Post Syndicated from Kai Zhao original https://aws.amazon.com/blogs/security/iam-policies-and-bucket-policies-and-acls-oh-my-controlling-access-to-s3-resources/

Updated on July 6, 2023: This post has been updated to reflect the current guidance around the usage of S3 ACL and to include S3 Access Points and the Block Public Access for accounts and S3 buckets.

Updated on April 27, 2023: Amazon S3 now automatically enables S3 Block Public Access and disables S3 access control lists (ACLs) for all new S3 buckets in all AWS Regions.

Updated on January 8, 2019: Based on customer feedback, we updated the third paragraph in the “What about S3 ACLs?” section to clarify permission management.

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In this post, we will discuss Amazon S3 Bucket Policies and IAM Policies and its different use cases. This post will assist you in distinguishing between the usage of IAM policies and S3 bucket policies. We will also discuss how these policies integrate with some default S3 bucket security settings like automatically enabling S3 Block Public Access and disabling S3 access control lists (ACLs).

IAM policies vs. S3 bucket policies

AWS access is managed by setting IAM policies and linking them to IAM identities (users, groups of users, or roles) or AWS resources. A policy is an object in AWS that when associated with an identity or resource, defines their permissions. IAM policies specify what actions are allowed or denied on what AWS resources (e.g. user Alice can read objects from the “Production” bucket but can’t write objects in the “Dev” bucket whereas user Bob can have full access to S3).

S3 bucket policies, on the other hand, are resource-based policies that you can use to grant access permissions to your Amazon S3 buckets and the objects in it. S3 bucket policies can allow or deny requests based on the elements in the policy.(e.g. allow user Alice to PUT but not DELETE objects in the bucket).

Note: You attach S3 bucket policies at the bucket level (i.e. you can’t attach a bucket policy to an S3 object), but the permissions specified in the bucket policy apply to all the objects in the bucket. You can also specify permissions at the object level by putting an object as the resource in the Bucket policy.

IAM policies and S3 bucket policies are both used for access control and they’re both written in JSON using the AWS access policy language. Let’s look at an example policy of each type:

Sample S3 Bucket Policy

This S3 bucket policy enables any IAM principal (user or role) in account 111122223333 to use the Amazon S3 GET Bucket (List Objects) operation.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": ["arn:aws:iam::111122223333:root"]
      },
      "Action": "s3:ListBucket",
      "Resource": ["arn:aws:s3:::my_bucket"]
    }
  ]
}

This S3 bucket policy enables the IAM role ‘Role-name’ under the account 111122223333 to use the Amazon S3 GET Bucket (List Objects) operation.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::111122223333:role/Role-name"
      },
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::my_bucket"
    }
  ]
}

Sample IAM Policy

This IAM policy grants the IAM principal it is attached to permission to perform any S3 operation on the contents of the bucket named “my_bucket”.

{
  "Version": "2012-10-17",
  "Statement":[{
    "Effect": "Allow",
    "Action": "s3:*",
    "Resource": ["arn:aws:s3:::my_bucket/*"]
    }
  ]
}

Note that the S3 bucket policy includes a “Principal” element, which lists the principals that bucket policy controls access for. The “Principal” element is unnecessary in an IAM policy, because the principal is by default the entity that the IAM policy is attached to.

S3 bucket policies (as the name would imply) only control access to S3 resources for the bucket they’re attached to, whereas IAM policies can specify nearly any AWS action. One of the neat things about AWS is that you can actually apply both IAM policies and S3 bucket policies simultaneously, with the ultimate authorization being the least-privilege union of all the permissions (more on this in the section below titled “How does authorization work with multiple access control mechanisms?”).

When to use IAM policies vs. S3 policies

Use IAM policies if:

• You need to control access to AWS services other than S3. IAM policies will be easier to manage since you can centrally manage all of your permissions in IAM, instead of spreading them between IAM and S3.
• You have numerous S3 buckets each with different permissions requirements. IAM policies will be easier to manage since you don’t have to define a large number of S3 bucket policies and can instead rely on fewer, more detailed IAM policies.
• You prefer to keep access control policies in the IAM environment.

Use S3 bucket policies if:

• You want a simple way to grant cross-account access to your S3 environment, without using IAM roles.
• Your IAM policies bump up against the size limit (up to 2 kb for users, 5 kb for groups, and 10 kb for roles). S3 supports bucket policies of up 20 kb.
• You prefer to keep access control policies in the S3 environment.
• You want to apply common security controls to all principals who interact with S3 buckets, such as restricting the IP addresses or VPC a bucket can be accessed from.

