diff --git a/README.md b/README.md
index cef96fb..7440242 100644
--- a/README.md
+++ b/README.md
@@ -160,6 +160,7 @@ The running server loads connector settings from the environment via
 | `FINANCE_DEFAULT_PAGE_SIZE` | Default page size when `limit` is omitted | `25` |
 | `FINANCE_MAX_PAGE_SIZE` | Upper bound applied to any requested `limit` | `100` |
 | `LOG_LEVEL` | Structured log level (`debug`/`info`/`warn`/`error`) | `info` |
+| `CORS_ALLOWED_ORIGINS` | Comma-separated list of origins allowed to read `/graphql`, `/graphql/stream`, and `/export` cross-origin | empty (all cross-origin reads denied) |
 
 Each connector endpoint that is **not** a `mock://` URL is served by the
 production HTTP client in `src/connectors/httpClient.js`, which adds bearer
@@ -448,12 +449,30 @@ client disconnects.
 
 ### CORS
 
-`/graphql`, `/graphql/stream`, and `/export` must be served behind an
-**explicit origin allow-list** — never a wildcard (`*`) `Access-Control-
-Allow-Origin` in production — so that only trusted front-ends can read
-responses cross-origin. Configure the CORS middleware with a fixed list (or
-an environment-driven list) of allowed origins instead of the permissive
-default before deploying publicly.
+`/graphql`, `/graphql/stream`, and `/export` are served behind an **explicit
+origin allow-list** — a wildcard (`*`) `Access-Control-Allow-Origin` is never
+emitted — so that only trusted front-ends can read responses cross-origin.
+
+The allow-list is built in `src/config/cors.js` from the
+`CORS_ALLOWED_ORIGINS` environment variable and applied by `src/index.js`:
+
+```bash
+CORS_ALLOWED_ORIGINS='https://app.example.com,https://admin.example.com' npm start
+```
+
+```js
+import { loadCorsOptions } from './config/cors.js';
+
+const corsOptions = loadCorsOptions();
+app.use('/graphql', cors(corsOptions), /* ... */);
+```
+
+An allowed `Origin` is echoed back verbatim; any other origin receives **no**
+CORS headers, so the browser blocks the response. When
+`CORS_ALLOWED_ORIGINS` is unset every cross-origin read is denied — this is
+the secure default, so the variable must be set for browser front-ends that
+live on a different origin. Requests without an `Origin` header (curl,
+server-to-server calls) are unaffected.
 
 Events are dispatched by an in-process pub/sub (`src/streaming/pubsub.js`).
 It is intentionally dependency-free, which means subscribers only see events
diff --git a/src/config/cors.js b/src/config/cors.js
new file mode 100644
index 0000000..2c89a4e
--- /dev/null
+++ b/src/config/cors.js
@@ -0,0 +1,33 @@
+/**
+ * Environment-driven CORS configuration.
+ *
+ * `CORS_ALLOWED_ORIGINS` holds a comma-separated allow-list of origins that may
+ * read responses cross-origin (e.g. `https://app.example.com,https://admin.example.com`).
+ * Anything outside the list gets no `Access-Control-Allow-Origin` header, so the
+ * browser blocks the response; the wildcard `*` is never emitted.
+ */
+export function parseAllowedOrigins(value) {
+  return String(value ?? '')
+    .split(',')
+    .map((origin) => origin.trim())
+    .filter(Boolean);
+}
+
+/** Builds the options object passed to the `cors()` middleware. */
+export function loadCorsOptions(env = process.env) {
+  const allowedOrigins = parseAllowedOrigins(env.CORS_ALLOWED_ORIGINS);
+
+  return {
+    // Requests without an `Origin` header (curl, server-to-server, same-origin
+    // navigations) are not subject to CORS, so they pass through unchanged and
+    // simply receive no CORS headers.
+    origin(origin, callback) {
+      if (!origin || allowedOrigins.includes(origin)) {
+        callback(null, origin ? [origin] : false);
+        return;
+      }
+      callback(null, false);
+    },
+    credentials: true,
+  };
+}
diff --git a/src/index.js b/src/index.js
index 8d3013b..db36ce8 100644
--- a/src/index.js
+++ b/src/index.js
@@ -2,6 +2,7 @@ import { expressMiddleware } from '@as-integrations/express4';
 import cors from 'cors';
 import express from 'express';
 
