Amazon S3 is widely used as a repository for backups. Backups are an important aspect of a resilient system, but they are also a potential security vulnerability. Unauthorized access to a database backup is still a PCI or HIPAA violation. The permissions on Amazon S3 should be configured to minimize access to critical backups. My strategy is to use IAM to create a backup user with no password (cannot log in to AWS) and a single access key. The backup user’s access key is used to put backup files into S3. The S3 bucket policy is configured to allow “write-only” access for the backup user. The backups cannot be obtained, even if the backup user’s credentials are compromised.

It is fairly difficult to figure out how to create a “write only” bucket policy. The policy shown below is the “write only” policy that I use. It consists of two statements: BucketPermissions gives a user ability to locate the bucket (necessary to do anything in the bucket) and list its contents (to verify that a backup was written). You may remove the s3:ListBucket action if true write-only access is desired. The statement called ObjectPermissions allows the user to create objects in the specified bucket.

{
 "Version": "2012-10-17",
 "Id": "YOUR_POLICY_ID_NUMBER",
 "Statement": [
 {
 "Sid": "BucketPermissions",
 "Effect": "Allow",
 "Principal": {
 "AWS": "arn:aws:iam::YOUR_ACCOUNT_ID:user/USERNAME"
 },
 "Action": [
 "s3:ListBucket",
 "s3:GetBucketLocation"
 ],
 "Resource": "arn:aws:s3:::BUCKET_NAME"
 },
 {
 "Sid": "ObjectPermissions",
 "Effect": "Allow",
 "Principal": {
 "AWS": "arn:aws:iam::YOUR_ACCOUNT_ID:user/USERNAME"
 },
 "Action": [
 "s3:PutObjectAcl",
 "s3:PutObject"
 ],
 "Resource": "arn:aws:s3:::BUCKET_NAME/*"
 }
 ]
}

S3 does one odd thing: this policy allows the user to verify that a particular object exists in S3, even if they don’t have permission to GET the object. For example, running the command:

s3cmd get s3://BUCKET_NAME/PATH/BACKUP_FILE_NAME.tgz

Produces the output:

s3://BUCKET_NAME/PATH/BACKUP_FILE_NAME.tgz -> ./BACKUP_FILE_NAME.tgz
ERROR: S3 error: Unknown error

It appears that the file is being downloaded, but in fact only an empty file is created! While it is generally a bad idea to allow unauthorized users to guess file names, it is not a real problem in this case, because the backup user’s credentials would have to be compromised even to confirm the existence of a file stored in S3.

References

  1. AWS Bucket Policy Reference
  2. S3 Encryption (high recommended)
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