In AWS s3, how to allow read and write only in specific prefixes (using a wildcard) - amazon-s3

Question

I have a bucket that has these prefixes:
1_user/
2_user/
10_user/
Users have access to all bucket by default, but I want to deny access to this specific bucket to all prefixes except those listed above (read/write), for which I tried something like this:
{
"Version": "2012-10-17",
"Id": "BlockReadWriteListAccessExceptUser",
"Statement": [
{
"Sid": "BlockReadWriteAccess",
"Effect": "Deny",
"Principal": "*",
"Action": [
"s3:GetObject",
"s3:PutObject"
],
"Resource": [
"arn:aws:s3:::mybucket",
"arn:aws:s3:::mybucket/*"
]
},
{
"Sid": "AllowReadWriteAccess",
"Effect": "Allow",
"Principal": "*",
"Action": [
"s3:GetObject",
"s3:PutObject"
],
"Resource": "arn:aws:s3:::mybucket/*_user"
}
]
}
I realized that for get and put, I cannot use conditions, so how can I achieve that?

Answer 1

Your bucket policy will not work because explicit deny always overrides the allows. I want to make this crystal clear. For example, say you want to add an object 999_user. The object ARN is arn:aws:s3:::mybucket/999_user, which matches the deny statement with sid "BlockReadWriteAccess". S3 will deny this request right away without even looking at the "AllowReadWriteAccess" statement.
How to achieve the behaviour you want? The policy element NotResource comes in handy in this case.
{
"Version": "2012-10-17",
"Id": "BlockReadWriteListAccessExceptUser",
"Statement": [
{
"Sid": "BlockReadWriteAccess",
"Effect": "Deny",
"Principal": "*",
"Action": [
"s3:GetObject",
"s3:PutObject"
],
"NotResource": "arn:aws:s3:::mybucket/*_user*"
}
]
}