Access control lists (ACLs)

Usage

This page provides an overview of access control lists (ACLs). To learn about other ways of controlling access to buckets and objects, read Overview of Access Control.

Should you use access control lists?

In most cases, you should avoid using ACLs, and you should enable uniform bucket-level access for your buckets, which prevents ACL usage:

  • ACLs can't be used exclusively to control access to your Google Cloud resources, because ACLs can't be set on the overall project or resources outside Cloud Storage.
  • Enabling uniform bucket-level access creates a simpler access control surface and lets you use additional Google Cloud features. For more information, see should you use uniform bucket-level access.
  • The domain restricted sharing organization policy and custom organization policies don't prevent access granted by ACLs, potentially leading to unintended access.
  • Unexpected behaviors and access can occur when using ACLs in projects that have IAM conditions set at or above the project level.

You most likely want to use ACLs in the following cases:

  • You need to customize access to individual objects within a bucket, such as if you want the uploader of an object to have full control over that object, but less access to other objects in your bucket.
  • You are exclusively using the XML API or require interoperability with Amazon S3.

Identity and Access Management (IAM) and ACLs work in tandem to grant access to your buckets and objects, which means a user only needs the relevant permission from either of these systems in order to access a bucket or object. In general, permissions granted by IAM policies don't appear in ACLs, and permissions granted by ACLs don't appear in IAM policies. See IAM relation to ACLs for more information.

What is an access control list?

An access control list (ACL) is a mechanism you can use to define who has access to your buckets and objects, as well as what level of access they have. In Cloud Storage, you apply ACLs to individual buckets and objects. Each ACL consists of one or more entries. An entry gives a specific user (or group) the ability to perform specific actions. Each entry consists of two pieces of information:

  • A permission, which defines what actions can be performed (for example, read or write).

  • A scope (sometimes referred to as a grantee), which defines who can perform the specified actions (for example, a specific user or group of users).

As an example, suppose you have a bucket that you want anyone to be able to access objects from, but you also want your collaborator to be able to add or remove objects from the bucket. In this case, your ACL would consist of two entries:

  • In one entry, you would give READER permission to a scope of allUsers.

  • In the other entry, you would give WRITER permission to the scope of your collaborator (there are several ways to specify this person, such as by their email).

The maximum number of ACL entries you can create for a bucket or object is 100. When the entry scope is a group or domain, it counts as one ACL entry regardless of how many users are in the group or domain.

When a user requests access to a bucket or object, the Cloud Storage system reads the bucket or object ACL and determines whether to allow or reject the access request. If the ACL grants the user permission for the requested operation, the request is allowed. If the ACL does not grant the user permission for the requested operation, the request fails and a 403 Forbidden error is returned.

Note that while ACLs can be used to manage most actions involving buckets and objects, the ability to create a bucket comes from having the appropriate project permission.

Permissions

Permissions describe what can be done to a given object or bucket.

Cloud Storage lets you assign the following concentric permissions for your buckets and objects, as shown in the following table:

Buckets Objects
READER Allows a user to list a bucket's contents. Also allows a user to read bucket metadata, excluding ACLs. Allows a user to download an object's data.
WRITER Gives a user all access that's granted by the READER permission. Additionally allows a user to create, replace, and delete objects in a bucket, including creating objects using multipart uploads. N/A. You cannot apply this permission to objects.
OWNER Gives a user all access that's granted by the WRITER permission. Also allows a user to read and write bucket metadata, including ACLs, and to work with tags on the bucket. Gives a user all access that's granted by the READER permission. Also allows a user to read and write object metadata, including ACLs.
Default Buckets have the predefined project-private ACL applied when they are created. Buckets are always owned by the project-owners group. Objects have the predefined project-private ACL applied when they are uploaded. Objects are always owned by the original requester who uploaded the object.

In this page, we generally refer to the permissions as READER, WRITER, and OWNER, which are how they are specified in the JSON API and the Google Cloud console. If you are using the XML API, the equivalent permissions are READ, WRITE, and FULL_CONTROL, respectively.

Scopes

Scopes specify who it is that has a given permission.