If you’re still unsure of which to use, consider which audit question is most important to you:

• If you’re more interested in “What can this user do in AWS?” then IAM policies are probably the way to go. You can easily answer this by looking up an IAM user and then examining their IAM policies to see what rights they have.
• If you’re more interested in “Who can access this S3 bucket?” then S3 bucket policies will likely suit you better. You can easily answer this by looking up a bucket and examining the bucket policy.

Whichever method you choose, we recommend staying as consistent as possible. Auditing permissions becomes more challenging as the number of IAM policies and S3 bucket policies grows.

What about S3 ACLs?

An S3 ACL is a sub-resource that’s attached to every S3 bucket and object. It defines which AWS accounts or groups are granted access and the type of access. You can attach S3 ACLs to both buckets and individual objects within a bucket to manage permissions for those objects. As a general rule, AWS recommends using S3 bucket policies or IAM policies for access control. S3 ACLs is a legacy access control mechanism that predates IAM. By default, Object Ownership is set to the Bucket owner enforced setting and all ACLs are disabled, as can be seen below.

A majority of modern use cases in Amazon S3 no longer require the use of ACLs, and we recommend that you keep ACLs disabled by applying the Bucket owner enforced setting. This approach simplifies permissions management: you can use policies to more easily control access to every object in your bucket, regardless of who uploaded the objects in your bucket. When ACLs are disabled, the bucket owner owns all the objects in the bucket and manages access to data exclusively using access management policies.

S3 bucket policies and IAM policies define object-level permissions by providing those objects in the Resource element in your policy statements. The statement will apply to those objects in the bucket. Consolidating object-specific permissions into one policy (as opposed to multiple S3 ACLs) makes it simpler for you to determine effective permissions for your users and roles.

You can disable ACLs on both newly created and already existing buckets. For newly created buckets, ACLs are disabled by default. In the case of an existing bucket that already has objects in it, after you disable ACLs, the object and bucket ACLs are no longer part of an access evaluation, and access is granted or denied on the basis of policies.

S3 Access Points and S3 Access

In some cases customers have use cases with complex entitlement: Amazon s3 is used to store shared datasets where data is aggregated and accessed by different applications, individuals or teams for different use cases. Managing access to this shared bucket requires a single bucket policy that controls access for dozens to hundreds of applications with different permission levels. As an application set grows, the bucket policy becomes more complex, time consuming to manage, and needs to be audited to make sure that changes don’t have an unexpected impact on another application.

These customers need additional policy space for access to their data, and that buckets. To support these use cases, Amazon S3 provides a feature called Amazon S3 Access Points. Amazon S3 access points simplify data access for any AWS service or customer application that stores data in S3.

Access points are named network endpoints that are attached to buckets that you can use to perform S3 object operations, such as GetObject and PutObject. Each access point has distinct permissions and network controls that S3 applies for any request that is made through that access point. Each access point enforces a customized access point policy that works in conjunction with the bucket policy that is attached to the underlying bucket.

Amazon S3 access points support AWS Identity and Access Management (IAM) resource policies that allow you to control the use of the access point by resource, user, or other conditions. For an application or user to be able to access objects through an access point, both the access point and the underlying bucket must permit the request.

Note that Adding an S3 access point to a bucket doesn’t change the bucket’s ehaviour when the bucket is accessed directly through the bucket’s name or Amazon Resource Name (ARN). All existing operations against the bucket will continue to work as before. Restrictions that you include in an access point policy apply only to requests made through that access point.

Sample Access point policy

This access point policy grants the IAM user Alice permissions to GET and PUT objects through the access point ‘my-access-point’ in account 111122223333.

{
  “Version”: “2012-10-17”,
  “Statement”:[{
    “Effect”: “Allow”,
    “Principal”: { “AWS”: “arn:aws:iam::111122223333:user/Alice” },
    “Action”: [“s3:GetObject”, “s3:PutObject”],
    “Resource”: “arn:aws:s3:us-west-2:111122223333:accesspoint/my-access-point/object/*”
    }
  ]
}

Blocking Public Access for accounts and buckets

Public access is granted to buckets and objects through access control lists (ACLs), bucket policies, access point policies, or all. In order to ensure that public access to this bucket and its objects is blocked, you can turn on Block all public on both the bucket level or the account level.

The Amazon S3 Block Public Access feature provides settings for access points, buckets, and accounts to help you manage public access to Amazon S3 resources. By default, new buckets, access points, and objects don’t allow public access. However, users can modify bucket policies, access point policies, or object permissions to allow public access. S3 Block Public Access settings override these policies and permissions so that you can limit public access to these resources.

