Learn to Secure Files

Cloud Storage provides a declarative path-based security model called Firebase Security Rules for Cloud Storage which allows you to quickly and easily secure your files.

Understand Rules

Firebase Security Rules for Cloud Storage are used to determine who has read and write access to files stored in Cloud Storage, as well as how files are structured and what metadata they contain. The basic type of rule is the allow rule, which allows read and write requests if an optionally specified condition is specified, for example:

// If no condition is specified, the rule evaluates to true
allow read;

// Rules can optionally specify a condition
allow write: if <condition>;

// Rules can also specify multiple request methods
allow read, write: if <condition>;

Matching Paths

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 evalutes 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";
}

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 User-Based Security section of the docs.

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

Property Type Description
auth map<string, string> When a user is logged in, provides uid, the user's unique ID, and token, a map of Firebase Authentication JWT claims. Otherwise, it will be null.
params map<string, string> Map containing the query parameters of the request.
path path A path representing the path the request is being performed at.
resource map<string, string> The new resource value, present only on write requests.
time timestamp A timestamp representing the server time the request is evaluated at.

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:

Property Type Description
name string The full name of the object
bucket string The name of the bucket this object resides in.
generation int The GCS object generation of this object.
metageneration int The GCS object metageneration of this object.
size int The size of the object in bytes.
timeCreated timestamp A timestamp representing the time an object was created.
updated timestamp A timestamp representing the time an object was last updated.
md5Hash string An MD5 hash of the object.
crc32c string A crc32c hash of the object.
etag string The etag associated with this object.
contentDisposition string The content disposition associated with this object.
contentEncoding string The content encoding associated with this object.
contentLanguage string The content language associated with this object.
contentType string The content type associated with this object.
metadata map<string, string> Key/value pairs of additional, developer specified custom metadata.

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

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
     }
   }
 }
}

Now, let's integrate Firebase Authentication for granular per user file access in the User Security section.

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