bugfix: ignore enumerable functions when copying headers in Pages-router

src/server/client.ts

@@ -682,17 +682,35 @@ export class Auth0Client {
     }
   }
 
+  /**
+   * createRequestCookies copies the incoming request's headers to a WHATWG
+   * `Headers` instance while ensuring we only include *actual* header values.
+   *
+   * In Node ≥ 20 the `req.headers` object is a proxy that also enumerates
+   * helper methods such as `append`, `delete`, `get`, etc.  Copying these
+   * function properties verbatim will cause the WHATWG `Headers` constructor
+   * to throw `TypeError: <fn> is an invalid header value`.
+   *
+   * We therefore iterate over `Object.entries(req.headers)` and copy only
+   * string or string[] values. This maintains backwards compatibility while
+   * fixing the runtime error reported in #2219.
+   */
   private createRequestCookies(req: PagesRouterRequest) {
     const headers = new Headers();
 
-    for (const key in req.headers) {
-      if (Array.isArray(req.headers[key])) {
-        for (const value of req.headers[key]) {
-          headers.append(key, value);
+    for (const [key, val] of Object.entries(req.headers)) {
+      if (val == null) {
+        continue; // skip undefined/null
+      }
+
+      if (Array.isArray(val)) {
+        for (const v of val) {
+          headers.append(key, v);
         }
-      } else {
-        headers.append(key, req.headers[key] ?? "");
+      } else if (typeof val === "string") {
+        headers.append(key, val);
       }
+      // ignore functions or other non-string values
     }
 
     return new RequestCookies(headers);

src/server/createRequestCookies-header-copy.test.ts

@@ -0,0 +1,63 @@
+import type { IncomingMessage } from "node:http";
+import { describe, expect, it } from "vitest";
+
+import { Auth0Client } from "./client.js";
+
+/**
+ * Regression tests for https://github.com/auth0/nextjs-auth0/issues/2219
+ * – "Headers.append: <fn> is an invalid header value".
+ */
+
+describe("#2219 – createRequestCookies header copying", () => {
+  /**
+   * Factory that creates an Auth0Client instance with minimal (fake) config
+   * sufficient for creating the class without throwing configuration errors.
+   */
+  function makeClient() {
+    return new Auth0Client({
+      domain: "example.auth0.com",
+      clientId: "client_id",
+      clientSecret: "client_secret",
+      secret: "0123456789abcdef0123456789abcdef",
+      appBaseUrl: "http://localhost:3000"
+    });
+  }
+
+  /**
+   * Creates a bare-bones object that satisfies the subset of `IncomingMessage`
+   * the private `createRequestCookies()` helper expects: a `headers` record.
+   */
+  function makePagesRouterReq(headers: Record<string, any>): IncomingMessage {
+    return { headers } as unknown as IncomingMessage;
+  }
+
+  it("ignores function properties on req.headers (no throw)", () => {
+    const client: any = makeClient();
+
+    const req = makePagesRouterReq({
+      cookie: "foo=bar",
+      foo: "bar",
+      append: () => {} // bogus enumerable function property that caused the crash
+    });
+
+    expect(() => client.createRequestCookies(req)).not.toThrow();
+  });
+
+  it("copies string and string[] values correctly", () => {
+    const client: any = makeClient();
+
+    const req = makePagesRouterReq({
+      cookie: "foo=bar",
+      "set-cookie": ["a=1", "b=2"]
+    });
+
+    const cookies = client.createRequestCookies(req);
+
+    // Should parse the cookie header
+    const foo = cookies.get("foo");
+    expect(foo?.value).toBe("bar");
+
+    // Still exposes at least one cookie object
+    expect(cookies.getAll().length).toBeGreaterThanOrEqual(1);
+  });
+});