<br />

Firebase Security Rules for Cloud Storage allow you to control access to objects stored
in Cloud Storage buckets. The flexible rules syntax allows you to create
rules to control any operation, from all writes to your Cloud Storage
bucket to operations on a specific file.
| **Note:** Buckets in Cloud Storage only contain files, they don't contain directories. On this page you'll see slashes `/` in filenames, like `/images/profilePhoto.png`, which suggest there is a hierarchy of directories and sub-directories in a bucket. However, filenames can include prefixes with slashes to allow you to define unique filenames that look like the full directory paths we're used to seeing in file systems.

This guide describes the basic syntax and structure of Cloud Storage Security Rules to
create complete rulesets.

## Service and database declaration

Firebase Security Rules for Cloud Storage always begin with the following declaration:  

    service firebase.storage {
        // ...
    }

The `service firebase.storage` declaration scopes the rules to
Cloud Storage, preventing conflicts between Cloud Storage Security Rules and
rules for other products such as Cloud Firestore.

## Basic read/write rules

Basic rules consist of a `match` statement identifying Cloud Storage
buckets, a match statement specifying a filename, and an `allow` expression
detailing when reading the specified data is allowed. `allow` expressions
specify the *access methods* (e.g., read, write) involved, and *conditions*
under which access is either allowed or denied.

In your default ruleset, the first `match` statement uses a `{bucket}` wildcard
expression to indicate the rules apply to all buckets in your project. We'll
discuss the idea of wildcard matches more in the next section.  

    service firebase.storage {
      // The {bucket} wildcard indicates we match files in all Cloud Storage buckets
      match /b/{bucket}/o {
        // Match filename
        match /filename {
          allow read: if <condition>;
          allow write: if <condition>;
        }
      }
    }

All match statements point to files. A match statement can point to a specific
file, as in `match /images/profilePhoto.png`.

### Match wildcards

In additiont to pointing to a single file, Security Rules can use **wildcards**
to point to any file with a given string prefix in its name, including slashes,
as in `match /images/{imageId}`.

In the example above, the match statement uses the `{imageId}` wildcard syntax.
This means the rule applies to any file with `/images/` at the start of its name,
such as `/images/profilePhoto.png` or `/images/croppedProfilePhoto.png`. When the
`allow` expressions in the match statement are evaluated, the `imageId` variable
will resolve to the image filename, such as `profilePhoto.png` or
`croppedProfilePhoto.png`.

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

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

| **Note:** Users can only access files that your Firebase Security Rules specifically allow you to access. For example, the rules shown above allow access only to files with `/images/` at the beginning of their names; as a result, they also deny access to files with different filename prefixes.

## Hierarchical data

As we said before, there is no hierarchical structure inside a
Cloud Storage bucket. But by using a file naming convention, often one
that includes slashes in filenames, we can mimic a structure that looks like a
nested series of directories and sub-directories. It is important to understand
how Firebase Security Rules interact with these filenames.

Consider the situation of a set of files with names that all begin with the
`/images/` stem. Firebase Security Rules apply only at the matched filename, so the access
controls defined on the `/images/` stem do not apply to the `/mp3s/` stem.
Instead, write explicit rules that match different filename patterns:  

    service firebase.storage {
      match /b/{bucket}/o {
        match /images/{imageId} {
          allow read, write: if <condition>;
        }

        // Explicitly define rules for the 'mp3s' pattern
        match /mp3s/{mp3Id} {
          allow read, write: if <condition>;
        }
      }
    }

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

    service firebase.storage {
      match /b/{bucket}/o {
        match /images {
          // Exact match for "images/profilePhoto.png"
          match /profilePhoto.png {
            allow write: if <condition>;
          }
        }
      }
    }

    service firebase.storage {
      match /b/{bucket}/o {
        // Exact match for "images/profilePhoto.png"
        match /images/profilePhoto.png {
          allow write: if <condition>;
          }
      }
    }

### Recursive match wildcards

In addition to wildcards that match and return strings at the end of a filename,
a **multiple segment wildcard** can be declared for more complex matching 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/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`.

