SCP add exception for AWS Services

Question

Hi,

We are trying to setup a **SCP**  which will deny some **DynamoDB** actions based on the **IP Ranes ** of our Network, the way that IAM Users for example can't Scan or Query a DynamoDB table outside of our Network.

In this SCP we need to add a**n exception** to some AWS Services (Like: **EC2** or **Lambda**) which can freely Query/Scan a DynamoDB table if they have the necessary permissions.

We tried with the following SCP and it worked fine for the first case "IAM Users" but failed for the Lambda case as we still recieving an AccessDenied Error trying to Query a DynamoDB table from a Lambda Function :

```
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Deny",
      "Action": "dynamodb:*",
      "Resource": "*",
      "Condition": {
        "Bool": {
          "aws:ViaAWSService": "false"
        },
        "NotIpAddress": {
          "aws:SourceIp": [
            "IP Range"
          ]
        }
      }
    }
  ]
}
```

Do you know how we can manage to add this exception for all AWS Services which need to perform any DynamoDB action without the need to use the ARN of specific IAM Role used by these service ?

Accepted Answer

Hi all,

we ended up using the following SCP :

```
{
	"Version": "2012-10-17",
	"Statement": [{
		"Effect": "Deny",
		"Action": "dynamodb:*",
		"Resource": "*",
		"Condition": {
			"ArnNotLikeIfExists": {
				"aws:PrincipalArn": [
					"arn:aws:iam::*:role/*"
				]
			},
			"Bool": {
				"aws:PrincipalIsAWSService": "false"
			}
		}
	}]
}
```

unfortunately, checks like **aws:PrincipalIsAWSService** or  **aws:ViaAWSService** wont work for AWS Services that uses a IAM Role to operate on **DynamoDB** like **Lambda** or an **EC2 Instance**

Answer

Hi Peter,

Thought I never used it myself, you can try to use the ViaAwsService as condition.

Below example exactly denies IPs from range BUT does not deny requests made by AWS services using the principal's credentials.

https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_examples_aws_deny-ip.html

Edit: aws:CalledVia could be evaluated too.

https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_condition-keys.html

Hope it helps ;)

Answer

You can use the IAM `PrincipalIsAWSService` key to check if the action is being performed by an AWS service principal, such as `lambda.amazonaws.com`.

More information here: https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-principalisawsservice.

SCP add exception for AWS Services

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