+import { loadCorsOptions } from './config/cors.js';
 import { store } from './data/store.js';
 import { createExportRouter } from './export/router.js';
 import { logger } from './observability/logger.js';
@@ -22,6 +23,10 @@ async function main() {
 
   const app = express();
 
+  // Cross-origin access is restricted to the CORS_ALLOWED_ORIGINS allow-list so
+  // a malicious page can never read GraphQL or export responses.
+  const corsOptions = loadCorsOptions();
+
   // Liveness probe: the process is up and serving.
   app.get('/health', (_req, res) => res.json({ status: 'ok' }));
 
@@ -38,13 +43,17 @@ async function main() {
   });
 
   // Spreadsheet downloads (.xlsx) for the same data the GraphQL API serves.
-  app.use('/export', cors(), createExportRouter({ store, finance: financeService, logger }));
+  app.use(
+    '/export',
+    cors(corsOptions),
+    createExportRouter({ store, finance: financeService, logger })
+  );
 
   // Streaming endpoint (Server-Sent Events) for subscriptions and one-shot
   // operations, mounted before /graphql so it keeps its own body parsing.
   app.use(
     '/graphql/stream',
-    cors(),
+    cors(corsOptions),
     express.json(),
     createStreamRouter({
       schema,
@@ -56,7 +65,7 @@ async function main() {
 