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

### Version 1

Firebase Security Rules use version 1 by default. In version 1, recursive wildcards
match one or more filename elements, not zero or more elements. Thus,
`match /images/{filenamePrefixWildcard}/{imageFilename=**}` matches a filename
like /images/profilePics/profile.png, but not /images/badge.png. Use
`/images/{imagePrefixorFilename=**}` instead.

Recursive wildcards must come at the end of a match statement.

We recommend you use version 2 for its more powerful features.

### Version 2

In version 2 of Firebase Security Rules, recursive wildcards match zero or more path
items. Thus, `/images/{filenamePrefixWildcard}/{imageFilename=**}` matches
filenames /images/profilePics/profile.png and /images/badge.png.

You must opt-in to version 2 by adding `rules_version = '2';` at the top of
your security rules:  

    rules_version = '2';
    service cloud.storage {
      match /b/{bucket}/o {
       ...
     }
    }

You can have at most one recursive wildcard per match statement, but in
version 2, you can place this wildcard anywhere in the match statement. For
example:  

    rules_version = '2';
    service firebase.storage {
     match /b/{bucket}/o {
       // Matches any file in a songs "subdirectory" under the
       // top level of your Cloud Storage bucket.
       match /{prefixSegment=**}/songs/{mp3filenames} {
         allow read, write: if <condition>;
       }
      }
    }

## Granular operations

In some situations, it's useful to break down `read` and `write` into more
granular operations. For example, your app may want to enforce different
conditions on file creation than on file deletion.

A `read` operation can be broken into `get` and `list`.
| **Note:** `list` Security Rules operations and hence the Cloud Storage List API are only available in projects that use Security Rules Version 2.

A `write` rule can be
broken into `create`, `update`, and `delete`:  

```css+lasso
service firebase.storage {
  match /b/{bucket}/o {
    // A read rule can be divided into read and list rules
    match /images/{imageId} {
      // Applies to single file read requests
      allow get: if <condition>;
      // Applies to list and listAll requests (Security Rules Version 2)
      allow list: if <condition>;

    // A write rule can be divided into create, update, and delete rules
    match /images/{imageId} {
      // Applies to writes to file contents
      allow create: if <condition>;

      // Applies to updates to (pre-existing) file metadata
      allow update: if <condition>;

      // Applies to delete operations
      allow delete: if <condition>;
    }
  }
 }
}
```

## Overlapping match statements

It's possible for a filename 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 firebase.storage {
      match b/{bucket}/o {
        // Matches file names directly inside of '/images/'.
        match /images/{imageId} {
          allow read, write: if false;
        }

        // Matches file names anywhere under `/images/`
        match /images/{imageId=**} {
          allow read, write: if true;
        }
      }
    }

In the example above, all reads and writes to files whose name starts with
`/images/` are allowed because the second rule is always `true`, even when
the first rule is `false`.

## Rules are not filters

Once you secure your data and begin to perform file operations, keep in mind
that security rules are not filters. You cannot perform operations on a set of
files matching a filename pattern and expect Cloud Storage to access only
the files that the current client has permission to access.

For example, take the following security rule:  

    service firebase.storage {
      match /b/{bucket}/o {
        // Allow the client to read files with contentType 'image/png'
        match /aFileNamePrefix/{aFileName} {
          allow read: if resource.contentType == 'image/png';
        }
      }
    }

Denied: This rule rejects the following
request because the result set can include files where `contentType` is
not `image/png`:  

##### Web

```css+lasso
filesRef = storage.ref().child("aFilenamePrefix");

filesRef.listAll()
    .then(function(result) {
      console.log("Success: ", result.items);
    })
});
```

Rules in Cloud Storage Security Rules evaluate each query against its potential
result and fails the request if it could return a file that the client does
not have permission to read. Access requests must follow the constraints set by
your rules.

## Next steps

You can deepen your understanding of Firebase Security Rules for Cloud Storage:

- Learn the next major concept of the Rules language, dynamic
  [conditions](https://firebase.google.com/docs/storage/security/rules-conditions), which let your Rules check user authorization,
  compare existing and incoming data, validate incoming data, and more.

- Review typical security use cases and the
  [Firebase Security Rules definitions that address them](https://firebase.google.com/docs/rules/basics).

You can explore Firebase Security Rules use cases specific to Cloud Storage: