How Security Rules work

Cloud Firestore

Basic structure

Firebase Security Rules in Cloud Firestore and Cloud Storage use the following structure and syntax:

service <<name>> {
  // Match the resource path.
  match <<path>> {
    // Allow the request if the following conditions are true.
    allow <<methods>> : if <<condition>>
  }
}

The following key concepts are important to understand as you build the rules:

• Request: The method or methods invoked in the allow statement. These are methods you're allowing to run. The standard methods are: get, list, create, update, and delete. The read and write convenience methods enable broad read and write access on the specified database or storage path.
• Path: The database or storage location, represented as a URI path.
• Rule: The allow statement, which includes a condition that permits a request if it evaluates to true.

Security rules version 2

As of May 2019, version 2 of the Firebase security rules is now available. Version 2 of the rules changes the behavior of recursive wildcards {name=**}. You must use version 2 if you plan to use collection group queries. You must opt-in to version 2 by making rules_version = '2'; the first line in your security rules:

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {

Matching paths

All match statements should point to documents, not collections. A match statement can point to a specific document, as in match /cities/SF or use wildcards to point to any document in the specified path, as in match /cities/{city}.

In the example above, the match statement uses the {city} wildcard syntax. This means the rule applies to any document in the cities collection, such as /cities/SF or /cities/NYC. When the allow expressions in the match statement are evaluated, the city variable will resolve to the city document name, such as SF or NYC.

Matching subcollections

Data in Cloud Firestore is organized into collections of documents, and each document may extend the hierarchy through subcollections. It is important to understand how security rules interact with hierarchical data.

Consider the situation where each document in the cities collection contains a landmarks subcollection. Security rules apply only at the matched path, so the access controls defined on the cities collection do not apply to the landmarks subcollection. Instead, write explicit rules to control access to subcollections:

service cloud.firestore {
  match /databases/{database}/documents {
    match /cities/{city} {
      allow read, write: if <condition>;

      // Explicitly define rules for the 'landmarks' subcollection
      match /landmarks/{landmark} {
        allow read, write: if <condition>;
      }
    }
  }
}

When nesting match statements, the path of the inner match statement is always relative to the path of the outer match statement. The following rulesets are therefore equivalent:

service cloud.firestore {
  match /databases/{database}/documents {
    match /cities/{city} {
      match /landmarks/{landmark} {
        allow read, write: if <condition>;
      }
    }
  }
}

service cloud.firestore {
  match /databases/{database}/documents {
    match /cities/{city}/landmarks/{landmark} {
      allow read, write: if <condition>;
    }
  }
}

Recursive wildcards

If you want rules to apply to an arbitrarily deep hierarchy, use the recursive wildcard syntax, {name=**}:

service cloud.firestore {
  match /databases/{database}/documents {
    // Matches any document in the cities collection as well as any document
    // in a subcollection.
    match /cities/{document=**} {
      allow read, write: if <condition>;
    }
  }
}

When using the recursive wildcard syntax, the wildcard variable will contain the entire matching path segment, even if the document is located in a deeply nested subcollection. For example, the rules listed above would match a document located at /cities/SF/landmarks/coit_tower, and the value of the document variable would be SF/landmarks/coit_tower.

Note, however, that the behavior of recursive wildcards depends on the rules version.

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    // Matches any document in the cities collection as well as any document
    // in a subcollection.
    match /cities/{city}/{document=**} {
      allow read, write: if <condition>;
    }
  }
}

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    // Matches any document in the songs collection group
    match /{path=**}/songs/{song} {
      allow read, write: if <condition>;
    }
  }
}

If you use collection group queries, you must use version 2, see securing collection group queries.

Overlapping match statements

It's possible for a document to match more than one match statement. In the case where multiple allow expressions match a request, the access is allowed if any of the conditions is true:

service cloud.firestore {
  match /databases/{database}/documents {
    // Matches any document in the 'cities' collection.
    match /cities/{city} {
      allow read, write: if false;
    }

    // Matches any document in the 'cities' collection or subcollections.
    match /cities/{document=**} {
      allow read, write: if true;
    }
  }
}

In the example above, all reads and writes to the cities collection will be allowed because the second rule is always true, even though the first rule is always false.

Security rule limits

As you work with security rules, note the following limits:

Cloud Storage

Basic structure

Firebase Security Rules in Cloud Firestore and Cloud Storage use the following structure and syntax:

service <<name>> {
  // Match the resource path.
  match <<path>> {
    // Allow the request if the following conditions are true.
    allow <<methods>> : if <<condition>>
  }
}

The following key concepts are important to understand as you build the rules:

• Request: The method or methods invoked in the allow statement. These are methods you're allowing to run. The standard methods are: get, list, create, update, and delete. The read and write convenience methods enable broad read and write access on the specified database or storage path.
• Path: The database or storage location, represented as a URI path.
• Rule: The allow statement, which includes a condition that permits a request if it evaluates to true.

Matching paths

Cloud Storage Security Rules match the file paths used to access files in Cloud Storage. Rules can match exact paths or wildcard paths, and rules can also be nested. If no match rule allows an request method, or the condition evaluates to false, the request is denied.

Exact matches

// Exact match for "images/profilePhoto.png"
match /images/profilePhoto.png {
  allow write: if <condition>;
}

// Exact match for "images/croppedProfilePhoto.png"
match /images/croppedProfilePhoto.png {
  allow write: if <other_condition>;
}

Nested matches

// Partial match for files that start with "images"
match /images {
  // Exact match for "images/profilePhoto.png"
  match /profilePhoto.png {
    allow write: if <condition>;
  }

  // Exact match for "images/croppedProfilePhoto.png"
  match /croppedProfilePhoto.png {
    allow write: if <other_condition>;
  }
}

Wildcard matches

Rules can also be used to match a pattern using wildcards. A wildcard is a named variable that represents either a single string such as profilePhoto.png, or multiple path segments, such as images/profilePhoto.png.

A wildcard is created by adding curly braces around the wildcard name, like {string}. A multiple segment wildcard can be declared by adding =** to the wildcard name, like {path=**}:

// Partial match for files that start with "images"
match /images {
  // Exact match for "images/*"
  // e.g. images/profilePhoto.png is matched
  match /{imageId} {
    // This rule only matches a single path segment (*)
    // imageId is a string that contains the specific segment matched
    allow read: if <condition>;
  }

  // Exact match for "images/**"
  // e.g. images/users/user:12345/profilePhoto.png is matched
  // images/profilePhoto.png is also matched!
  match /{allImages=**} {
    // This rule matches one or more path segments (**)
    // allImages is a path that contains all segments matched
    allow read: if <other_condition>;
  }
}

If multiple rules match a file, the result is the OR of the result of all rules evaluations. That is, if any rule the file matches evaluates to true, the result is true.

In the rules above, the file "images/profilePhoto.png" can be read if either condition or other_condition evaluate to true, while the file "images/users/user:12345/profilePhoto.png" is only subject to the result of other_condition.

A wildcard variable can be referenced from within the match provide file name or path authorization:

// Another way to restrict the name of a file
match /images/{imageId} {
  allow read: if imageId == "profilePhoto.png";
}

Cloud Storage Security Rules do not cascade, and rules are only evaluated when the request path matches a path with rules specified.

Request evaluation

Uploads, downloads, metadata changes, and deletes are evaluated using the request sent to Cloud Storage. The request variable contains the file path where the request is being performed, the time when the request is received, and the new resource value if the request is a write. HTTP headers and authentication state are also included.

The request object also contains the user's unique ID and the Firebase Authentication payload in the request.auth object, which will be explained further in the Authentication section of the docs.

A full list of properties in the request object is available below:

Resource evaluation

When evaluating rules, you may also want to evaluate the metadata of the file being uploaded, downloaded, modified, or deleted. This allows you to create complex and powerful rules that do things like only allow files with certain content types to be uploaded, or only files greater than a certain size to be deleted.

Firebase Security Rules for Cloud Storage provides file metadata in the resource object, which contains key/value pairs of the metadata surfaced in a Cloud Storage object. These properties can be inspected on read or write requests to ensure data integrity.

On write requests (such as uploads, metadata updates, and deletes), in addition to the resource object, which contains file metadata for the file that currently exists at the request path, you also have the ability to use the request.resource object, which contains a subset of the file metadata to be written if the write is allowed. You can use these two values to ensure data integrity or enforce application constraints such as file type or size.

A full list of properties in the resource object is available below:

request.resource contains all of these with the exception of generation, metageneration, etag, timeCreated, and updated.

Security Rules limits

As you work with security rules, note the following limits:

Full Example

Putting it all together, you can create a full example of rules for an image storage solution:

service firebase.storage {
 match /b/{bucket}/o {
   match /images {
     // Cascade read to any image type at any path
     match /{allImages=**} {
       allow read;
     }

     // Allow write files to the path "images/*", subject to the constraints:
     // 1) File is less than 5MB
     // 2) Content type is an image
     // 3) Uploaded content type matches existing content type
     // 4) File name (stored in imageId wildcard variable) is less than 32 characters
     match /{imageId} {
       allow write: if request.resource.size < 5 * 1024 * 1024
                    && request.resource.contentType.matches('image/.*')
                    && request.resource.contentType == resource.contentType
                    && imageId.size() < 32
     }
   }
 }
}

Realtime Database

Basic structure

In Realtime Database, Firebase Security Rules consist of JavaScript-like expressions contained in a JSON document.

They use the following syntax:

{
  "rules": {
    "<<path>>": {
    // Allow the request if the condition for each method is true.
      ".read": <<condition>>,
      ".write": <<condition>>,
      ".validate": <<condition>>
    }
  }
}

There are three basic elements in the rule:

• Path: The database location. This mirrors your database's JSON structure.
• Request: These are the methods the rule uses to grant access. The read and write rules grant broad read and write access, while validate rules act as a secondary verification to grant access based on incoming or existing data.
• Condition: The condition that permits a request if it evaluates to true.