An ACL consists of one or more entries, where each entry grants permissions to a scope. You can specify an ACL scope using any of the following entities:

Scope ("grantee") Entity Type(s) Example
Special identifier for all entities User allUsers
Special identifier for all valid accounts User allAuthenticatedUsers
User account email address User [email protected]
Service account email address User [email protected]
Google group email address Group [email protected]
Convenience values for projects Project owners-123456789012
Google Workspace domain Domain [email protected]
Cloud Identity domain Domain [email protected]

  • Special identifier for all entities:

    The special scope identifier allUsers represents any entity on the Internet. Note that while this identifier is a User entity type, when using the Google Cloud console it's labeled as a Public entity type.

  • Special identifier for all valid accounts:

    The special scope identifier allAuthenticatedUsers represents most authenticated accounts, including service accounts. For more information, see IAM overview. Note that while this identifier is a User entity type, when using the Google Cloud console it's labeled as a Public entity type.

  • User account email address:

    Every user who has a user account must have a unique email address associated with that account. You can specify a scope by using any email address that is associated with a user account.

    Cloud Storage remembers email addresses as they are provided in ACLs until the entries are removed or replaced. If a user changes email addresses, you should update ACL entries to reflect these changes.

  • Service account email address:

    Every service account has a unique email address associated with it. You can specify a scope by using the email address that is associated with the service account.

  • Google group email address:

    Every Google group has a unique email address that is associated with the group. For example, the Cloud Storage Announce group has the following email address: [email protected]. You can find the email address that is associated with a Google group by clicking About on the homepage of every Google group.

    Like user account email addresses, Cloud Storage remembers group email addresses as they are provided in ACLs until the entries are removed. You don't need to worry about updating Google Group email addresses, because Google Group email addresses are permanent and unlikely to change.

  • Convenience values for projects:

    Convenience values allow you to grant bulk access to your project's viewers, editors, and owners. Convenience values combine a project role and an associated project number. For example, in project 867489160491, editors are identified as editors-867489160491. You can find your project number on the homepage of the Google Cloud console.

    You should generally avoid using convenience values in production environments, because they require granting basic roles, a practice which is discouraged in production environments.

  • Google Workspace or Cloud Identity:

    Google Workspace and Cloud Identity customers can associate their email accounts with an Internet domain name. When you do this, each email account takes the form USERNAME@YOUR_DOMAIN.com. You can specify a scope by using any Internet domain name that is associated with Google Workspace or Cloud Identity.

Concentric permissions and scopes

When specifying ACLs in Cloud Storage, you do not need to list multiple scopes to grant multiple permissions. Cloud Storage uses concentric permissions, so when you grant WRITER permission, you also grant READER permission, and if you grant OWNER permission, you also grant READER and WRITER permission.

When specifying an ACL, most tools allow you to specify multiple scopes for the same entry. The most permissive permission is the access granted to the scope. For example, if you provide two entries for a user, one with READER permission and one with WRITER permission on a bucket, the user will have WRITER permission on the bucket.

In the XML API, it is not possible to provide two ACL entries with the same scope. For example, granting a user READ permission and WRITE permission on a bucket results in an error. Instead, grant the user WRITE permission, which also grants the user READ permission.

Predefined ACLs

A predefined ACL, also sometimes known as a canned ACL, is an alias for a set of specific ACL entries that you can use to quickly apply many ACL entries at once to a bucket or object.

The table below lists predefined ACLs and shows which ACL entries are applied for each predefined ACL. When using the table below, note that:

  • The project owners group has ownership of buckets in the project, and the user that creates an object has ownership of that object. If an object was created by an anonymous user, then the project owners group has ownership of the object.

  • In the table, the JSON API descriptions of permissions, OWNER, WRITER, and READER, are used. The equivalent XML API scopes are FULL_CONTROL, WRITE, and READ.

    JSON API/gcloud storage XML API Description
    private private Gives the bucket or object owner OWNER permission for a bucket or object.
    bucketOwnerRead bucket-owner-read Gives the object owner OWNER permission, and gives the bucket owner READER permission. This is used only with objects.
    bucketOwnerFullControl bucket-owner-full-control Gives the object and bucket owners OWNER permission. This is used only with objects.
    projectPrivate project-private Gives permission to the project team based on their roles. Anyone who is part of the team has READER permission. Project owners and project editors have OWNER permission. This is the default ACL for newly created buckets. This is also the default ACL for newly created objects unless the default object ACL for that bucket has been changed.
    authenticatedRead authenticated-read Gives the bucket or object owner OWNER permission, and gives all authenticated user account holders READER permission.
    publicRead public-read Gives the bucket or object owner OWNER permission, and gives all users, both authenticated and anonymous, READER permission. When you apply this to an object, anyone on the Internet can read the object without authenticating. When you apply this to a bucket, anyone on the Internet can list objects without authenticating.

