title: Firebase security rules cross project
body: <p>Project A has Firebase Authentication and Project B has Firebase Firestore.
  Both Authentication and Firestore are working correctly using:
  <a href="https://firebase.google.com/docs/projects/multiprojects" rel="nofollow noreferrer">https://firebase.google.com/docs/projects/multiprojects</a> (Web)</p>
  
  <p>However when writing the following Project B Firestore Security Rules:</p>
  
  <pre><code>rules_version = '2';
  service cloud.firestore {
    match /databases/{database}/documents {
      match /users/{uid} {
        allow get: if request.auth.uid == uid;
      }
    }
  }
  </code></pre>
  
  <p>I get an error:</p>
  
  <pre><code>Missing or insufficient permissions
  </code></pre>
  
  <p>Because the Authentication data is with Project A, there is no <code>auth</code> with the Project B Firestore request.</p>
  
  <p>How do I give Project B the authentication information from Project A using JavaScript/Web?</p>