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fix(server): read HTTP request bodies with a size limit and bound JSON-RPC batch length (#2698)

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Co-authored-by: Felix Weinberger <fweinberger@anthropic.com>
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Max 2026-08-25 14:23:40 +01:00 • committed by GitHub
commit 7b781ed4e2
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2 changed files with 651 additions and 53 deletions
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5
packages/middleware/express/src/express.ts
View file

@ -76,7 +76,6 @@ export function createMcpExpressApp(options: CreateMcpExpressAppOptions = {}): E
const { host = '127.0.0.1', allowedHosts, allowedOrigins, jsonLimit } = options;
const app = express();
app.use(express.json(jsonLimit ? { limit: jsonLimit } : undefined));
// If allowedHosts is explicitly provided, use that for validation
if (allowedHosts) {
@ -106,5 +105,9 @@ export function createMcpExpressApp(options: CreateMcpExpressAppOptions = {}): E
app.use(localhostOriginValidation());
}
// The JSON body parser runs after the Host/Origin validation, so a request
// from a disallowed origin is answered 403 without its body being read.
app.use(express.json(jsonLimit ? { limit: jsonLimit } : undefined));
return app;
}

38
packages/middleware/express/test/express.test.ts
View file

@ -1,4 +1,5 @@
import type { NextFunction, Request, Response } from 'express';
import supertest from 'supertest';
import { vi } from 'vitest';
import { createMcpExpressApp } from '../src/express';
@ -107,6 +108,43 @@ describe('@modelcontextprotocol/express', () => {
});
describe('createMcpExpressApp', () => {
test('validates Host/Origin before the JSON body parser runs', async () => {
const app = createMcpExpressApp();
app.post('/mcp', (req: Request, res: Response) => {
res.json({ parsed: req.body });
});
const disallowedHost = await supertest(app)
.post('/mcp')
.set('Host', 'evil.example.com')
.set('Content-Type', 'application/json')
.send('{not json');
expect(disallowedHost.status).toBe(403);
const disallowedOrigin = await supertest(app)
.post('/mcp')
.set('Host', 'localhost:3000')
.set('Origin', 'http://evil.example.com')
.set('Content-Type', 'application/json')
.send('{not json');
expect(disallowedOrigin.status).toBe(403);
const invalidJson = await supertest(app)
.post('/mcp')
.set('Host', 'localhost:3000')
.set('Content-Type', 'application/json')
.send('{not json');
expect(invalidJson.status).toBe(400);
const allowed = await supertest(app)
.post('/mcp')
.set('Host', 'localhost:3000')
.set('Content-Type', 'application/json')
.send({ ok: true });
expect(allowed.status).toBe(200);
expect(allowed.body).toEqual({ parsed: { ok: true } });
});
test('should enable localhost DNS rebinding protection by default', () => {
const app = createMcpExpressApp();