# Denying the ModifyInstanceGroup action in Amazon EMR
<a name="emr-cluster-deny-modifyinstancegroup"></a>

The [ModifyInstanceGroups](https://docs.aws.amazon.com/emr/latest/APIReference/API_ModifyInstanceGroups.html) action in Amazon EMR does not require that you provide a cluster ID with the action. Instead, you can specify only an instance group ID. For this reason, an apparently simple deny policy for this action based on cluster ID or a cluster tag may not have the intended effect. Consider the following example policy.

------
#### [ JSON ]

****  

```
{
  "Version":"2012-10-17",		 	 	 
  "Statement": [
    {
      "Action": [
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Effect": "Allow",
      "Resource": [
        "*"
      ],
      "Sid": "AllowELASTICMAPREDUCEModifyinstancegroups"
    },
    {
      "Action": [
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Effect": "Deny",
      "Resource": [
        "arn:aws:elasticmapreduce:us-east-1:123456789012:cluster/j-12345ABCDEFG67"
      ],
      "Sid": "DenyELASTICMAPREDUCEModifyinstancegroups"
    }
  ]
}
```

------

If a user with this policy attached performs a `ModifyInstanceGroup` action and specifies only the instance group ID, the policy does not apply. Because the action is allowed on all other resources, the action is successful.

A solution to this issue is to attach a policy statement to the identity that uses a [NotResource](https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_elements_notresource.html) element to deny any `ModifyInstanceGroup` action issued without a cluster ID. The following example policy adds such a deny statement so that any `ModifyInstanceGroups` request fails unless a cluster ID is specified. Because an identity must specify a cluster ID with the action, deny statements based on cluster ID are therefore effective.

------
#### [ JSON ]

****  

```
{
  "Version":"2012-10-17",		 	 	 
  "Statement": [
    {
      "Action": [
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Effect": "Allow",
      "Resource": [
        "*"
      ],
      "Sid": "AllowELASTICMAPREDUCEModifyinstancegroups"
    },
    {
      "Action": [
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Effect": "Deny",
      "Resource": [
        "arn:aws:elasticmapreduce:us-east-1:123456789012:cluster/j-12345ABCDEFG67"
      ],
      "Sid": "DenyELASTICMAPREDUCEModifyinstancegroupsSpecificCluster"
    },
    {
      "Action": [
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Effect": "Deny",
      "NotResource": "arn:*:elasticmapreduce:*:*:cluster/*",
      "Sid": "DenyELASTICMAPREDUCEModifyinstancegroupsNonCluster"
    }
  ]
}
```

------

A similar issue exists when you want to deny the `ModifyInstanceGroups` action based on the value associated with a cluster tag. The solution is similar. In addition to a deny statement that specifies the tag value, you can add a policy statement that denies the `ModifyInstanceGroup` action if the tag that you specify is not present, regardless of value.

The following example demonstrates a policy that, when attached to an identity, denies the identity the `ModifyInstanceGroups` action any cluster with the tag `department` set to `dev`. This statement is only effective because of the deny statement that uses the `StringNotLike` condition to deny the action unless the `department` tag is present.

------
#### [ JSON ]

****  

```
{
  "Version":"2012-10-17",		 	 	 
  "Statement": [
    {
      "Action": [
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Effect": "Allow",
      "Resource": [
        "*"
      ],
      "Sid": "AllowELASTICMAPREDUCEModifyinstancegroups"
    },
    {
      "Action": [
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Condition": {
        "StringEquals": {
          "aws:ResourceTag/department": "dev"
        }
      },
      "Effect": "Deny",
      "Resource": [
        "*"
      ],
      "Sid": "DenyELASTICMAPREDUCEModifyinstancegroupsDevDepartment"
    },
    {
      "Action": [
        "elasticmapreduce:ModifyInstanceGroups"
      ],
      "Condition": {
        "StringNotLike": {
          "aws:ResourceTag/department": "?*"
        }
      },
      "Effect": "Deny",
      "Resource": [
        "*"
      ],
      "Sid": "DenyELASTICMAPREDUCEModifyinstancegroupsNoDepartmentTag"
    }
  ]
}
```

------