With S3 Block Public Access, account administrators and bucket owners can easily set up centralized controls to limit public access to their Amazon S3 resources that are enforced regardless of how the resources are created.

If you apply a setting to an account, it applies to all buckets and access points that are owned by that account. Similarly, if you apply a setting to a bucket, it applies to all access points associated with that bucket.

Block Public Access for buckets

These settings apply only to this bucket and its access points. AWS recommends that you turn on Block all public access, but before applying any of these settings, ensure that your applications will work correctly without public access. If you require some level of public access to this bucket or objects within, you can customize the individual settings below to suit your specific storage use cases.

You can use the S3 console, AWS CLI, AWS SDKs, and REST API to grant public access to one or more buckets. This setting is on by default at the account creation, as can be seen below (using the S3 console).

Turning off this session will create a warning in the account, as AWS recommends this setting to be turned un unless public access is required for specific and verified use cases such as static website hosting.

This setting can also be turned on for existing buckets. In the AWS Management Console this is done by opening the Amazon S3 console at https://console.aws.amazon.com/s3/, choosing the name of the bucket you want, choosing the Permissions tab. And Choosing Edit to change the public access settings for the bucket.

Block Public Access for accounts

In order to ensure that public access to all your S3 buckets and objects is blocked, turn on Block all public access. These settings apply account-wide for all current and future buckets and access points. AWS recommends that you turn on Block all public access, but before applying any of these settings, ensure that your applications will work correctly without public access. If you require some level of public access to your buckets or objects, you can customize the individual settings below to suit your specific storage use cases.

You can use the S3 console, AWS CLI, AWS SDKs, and REST API to configure block public access settings for all the buckets in your account. This setting can be turned on in the AWS Management Console by opening the Amazon S3 console at https://console.aws.amazon.com/s3/, and clicking Block Public Access setting for this account on the left panel. And Choosing Edit to change the public access settings for the bucket.

When working with AWS organizations, you can prevent people from modifying the Block Public Access on the account level by adding a Service control policy (SCP) that denies editing this. An example of such a SCP can be seen below:

{
  “Version”: “2012-10-17”,
  “Statement”:[{
    “Sid”: “DenyTurningOffBlockPublicAccessForThisAccount”,
    “Effect”: “Deny”,
    “Action”: “s3:PutAccountPublicAccessBlock”,
    “Resource”: “arn:aws:s3:::*”
    }
  ]
}

How does authorization work with multiple access control mechanisms?

Whenever an AWS principal issues a request to S3, the authorization decision depends on the union of all the IAM policies, S3 bucket policies, and S3 ACLs that apply as well as if Block Public Access is enabled on either the account, bucket or access point.

In accordance with the principle of least-privilege, decisions default to DENY and an explicit DENY always trumps an ALLOW. For example, if an IAM policy grants access to an object, the S3 bucket policies denies access to that object, and there is no S3 ACL, then access will be denied. Similarly, if no method specifies an ALLOW, then the request will be denied by default. Only if no method specifies a DENY and one or more methods specify an ALLOW will the request be allowed.

When Amazon S3 receives a request to access a bucket or an object, it determines whether the bucket or the bucket owner’s account has a block public access setting applied. If the request was made through an access point, Amazon S3 also checks for block public access settings for the access point. If there is an existing block public access setting that prohibits the requested access, Amazon S3 rejects the request.

This diagram illustrates the authorization process.

We hope that this post clarifies some of the confusion around the various ways you can control access to your S3 environment.

Using IAM Access Analyzer for S3 to review bucket access

Another interesting feature that can be used is IAM Access Analyzer for S3 to review bucket access. You can use IAM Access Analyzer for S3 to review buckets with bucket ACLs, bucket policies, or access point policies that grant public access. IAM Access Analyzer for S3 alerts you to buckets that are configured to allow access to anyone on the internet or other AWS accounts, including AWS accounts outside of your organization. For each public or shared bucket, you receive findings that report the source and level of public or shared access.

In IAM Access Analyzer for S3, you can block all public access to a bucket with a single click. You can also drill down into bucket-level permission settings to configure granular levels of access. For specific and verified use cases that require public or shared access, you can acknowledge and record your intent for the bucket to remain public or shared by archiving the findings for the bucket.

Additional Resources

• IAM policies documentation
• S3 bucket policies documentation
• S3 ACLs documentation
• Block Public Access documentation
• IAM Access Analyzer documentation

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