Amazon S3 bucket policy for CloudTrail

By default, Amazon S3 buckets and objects are private. Only the resource owner (the AWS account that created the bucket) can access the bucket and objects it contains. The resource owner can grant access permissions to other resources and users by writing an access policy.

To create or modify an Amazon S3 bucket to receive log files for an organization trail, you must change the bucket policy. For more information, see Creating a trail for an organization with the AWS Command Line Interface.

To deliver log files to an S3 bucket, CloudTrail must have the required permissions, and it cannot be configured as a Requester Pays bucket.

CloudTrail adds the following fields in the policy for you:

• The allowed SIDs

• The bucket name

• The service principal name for CloudTrail

• The name of the folder where the log files are stored, including the bucket name, a prefix (if you specified one), and your AWS account ID

As a security best practice, add an aws:SourceArn condition key to the Amazon S3 bucket policy. The IAM global condition key aws:SourceArn helps ensure that CloudTrail writes to the S3 bucket only for a specific trail or trails. The value of aws:SourceArn is always the ARN of the trail (or array of trail ARNs) that is using the bucket to store logs. Be sure to add the aws:SourceArn condition key to S3 bucket policies for existing trails.

The following policy allows CloudTrail to write log files to the bucket from supported AWS Regions. Replace myBucketName, [optionalPrefix]/, myAccountID, region, and trailName with the appropriate values for your configuration.

S3 bucket policy

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AWSCloudTrailAclCheck20150319",
            "Effect": "Allow",
            "Principal": {"Service": "cloudtrail.amazonaws.com"},
            "Action": "s3:GetBucketAcl",
            "Resource": "arn:aws:s3:::myBucketName",
            "Condition": {
                "StringEquals": {
                    "aws:SourceArn": "arn:aws:cloudtrail:region:myAccountID:trail/trailName"
                }
            }
        },
        {
            "Sid": "AWSCloudTrailWrite20150319",
            "Effect": "Allow",
            "Principal": {"Service": "cloudtrail.amazonaws.com"},
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::myBucketName/[optionalPrefix]/AWSLogs/myAccountID/*",
            "Condition": {
                "StringEquals": {
                    "s3:x-amz-acl": "bucket-owner-full-control",
                    "aws:SourceArn": "arn:aws:cloudtrail:region:myAccountID:trail/trailName"
                }
            }
        }
    ]
}

For more information about AWS Regions, see CloudTrail supported Regions.

Contents

• Specifying an existing bucket for CloudTrail log delivery
• Receiving log files from other accounts
• Create or update an Amazon S3 bucket to use to store the log files for an organization trail
• Troubleshooting the Amazon S3 bucket policy
  • Common Amazon S3 policy configuration errors
  • Changing a prefix for an existing bucket
• Additional resources

Specifying an existing bucket for CloudTrail log delivery

If you specified an existing S3 bucket as the storage location for log file delivery, you must attach a policy to the bucket that allows CloudTrail to write to the bucket.

Note

As a best practice, use a dedicated S3 bucket for CloudTrail logs.

To add the required CloudTrail policy to an Amazon S3 bucket

1. Open the Amazon S3 console at https://console.aws.amazon.com/s3/.

2. Choose the bucket where you want CloudTrail to deliver your log files, and then choose Permissions.

3. Choose Edit.

4. Copy the S3 bucket policy to the Bucket Policy Editor window. Replace the placeholders in italics with the names of your bucket, prefix, and account number. If you specified a prefix when you created your trail, include it here. The prefix is an optional addition to the S3 object key that creates a folder-like organization in your bucket.

   Note

   If the existing bucket already has one or more policies attached, add the statements for CloudTrail access to that policy or policies. Evaluate the resulting set of permissions to be sure that they are appropriate for the users who will access the bucket.

Receiving log files from other accounts

You can configure CloudTrail to deliver log files from multiple AWS accounts to a single S3 bucket. For more information, see Receiving CloudTrail log files from multiple accounts.

Create or update an Amazon S3 bucket to use to store the log files for an organization trail

You must specify an Amazon S3 bucket to receive the log files for an organization trail. This bucket must have a policy that allows CloudTrail to put the log files for the organization into the bucket.

The following is an example policy for an Amazon S3 bucket named myOrganizationBucket, which is owned by the organization's management account. Replace myOrganizationBucket, region, managementAccountID, trailName, and o-organizationID with the values for your organization

This bucket policy contains three statements.

• The first statement allows CloudTrail to call the Amazon S3 GetBucketAcl action on the Amazon S3 bucket.

• The second statement allows logging in the event the trail is changed from an organization trail to a trail for that account only.

• The third statement allows logging for an organization trail.