    * See the note at the end of the table regarding caching.
    publicReadWrite public-read-write Gives the bucket owner OWNER permission, and gives all users, both authenticated and anonymous, READER and WRITER permission. This ACL applies only to buckets. When you apply this to a bucket, anyone on the Internet can list, create, replace, and delete objects without authenticating.

    * See the note at the end of the table regarding caching.

* By default, publicly readable objects are served with a Cache-Control header that allows the objects to be cached for 3600 seconds. If you need to ensure that updates become visible immediately, you should set the Cache-Control metadata for the objects to Cache-Control:private, max-age=0, no-transform.

Default ACLs

When buckets are created or objects are uploaded, if you do not explicitly assign an ACL to them, they are given the default ACL. You can change the default ACL given to an object; the process to do so is described in Changing default object ACLs. Note that when you change the default ACL, the ACLs of objects that already exist in the bucket or buckets that already exist in the project remain unchanged.

Default bucket ACLs

All buckets are owned by the project owners group. Additionally, project owners are granted OWNER permission for any buckets inside their project that use a predefined ACL.

If you create a bucket with the default bucket ACL—that is, you do not specify a predefined ACL when you create the bucket—your bucket has the predefined projectPrivate ACL applied to it.

Default object ACLs

By default, anyone who has OWNER permission or WRITER permission on a bucket can upload objects into that bucket. When you upload an object, you can provide a predefined ACL or not specify an ACL at all. If you don't specify an ACL, Cloud Storage applies the bucket's default object ACL to the object. Every bucket has a default object ACL, and this ACL is applied to all objects uploaded to that bucket without a predefined ACL or an ACL specified in the request (JSON API only). The initial value for the default object ACL of every bucket is projectPrivate.

Based on how objects are uploaded, object ACLs are applied accordingly:

  • Authenticated Uploads

    If you make an authenticated request to upload an object and do not specify any object ACLs when you upload it, then you are listed as the owner of the object and the predefined projectPrivate ACL is applied to the object by default. This means:

    • You (the person who uploaded the object) are listed as the object owner. Object ownership cannot be changed by modifying ACLs. You can change object ownership only by replacing an object.

    • You (the object owner) are granted OWNER permission on the object. If you attempt to give less than OWNER permission to the owner, Cloud Storage automatically escalates the permission to OWNER.

    • The project owners and project editors group have OWNER permission on the object.

    • The project team members group has READER permission on the object.

  • Anonymous Uploads

    If an unauthenticated (anonymous) user uploads an object, which is possible if a bucket grants the allUsers group WRITER or OWNER permission, then the default bucket ACLs are applied to the object as described above.

    Anonymous users cannot specify a predefined ACL during object upload.

ACL behavior

Cloud Storage helps you adhere to best practices by enforcing some ACL modification rules, which prevent you from setting ACLs that make data inaccessible:

  • You cannot apply an ACL that specifies a different bucket or object owner.

    Bucket and object ownership cannot be changed by modifying ACLs. If you apply a new ACL to a bucket or object, be sure that the bucket or object owner remains unchanged in the new ACL.

  • The bucket or object owner always has OWNER permission of the bucket or object.

    The owner of a bucket is the project owners group, and the owner of an object is either the user who uploaded the object, or the project owners group if the object was uploaded by an anonymous user.

    When you apply a new ACL to a bucket or object, Cloud Storage respectively adds OWNER permission to the bucket or object owner if you omit the grants. It does not grant the project owners group OWNER permission for an object (unless the object was created by an anonymous user), so you must explicitly include it.

You cannot apply ACLs that change the ownership of a bucket or object (which should not be confused with the OWNER permission). Once created in Cloud Storage, bucket and object ownership are permanent. You can, however, effectively change the ownership of objects (but not buckets) by replacing them. Replacement is basically a delete operation followed immediately by an upload operation. During an upload operation, the person who is performing the upload becomes the owner of the object. Keep in mind that to replace an object, the person performing the replacement (and gaining ownership of the object by doing so) must have WRITER or OWNER permission on the bucket in which the object is being uploaded.

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