Add origin filtering for expose

src/comlink.ts

@@ -281,11 +281,34 @@ export const transferHandlers = new Map<
   ["throw", throwTransferHandler],
 ]);
 
-export function expose(obj: any, ep: Endpoint = self as any) {
+function isAllowedOrigin(
+  allowedOrigins: (string | RegExp)[],
+  origin: string
+): boolean {
+  for (const allowedOrigin of allowedOrigins) {
+    if (origin === allowedOrigin || allowedOrigin === "*") {
+      return true;
+    }
+    if (allowedOrigin instanceof RegExp && allowedOrigin.test(origin)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+export function expose(
+  obj: any,
+  ep: Endpoint = self as any,
+  allowedOrigins: (string | RegExp)[] = ["*"]
+) {
   ep.addEventListener("message", function callback(ev: MessageEvent) {
     if (!ev || !ev.data) {
       return;
     }
+    if (!isAllowedOrigin(allowedOrigins, ev.origin)) {
+      console.warn(`Invalid origin '${ev.origin}' for comlink proxy`);
+      return;
+    }
     const { id, type, path } = {
       path: [] as string[],
       ...(ev.data as Message),

tests/cross-origin.comlink.test.js

@@ -0,0 +1,83 @@
+/**
+ * Copyright 2017 Google Inc. All Rights Reserved.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import * as Comlink from "/base/dist/esm/comlink.mjs";
+
+describe("Comlink origin filtering", function () {
+  it("rejects messages from unknown origin", async function () {
+    // expose on our window so comlink is listening to window postmessage
+    const obj = { my: "value" };
+    Comlink.expose(obj, self, [/^http:\/\/localhost(:[0-9]+)?\/?$/]);
+
+    let handler;
+    // juggle async timings to get the attack started
+    const attackComplete = new Promise((resolve, reject) => {
+      handler = (ev) => {
+        if (ev.data === "ready" && ev.origin === "null") {
+          // tell the iframe it can start the attack
+          ifr.contentWindow.postMessage("start", "*");
+        } else if (ev.data === "done") {
+          // confirm the attack failed, the prototype was not updated
+          expect(Object.prototype.foo).to.be.undefined;
+          expect(obj.my).to.equal("value");
+          resolve();
+        }
+      };
+      window.addEventListener("message", handler);
+    });
+    // create a sandboxed iframe for the attack
+    const ifr = document.createElement("iframe");
+    ifr.sandbox.add("allow-scripts");
+    ifr.src = "/base/tests/fixtures/attack-iframe.html";
+    document.body.appendChild(ifr);
+    // wait for the iframe to load
+    await new Promise((resolve) => (ifr.onload = resolve));
+    // and wait for the attack to complete
+    await attackComplete;
+    window.removeEventListener("message", handler);
+    ifr.remove();
+  });
+  it("accepts messages from matching origin", async function () {
+    // expose on our window so comlink is listening to window postmessage
+    const obj = { my: "value" };
+    Comlink.expose(obj, self, [/^http:\/\/localhost(:[0-9]+)?\/?$/]);
+
+    let handler;
+    // juggle async timings to get the attack started
+    const attackComplete = new Promise((resolve, reject) => {
+      handler = (ev) => {
+        if (ev.data === "ready" && ev.origin === window.origin) {
+          // tell the iframe it can start the attack
+          ifr.contentWindow.postMessage("start", "*");
+        } else if (ev.data === "done") {
+          // confirm the attack succeeded, the prototype was updated
+          expect(Object.prototype.foo).to.equal("x");
+          expect(obj.my).to.equal("value");
+          resolve();
+        }
+      };
+      window.addEventListener("message", handler);
+    });
+    // create a sandboxed iframe for the attack, but with same origin
+    const ifr = document.createElement("iframe");
+    ifr.sandbox.add("allow-scripts", "allow-same-origin");
+    ifr.src = "/base/tests/fixtures/attack-iframe.html";
+    document.body.appendChild(ifr);
+    // wait for the iframe to load
+    await new Promise((resolve) => (ifr.onload = resolve));
+    // and wait for the attack to complete
+    await attackComplete;
+    window.removeEventListener("message", handler);
+    ifr.remove();
+  });
+});

tests/fixtures/attack-iframe.html

@@ -0,0 +1,21 @@
+<html>
+  <body>
+    <script>
+      window.addEventListener("message", (ev) => {
+        if (ev.data === "start") {
+          // send back a message to modify the prototype
+          parent.postMessage(
+            {
+              type: "SET",
+              value: { type: "RAW", value: "x" },
+              path: ["__proto__", "foo"],
+            },
+            "*"
+          );
+          parent.postMessage("done", "*");
+        }
+      });
+      parent.postMessage("ready", "*");
+    </script>
+  </body>
+</html>