Understand Firebase Security Rules for Cloud Storage

Traditionally, security has been one of the most complex parts of app development. In most applications, developers must build and run a server that handles authentication (who a user is) and authorization (what a user can do). Authentication and authorization are hard to set up, harder to get right, and critical to the success of your product.

Similar to how Firebase Authentication makes it easy for you to authenticate your users, Firebase Security Rules for Cloud Storage makes it easy for you to authorize users and validate requests. Storage Security Rules manage the complexity for you by allowing you to specify path based permissions. In just a few lines of code, you can write authorization rules that restrict Cloud Storage requests to a certain user or limit the size of an upload.

The Firebase Realtime Database has a similar feature, called Firebase Realtime Database Rules

Authentication

Knowing who your users are is an important part of building an application, and Firebase Authentication provides an easy to use, secure, client side only solution to authentication. Firebase Security Rules for Cloud Storage ties in to Firebase Authentication for user based security. When a user is authenticated with Firebase Authentication, the request.auth variable in Storage Security Rules becomes an object that contains the user's unique ID (request.auth.uid) and all other user information in the token (request.auth.token). When the user is not authenticated, request.auth is null. This allows you to securely control data access on a per-user basis. You can learn more in the User Security section.

Authorization

Identifying your user is only part of security. Once you know who they are, you need a way to control their access to files in Cloud Storage.

Cloud Storage lets you specify per file and per path authorization rules that live on our servers and determine access to the files in your app. For example, the default Storage Security Rules require Firebase Authentication in order to perform any read or write operations on all files:

service firebase.storage {
  match /b/{bucket}/o {
    match /{allPaths=**} {
      allow read, write: if request.auth != null;
    }
  }
}

You can edit these rules by selecting a Firebase app in Firebase console and viewing the Rules tab of the Storage section.

Data Validation

Firebase Security Rules for Cloud Storage can also be used for data validation, including validating file name and path as well as file metadata properties such as contentType and size.

service firebase.storage {
  match /b/{bucket}/o {
    match /images/{imageId} {
      // Only allow uploads of any image file that's less than 5MB
      allow write: if request.resource.size < 5 * 1024 * 1024
                   && request.resource.contentType.matches('image/.*');
    }
  }
}

Now that you've learned the basics of what Storage Security Rules can do, Get Started!

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