AWS Security Blog
How to use policies to restrict where EC2 instance credentials can be used from
April 5, 2023: A fix has been added to the Service Control Policy examples to allow EC2 instances to mount encrypted EBS volumes.
March 7, 2023: We’ve added language clarifying the requirement around using VPC Endpoints, and we’ve corrected a typo in the S3 bucket policy example.
Today AWS launched two new global condition context keys that make it simpler for you to write policies in which Amazon Elastic Compute Cloud (Amazon EC2) instance credentials work only when used on the instance to which they are issued. These new condition keys are available today in all AWS Regions, as well as AWS GovCloud and China partitions.
Using these new condition keys, you can write service control policies (SCPs) or AWS Identity and Access Management (IAM) policies that restrict the virtual private clouds (VPCs) and private IP addresses from which your EC2 instance credentials can be used, without hard-coding VPC IDs or IP addresses in the policy. Previously, you had to list specific VPC IDs and IP addresses in the policy if you wanted to use it to restrict where EC2 credentials were used. With this new approach, you can use less policy space and reduce the time spent on updates when your list of VPCs and network ranges changes.
In this blog post, we will show you how to use these new condition keys in an SCP and a resource policy to help ensure that the IAM role credentials assigned to your EC2 instances can only be used from the instances to which they were issued.
New global condition keys
The two new condition keys are as follows:
- aws:EC2InstanceSourceVPC — This single-valued condition key contains the VPC ID to which an EC2 instance is deployed.
- aws:EC2InstanceSourcePrivateIPv4 — This single-valued condition key contains the primary IPv4 address of an EC2 instance.
These new conditions are available only for use with credentials issued to an EC2 instance. You don’t have to make configuration changes to activate the new condition keys.
Let’s start by reviewing some existing IAM conditions and how to combine them with the new conditions. When requests are made to an AWS service over a VPC endpoint, the value of the aws:SourceVpc condition key is the ID of the VPC into which the endpoint is deployed. The value of the aws:VpcSourceIP condition key is the IP address from which the endpoint receives the request. The aws:SourceVpc and aws:VpcSourceIP keys are null when requests are made through AWS public service endpoints. These condition keys relate to dynamic properties of the network path by which your AWS Signature Version 4-signed request reached the API endpoint. For a list of AWS services that support VPC endpoints, see AWS services that integrate with AWS PrivateLink.
The two new condition keys relate to dynamic properties of the EC2 role credential itself. By using the two new credential-relative condition keys with the existing network path-relative aws:SourceVPC and aws:VpcSourceIP condition keys, you can create SCPs to help ensure that credentials for EC2 instances are only used from the EC2 instances to which they were issued. By writing policies that compare the two sets of dynamic values, you can configure your environment such that requests signed with an EC2 instance credential are denied if they are used anywhere other than the EC2 instance to which they were issued.
Policy examples
In the following SCP example, access is denied if the value of aws:SourceVpc is not equal to the value of aws:ec2InstanceSourceVPC, or if the value of aws:VpcSourceIp is not equal to the value of aws:ec2InstanceSourcePrivateIPv4. This means that the request to an AWS service must pass through a VPC endpoint, and originate from the same EC2 instance that an instances credential was issued to in order to allow access.
The policy uses aws:ViaAWSService to allow certain AWS services to take action on your behalf when they use your role’s identity to call services, such as when Amazon Athena queries Amazon Simple Storage Service (Amazon S3).
Because we encase aws:SourceVpc and aws:VpcSourceIp in “${}” in these policies, they are treated as a variable using the value in the request being made. However, in the IAM policy language, the operator on the left side of a comparison is implicitly treated as a variable, while the operator on the right side must be explicitly declared as a variable. The “Null” operator on the ec2:SourceInstanceARN condition key is designed to ensure that this policy only applies to EC2 instance roles, and not roles used for other purposes, such as those used in AWS Lambda functions. The ArnNotLike operator is used to allow your EC2 instances to communicate with KMS when mounting an encrypted volume.
The two deny statements in this example form a logical “or” statement, such that either a request from a different VPC or a different IP address evaluates in a deny. But functionally, they act in an “and” fashion. To be allowed, a request must satisfy both the VPC-based and the IP-based conditions because failure of either denies the call. Because VPC IDs are globally unique values, it’s reasonable to use the VPC-based condition without the private IP condition. However, you should avoid evaluating only the private IP condition without also evaluating the VPC condition. Private IPs can be the same across different environments, so aws:ec2InstanceSourcePrivateIPv4 is safe to use only in conjunction with the VPC-based condition.
Note: SCPs do not grant IAM permissions; they only remove them. Thus, you must permit your EC2 instances to use AWS services by using IAM policies associated with their roles. For more information, see Determining whether a request is allowed or denied within an account.
Note: This policy will deny all requests using the EC2 instances credential if the requests are not made through a VPC endpoint.
If you have specific EC2 instance roles that you want to exclude from the statement, you can apply exception logic through tags or role names.
The following example applies to roles used as EC2 instance roles, except those with a tag of exception-to-vpc-ip where the value is equal to true by using the aws:PrincipalTag condition key. The three condition operators (StringNotEquals, Null, and BoolIfExists) in the same condition block are evaluated with a logical AND operation, and if either of the tests doesn’t evaluate, then the deny statement doesn’t apply. Hence, EC2 instance roles with a principal tag of exception-to-vpc-ip equal to true are not subject to this SCP.
You can apply exception logic to other attributes of your IAM roles. For example, you can use the aws:PrincipalArn condition key to exempt certain roles based on their AWS account. You can also specify where you want this SCP to be applied in your AWS Organizations organization. You can apply SCPs directly to accounts, organizational units, or organizational roots. For more information about inheritance when applying SCPs in Organizations, see Understanding policy inheritance.
You can also apply exception logic to your SCP statements at the IAM Action. The following example statement restricts an EC2 instance’s credential usage to only the instance from which it was issued, except for calls to IAM by using a NotAction element. You should use this exception logic if an AWS service doesn’t have a VPC endpoint, or if you don’t want to use VPC endpoints to access a particular service.
Because these new condition keys are global condition keys, you can use the keys in all relevant AWS policy types, such as the following policy for an S3 bucket. When using this as a bucket policy, make sure to replace <DOC-EXAMPLE-BUCKET> with the ARN of your S3 bucket.
This policy restricts access to your S3 bucket to EC2 instance roles that are used only from the instance to which they were vended. Like the previous policy examples, there are two deny statements in this example to form a logical “or” statement but a functional “and” statement, because a request must come from the same VPC and same IP address of the instance that it was issued to, or else it evaluates to a deny.
Conclusion
In this blog post, you learned about the newly launched aws:ec2InstanceSourceVPC and aws:ec2InstanceSourcePrivateIPv4 condition keys. You also learned how to use them with SCPs and resource policies to limit the usage of your EC2 instance roles to the instances from which they originated when requests are made over VPC endpoints. Because these new condition keys are global condition keys, you can use them in all relevant AWS policy types. These new condition keys are available today in all Regions, as well as AWS GovCloud and China partitions.
If you have questions, comments, or concerns, contact AWS Support or start a new thread at AWS Identity and Access Management or Compute re:Post.
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