   app.use(
     '/graphql',
-    cors(),
+    cors(corsOptions),
     express.json(),
     expressMiddleware(apolloServer, {
       // Every request shares the same in-memory store.
diff --git a/test/cors.test.js b/test/cors.test.js
new file mode 100644
index 0000000..27fe83d
--- /dev/null
+++ b/test/cors.test.js
@@ -0,0 +1,157 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+
+import { expressMiddleware } from '@as-integrations/express4';
+import cors from 'cors';
+import express from 'express';
+
+import { loadCorsOptions, parseAllowedOrigins } from '../src/config/cors.js';
+import { createStore } from '../src/data/store.js';
+import { createExportRouter } from '../src/export/router.js';
+import { createSchema } from '../src/schema.js';
+import { createFinanceService } from '../src/services/financeService.js';
+import { createApolloServer } from '../src/server.js';
+import { createPubSub } from '../src/streaming/pubsub.js';
+import { createStreamRouter } from '../src/streaming/sseRouter.js';
+
+/** Starts a throwaway app guarded by the allow-list CORS middleware. */
+async function withServer(env, run) {
+  const app = express();
+  app.use(cors(loadCorsOptions(env)));
+  app.get('/probe', (_req, res) => res.json({ ok: true }));
+
+  const server = await new Promise((resolve) => {
+    const listener = app.listen(0, () => resolve(listener));
+  });
+  const { port } = server.address();
+  try {
+    await run(`http://127.0.0.1:${port}/probe`);
+  } finally {
+    await new Promise((resolve) => server.close(resolve));
+  }
+}
+
+/** Starts the actual /export, /graphql/stream, and /graphql mounts wired the same way as src/index.js. */
+async function withApp(env, run) {
+  const store = createStore();
+  const finance = createFinanceService();
+  const pubsub = createPubSub();
+  const schema = createSchema();
+  const apolloServer = createApolloServer({ schema });
+  await apolloServer.start();
+
+  const app = express();
+  const corsOptions = loadCorsOptions(env);
+
+  app.use('/export', cors(corsOptions), createExportRouter({ store, finance }));
+  app.use(
+    '/graphql/stream',
+    cors(corsOptions),
+    express.json(),
+    createStreamRouter({ schema, contextValue: () => ({ store, finance, pubsub }) })
+  );
+  app.use(
+    '/graphql',
+    cors(corsOptions),
+    express.json(),
+    expressMiddleware(apolloServer, { context: async () => ({ store, finance, pubsub }) })
+  );
+
+  const server = await new Promise((resolve) => {
+    const listener = app.listen(0, () => resolve(listener));
+  });
+  const { port } = server.address();
+  try {
+    await run(`http://127.0.0.1:${port}`);
+  } finally {
+    await new Promise((resolve) => server.close(resolve));
+    await apolloServer.stop();
+  }
+}
+
+describe('cors configuration', () => {
+  it('parses a comma-separated allow-list and ignores blanks', () => {
+    assert.deepEqual(parseAllowedOrigins(' https://a.example , ,https://b.example '), [
+      'https://a.example',
+      'https://b.example',
+    ]);
+    assert.deepEqual(parseAllowedOrigins(undefined), []);
+  });
+
+  it('echoes allowed origins and never emits a wildcard', async () => {
+    await withServer({ CORS_ALLOWED_ORIGINS: 'https://app.example' }, async (url) => {
+      const allowed = await fetch(url, { headers: { origin: 'https://app.example' } });
+      assert.equal(allowed.headers.get('access-control-allow-origin'), 'https://app.example');
+
+      const denied = await fetch(url, { headers: { origin: 'https://evil.example' } });
+      assert.equal(denied.headers.get('access-control-allow-origin'), null);
+
+      const noOrigin = await fetch(url);
+      assert.equal(noOrigin.headers.get('access-control-allow-origin'), null);
+      assert.equal(noOrigin.status, 200);
+    });
+  });
+
+  it('denies every origin when the allow-list is unset', async () => {
+    await withServer({}, async (url) => {
+      const response = await fetch(url, { headers: { origin: 'https://app.example' } });
+      assert.equal(response.headers.get('access-control-allow-origin'), null);
+      assert.equal(response.status, 200);
+    });
+  });
+
+  it('enforces the allow-list on the production /export, /graphql/stream, and /graphql mounts', async () => {
+    const env = { CORS_ALLOWED_ORIGINS: 'https://app.example' };
+    await withApp(env, async (baseUrl) => {
+      const routes = [
+        { path: '/export', init: {} },
+        {
+          // A valid one-shot query completes and closes the response itself,
+          // instead of leaving an SSE stream open for the test to clean up.
+          path: `/graphql/stream?query=${encodeURIComponent('{ __typename }')}`,
+          init: { headers: { accept: 'text/event-stream' }, method: 'GET' },
+        },
+        {
+          path: '/graphql',
+          init: {
+            method: 'POST',
+            headers: { 'content-type': 'application/json' },
+            body: JSON.stringify({ query: '{ __typename }' }),
+          },
+        },
+      ];
+
+      for (const { path, init } of routes) {
+        const allowed = await fetch(`${baseUrl}${path}`, {
+          ...init,
+          headers: { ...init.headers, origin: 'https://app.example' },
+        });
+        assert.equal(allowed.status, 200, `${path} should succeed for an allowed origin`);
+        assert.equal(
+          allowed.headers.get('access-control-allow-origin'),
+          'https://app.example',
+          `${path} should echo the allowed origin`
+        );
+
+        const denied = await fetch(`${baseUrl}${path}`, {
+          ...init,
+          headers: { ...init.headers, origin: 'https://evil.example' },
+        });
+        assert.equal(denied.status, 200, `${path} should still respond for a denied origin`);
+        assert.equal(
+          denied.headers.get('access-control-allow-origin'),
+          null,
+          `${path} should not echo a denied origin`
+        );
+
+        const noOrigin = await fetch(`${baseUrl}${path}`, init);
+        assert.equal(noOrigin.status, 200, `${path} should succeed without an Origin header`);
+        assert.equal(
+          noOrigin.headers.get('access-control-allow-origin'),
+          null,
+          `${path} should not emit a CORS header without an Origin header`
+        );
+      }
+    });
+  });
+});