diff --git a/packages/router/src/url_tree.ts b/packages/router/src/url_tree.ts
index ee6853cc0872..3a9e62175d28 100644
--- a/packages/router/src/url_tree.ts
+++ b/packages/router/src/url_tree.ts
@@ -453,7 +453,8 @@ export class DefaultUrlSerializer implements UrlSerializer {
 
   /** Converts a `UrlTree` into a url */
   serialize(tree: UrlTree): string {
-    const segment = `/${serializeSegment(tree.root, true)}`;
+    // Empty leading path segments would make the result protocol-relative in browsers.
+    const segment = `/${serializeSegment(tree.root, true).replace(/^\/+/, '')}`;
     const query = serializeQueryParams(tree.queryParams);
     const fragment =
       typeof tree.fragment === `string` ? `#${encodeUriFragment(tree.fragment)}` : '';
diff --git a/packages/router/test/create_url_tree.spec.ts b/packages/router/test/create_url_tree.spec.ts
index 8132136d1ed5..668c3fe4902f 100644
--- a/packages/router/test/create_url_tree.spec.ts
+++ b/packages/router/test/create_url_tree.spec.ts
@@ -107,6 +107,12 @@ describe('createUrlTree', () => {
     expect(serializer.serialize(t)).toEqual('/');
   });
 
+  it('should not serialize leading empty path commands as a protocol-relative URL', async () => {
+    const p = serializer.parse('/');
+    const t = await createRoot(p, ['/', '', 'attacker.example', 'collect'], {session: 'secret'});
+    expect(serializer.serialize(t)).toEqual('/attacker.example/collect?session=secret');
+  });
+
   it('should error when navigating to the root segment with params', async () => {
     const p = serializer.parse('/');
     await expectAsync(createRoot(p, ['/', {p: 11}])).toBeRejectedWithError(