+        const response = await postToTransport({ 'Content-Type': 'text/plain; a=application/json' });
+        expect(response.status).toBe(415);
+        const body = (await response.json()) as { error: { code: number; message: string } };
+        expect(body.error.code).toBe(-32000);
+        expect(body.error.message).toContain('Content-Type must be application/json');
+        expect(executed).toEqual([]);
+    });
+
+    it('rejects a POST with no Content-Type header with 415', async () => {
+        // A string body makes Request auto-attach `text/plain;charset=UTF-8`;
+        // delete it so this actually exercises the absent-header branch.
+        const request = postRequest(TOOL_CALL, {});
+        request.headers.delete('content-type');
+        expect(request.headers.get('content-type')).toBeNull();
+        const response = await sendToTransport(request);
+        expect(response.status).toBe(415);
+        expect(executed).toEqual([]);
+    });
+
+    it('accepts an unambiguous media type with a malformed parameter section (trailing semicolon)', async () => {
+        const response = await postToTransport({ 'Content-Type': 'application/json;' });
+        expect(response.status).toBe(200);
+        expect(executed).toEqual(['hello']);
+    });
+
+    it('rejects joined duplicate Content-Type headers', async () => {
+        const response = await postToTransport({ 'Content-Type': 'application/json, application/json' });
+        expect(response.status).toBe(415);
+        expect(executed).toEqual([]);
+    });
+});

test/shared/mediaType.test.ts

@@ -0,0 +1,66 @@
+import { isJsonContentType, mediaTypeEssence } from '../../src/shared/mediaType.js';
+
+describe('mediaTypeEssence', () => {
+    it('parses well-formed headers', () => {
+        expect(mediaTypeEssence('application/json')).toBe('application/json');
+        expect(mediaTypeEssence('text/event-stream; charset=utf-8')).toBe('text/event-stream');
+        expect(mediaTypeEssence('Application/JSON; charset=utf-8')).toBe('application/json');
+    });
+
+    it('falls back to the pre-parameter segment for malformed parameter sections', () => {
+        expect(mediaTypeEssence('application/json;')).toBe('application/json');
+        expect(mediaTypeEssence('application/json; charset=')).toBe('application/json');
+        expect(mediaTypeEssence('text/plain;')).toBe('text/plain');
+    });
+
+    it('returns undefined for missing or empty headers', () => {
+        expect(mediaTypeEssence(null)).toBeUndefined();
+        expect(mediaTypeEssence(undefined)).toBeUndefined();
+        expect(mediaTypeEssence('')).toBeUndefined();
+        expect(mediaTypeEssence('   ')).toBeUndefined();
+        expect(mediaTypeEssence(';charset=utf-8')).toBeUndefined();
+    });
+
+    it('yields no essence for joined duplicate headers, with or without parameters', () => {
+        // Headers.get() joins repeated headers with ', '. Without parameters
+        // the comma lands in the first segment; with parameters it hides in
+        // the tail — both must behave the same.
+        expect(mediaTypeEssence('application/json, application/json')).toBe('application/json, application/json');
+        expect(mediaTypeEssence('application/json; charset=utf-8, text/plain')).toBeUndefined();
+        expect(mediaTypeEssence('application/json; charset=utf-8, application/json')).toBeUndefined();
+    });
+});
+
+describe('isJsonContentType', () => {
+    it('accepts application/json with or without parameters', () => {
+        expect(isJsonContentType('application/json')).toBe(true);
+        expect(isJsonContentType('application/json; charset=utf-8')).toBe(true);
+        expect(isJsonContentType('Application/JSON')).toBe(true);
+        expect(isJsonContentType('  application/json ; charset=utf-8')).toBe(true);
+    });
+
+    it('accepts unambiguous media types with malformed parameter sections', () => {
+        expect(isJsonContentType('application/json;')).toBe(true);
+        expect(isJsonContentType('application/json; charset=')).toBe(true);
+        expect(isJsonContentType('application/json; charset=utf-8; charset=x')).toBe(true);
+    });
+
+    it('never matches on substrings: parameters and sibling types are not application/json', () => {
+        expect(isJsonContentType('text/plain; a=application/json')).toBe(false);
+        expect(isJsonContentType('text/plain;')).toBe(false);
+        expect(isJsonContentType('text/plain, application/json')).toBe(false);
+        expect(isJsonContentType('application/json, application/json')).toBe(false);
+        expect(isJsonContentType('application/json; charset=utf-8, text/plain')).toBe(false);
+        expect(isJsonContentType('text/plain; charset=utf-8, application/json')).toBe(false);
+        expect(isJsonContentType('application/json-patch+json')).toBe(false);
+        expect(isJsonContentType('application/jsonp')).toBe(false);
+    });
+
+    it('rejects missing, empty, and non-JSON types', () => {
+        expect(isJsonContentType(null)).toBe(false);
+        expect(isJsonContentType(undefined)).toBe(false);
+        expect(isJsonContentType('')).toBe(false);
+        expect(isJsonContentType('text/plain')).toBe(false);
+        expect(isJsonContentType('multipart/form-data')).toBe(false);
+    });
+});