How rules apply to paths

In Realtime Database, Rules apply atomically, meaning that rules at higher-level parent nodes override rules at more granular child nodes and rules at a deeper node can't grant access to a parent path. You can't refine or revoke access at a deeper path in your database structure if you've already granted it for one of the parent paths.

Consider the following rules:

{
  "rules": {
     "foo": {
        // allows read to /foo/*
        ".read": "data.child('baz').val() === true",
        "bar": {
          // ignored, since read was allowed already
          ".read": false
        }
     }
  }
}

This security structure allows /bar/ to be read from whenever /foo/ contains a child baz with value true. The ".read": false rule under /foo/bar/ has no effect here, since access cannot be revoked by a child path.

While it may not seem immediately intuitive, this is a powerful part of the rules language and allows for very complex access privileges to be implemented with minimal effort. This is particularly useful for user-based security.

However, .validate rules do not cascade. All validate rules must be satisfied at all levels of the hierarchy for a write to be allowed.

Additionally, because rules do not apply back to a parent path, read or write operation fail if there isn't a rule at the requested location or at a parent location that grants access. Even if every affected child path is accessible, reading at the parent location will fail completely. Consider this structure:

{
  "rules": {
    "records": {
      "rec1": {
        ".read": true
      },
      "rec2": {
        ".read": false
      }
    }
  }
}

Without understanding that rules are evaluated atomically, it might seem like fetching the /records/ path would return rec1 but not rec2. The actual result, however, is an error:

var db = firebase.database();
db.ref("records").once("value", function(snap) {
  // success method is not called
}, function(err) {
  // error callback triggered with PERMISSION_DENIED
});

FIRDatabaseReference *ref = [[FIRDatabase database] reference];
[[_ref child:@"records"] observeSingleEventOfType:FIRDataEventTypeValue withBlock:^(FIRDataSnapshot *snapshot) {
  // success block is not called
} withCancelBlock:^(NSError * _Nonnull error) {
  // cancel block triggered with PERMISSION_DENIED
}];

var ref = FIRDatabase.database().reference()
ref.child("records").observeSingleEventOfType(.Value, withBlock: { snapshot in
    // success block is not called
}, withCancelBlock: { error in
    // cancel block triggered with PERMISSION_DENIED
})

FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference ref = database.getReference("records");
ref.addListenerForSingleValueEvent(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot snapshot) {
    // success method is not called
  }

  @Override
  public void onCancelled(FirebaseError firebaseError) {
    // error callback triggered with PERMISSION_DENIED
  });
});

curl https://docs-examples.firebaseio.com/rest/records/
# response returns a PERMISSION_DENIED error

Since the read operation at /records/ is atomic, and there's no read rule that grants access to all of the data under /records/, this will throw a PERMISSION_DENIED error. If we evaluate this rule in the security simulator in our Firebase console, we can see that the read operation was denied:

Attempt to read /records with auth=Success(null)
    /
    /records

No .read rule allowed the operation.
Read was denied.

The operation was denied because no read rule allowed access to the /records/ path, but note that the rule for rec1 was never evaluated because it wasn't in the path we requested. To fetch rec1, we would need to access it directly:

var db = firebase.database();
db.ref("records/rec1").once("value", function(snap) {
  // SUCCESS!
}, function(err) {
  // error callback is not called
});

FIRDatabaseReference *ref = [[FIRDatabase database] reference];
[[ref child:@"records/rec1"] observeSingleEventOfType:FEventTypeValue withBlock:^(FIRDataSnapshot *snapshot) {
    // SUCCESS!
}];

var ref = FIRDatabase.database().reference()
ref.child("records/rec1").observeSingleEventOfType(.Value, withBlock: { snapshot in
    // SUCCESS!
})

FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference ref = database.getReference("records/rec1");
ref.addListenerForSingleValueEvent(new ValueEventListener() {
  @Override
  public void onDataChange(DataSnapshot snapshot) {
    // SUCCESS!
  }

  @Override
  public void onCancelled(FirebaseError firebaseError) {
    // error callback is not called
  }
});

curl https://docs-examples.firebaseio.com/rest/records/rec1
# SUCCESS!

Location variable

Realtime Database Rules support a $location variable to match path segments. Use the $ prefix in front of your path segment to match your rule to any child nodes along the path.

  {
    "rules": {
      "rooms": {
        // This rule applies to any child of /rooms/, the key for each room id
        // is stored inside $room_id variable for reference
        "$room_id": {
          "topic": {
            // The room's topic can be changed if the room id has "public" in it
            ".write": "$room_id.contains('public')"
          }
        }
      }
    }
  }

You can also use the $variable in parallel with constant path names.

  {
    "rules": {
      "widget": {
        // a widget can have a title or color attribute
        "title": { ".validate": true },
        "color": { ".validate": true },

        // but no other child paths are allowed
        // in this case, $other means any key excluding "title" and "color"
        "$other": { ".validate": false }
      }
    }
  }