The example policy includes an aws:SourceArn condition key for the Amazon S3 bucket policy. The IAM global condition key aws:SourceArn helps ensure that CloudTrail writes to the S3 bucket only for a specific trail or trails. In an organization trail, the value of aws:SourceArn must be a trail ARN that is owned by the management account, and uses the management account ID.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AWSCloudTrailAclCheck20150319",
            "Effect": "Allow",
            "Principal": {
                "Service": [
                    "cloudtrail.amazonaws.com"
                ]
            },
            "Action": "s3:GetBucketAcl",
            "Resource": "arn:aws:s3:::myOrganizationBucket",
            "Condition": {
                "StringEquals": {
                    "aws:SourceArn": "arn:aws:cloudtrail:region:managementAccountID:trail/trailName"
                }
            }
        },
        {
            "Sid": "AWSCloudTrailWrite20150319",
            "Effect": "Allow",
            "Principal": {
                "Service": [
                    "cloudtrail.amazonaws.com"
                ]
            },
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::myOrganizationBucket/AWSLogs/managementAccountID/*",
            "Condition": {
                "StringEquals": {
                    "s3:x-amz-acl": "bucket-owner-full-control",
                    "aws:SourceArn": "arn:aws:cloudtrail:region:managementAccountID:trail/trailName"
                }
            }
        },
        {
            "Sid": "AWSCloudTrailOrganizationWrite20150319",
            "Effect": "Allow",
            "Principal": {
                "Service": [
                    "cloudtrail.amazonaws.com"
                ]
            },
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::myOrganizationBucket/AWSLogs/o-organizationID/*",
            "Condition": {
                "StringEquals": {
                    "s3:x-amz-acl": "bucket-owner-full-control",
                    "aws:SourceArn": "arn:aws:cloudtrail:region:managementAccountID:trail/trailName"
                }
            }
        }
    ]
}

This example policy does not allow any users from member accounts to access the log files created for the organization. By default, organization log files are accessible only to the management account. For information about how to allow read access to the Amazon S3 bucket for IAM users in member accounts, see Sharing CloudTrail log files between AWS accounts.

Troubleshooting the Amazon S3 bucket policy

The following sections describe how to troubleshoot the S3 bucket policy.

Common Amazon S3 policy configuration errors

When you create a new bucket as part of creating or updating a trail, CloudTrail attaches the required permissions to your bucket. The bucket policy uses the service principal name, "cloudtrail.amazonaws.com", which allows CloudTrail to deliver logs for all Regions.

If CloudTrail is not delivering logs for a Region, it's possible that your bucket has an older policy that specifies CloudTrail account IDs for each Region. This policy gives CloudTrail permission to deliver logs only for the Regions specified.

As a best practice, update the policy to use a permission with the CloudTrail service principal. To do this, replace the account ID ARNs with the service principal name: "cloudtrail.amazonaws.com". This gives CloudTrail permission to deliver logs for current and new Regions. As a security best practice, add an aws:SourceArn or aws:SourceAccount condition key to the Amazon S3 bucket policy. This helps prevent unauthorized account access to your S3 bucket. If you have existing trails, be sure to add one or more condition keys. The following example shows a recommended policy configuration. Replace myBucketName, [optionalPrefix]/, myAccountID, region, and trailName with the appropriate values for your configuration.

Example bucket policy with service principal name

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AWSCloudTrailAclCheck20150319",
            "Effect": "Allow",
            "Principal": {"Service": "cloudtrail.amazonaws.com"},
            "Action": "s3:GetBucketAcl",
            "Resource": "arn:aws:s3:::myBucketName",
            "Condition": {
                "StringEquals": {
                    "aws:SourceArn": "arn:aws:cloudtrail:region:myAccountID:trail/trailName"
                }
            }
        },
        {
            "Sid": "AWSCloudTrailWrite20150319",
            "Effect": "Allow",
            "Principal": {"Service": "cloudtrail.amazonaws.com"},
            "Action": "s3:PutObject",
            "Resource": "arn:aws:s3:::myBucketName/[optionalPrefix]/AWSLogs/myAccountID/*",
            "Condition": {"StringEquals": {
                "s3:x-amz-acl": "bucket-owner-full-control",
                "aws:SourceArn": "arn:aws:cloudtrail:region:myAccountID:trail/trailName"
                }
            }
        }
    ]
}

Changing a prefix for an existing bucket

If you try to add, modify, or remove a log file prefix for an S3 bucket that receives logs from a trail, you might see the error: There is a problem with the bucket policy. A bucket policy with an incorrect prefix can prevent your trail from delivering logs to the bucket. To resolve this issue, use the Amazon S3 console to update the prefix in the bucket policy, and then use the CloudTrail console to specify the same prefix for the bucket in the trail.

To update the log file prefix for an Amazon S3 bucket

1. Open the Amazon S3 console at https://console.aws.amazon.com/s3/.

2. Choose the bucket for which you want to modify the prefix, and then choose Permissions.

3. Choose Edit.

4. In the bucket policy, under the s3:PutObject action, edit the Resource entry to add, modify, or remove the log file prefix/ as needed.

   "Action": "s3:PutObject",
         "Resource": "arn:aws:s3:::myBucketName/prefix/AWSLogs/myAccountID/*",

5. Choose Save.

6. Open the CloudTrail console at https://console.aws.amazon.com/cloudtrail/.

7. Choose your trail and for Storage location, click the pencil icon to edit the settings for your bucket.

8. For S3 bucket, choose the bucket with the prefix you are changing.

9. For Log file prefix, update the prefix to match the prefix that you entered in the bucket policy.

10. Choose Save.

Additional resources

For more information about S3 buckets and policies, see the Amazon Simple Storage Service User Guide.