Complete AWS IAM Reference

Michael Wittig – 18 Aug 2016

Writing IAM policies is hard. Following the principle of least privilege is even harder. To write a secure IAM policy you need to know:

  • What actions are needed?
  • Are resource-level permissions supported and on what levels?
  • Are conditions supported to restrict access?

That’s a lot of stuff and the information is spread all across the AWS documentation. That’s why we created the Complete AWS IAM Reference.

Example

For example, you want to allow the launch of new EC2 instances.

First you need to find out what action is needed. You can use the Complete AWS IAM Reference to search for launch in the description field. Now you know, that the action is called ec2:RunInstances .

{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": "ec2:RunInstances",
"Resource": "*"
}
]
}

Now you can restrict access further. What options do you have? The Complete AWS IAM Reference shows you that you can use many resource-level permissions. For example, you can restrict that it is only allowed to use a certain AMI like Amazon Linux 2016.03.3 (64bit, gp2).

{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": "ec2:RunInstances",
"Resource": [
"arn:aws:ec2:us-east-1::image/ami-6869aa05",
"arn:aws:ec2:us-east-1:$account-id:instance/*",
"arn:aws:ec2:us-east-1:$account-id:key-pair/*",
"arn:aws:ec2:us-east-1:$account-id:network-interface/*",
"arn:aws:ec2:us-east-1:$account-id:placement-group/*",
"arn:aws:ec2:us-east-1:$account-id:security-group/*",
"arn:aws:ec2:us-east-1::snapshot/*",
"arn:aws:ec2:us-east-1:$account-id:subnet/*",
"arn:aws:ec2:us-east-1:$account-id:volume/*"
]
}
]
}

You can take it even further. The Complete AWS IAM Reference shows that you can use a condition to restrict based on instance type. To save money in your dev account you may only allow t2.micro instances.

{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": "ec2:RunInstances",
"Resource": [
"arn:aws:ec2:us-east-1:$account-id:instance/*"
],
"Condition": {
"StringEquals": {
"ec2:InstanceType": "t2.micro"
}
}
}, {
"Effect": "Allow",
"Action": "ec2:RunInstances",
"Resource": [
"arn:aws:ec2:us-east-1::image/ami-6869aa05",
"arn:aws:ec2:us-east-1:$account-id:key-pair/*",
"arn:aws:ec2:us-east-1:$account-id:network-interface/*",
"arn:aws:ec2:us-east-1:$account-id:placement-group/*",
"arn:aws:ec2:us-east-1:$account-id:security-group/*",
"arn:aws:ec2:us-east-1::snapshot/*",
"arn:aws:ec2:us-east-1:$account-id:subnet/*",
"arn:aws:ec2:us-east-1:$account-id:volume/*"
]
}
]
}

Keep in mind that different resource-level permissions support different service specific conditions. That’s why the following policy is not working:

{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": "ec2:RunInstances",
"Resource": [
"arn:aws:ec2:us-east-1::image/*",
"arn:aws:ec2:us-east-1:$account-id:instance/*"
"arn:aws:ec2:us-east-1:$account-id:key-pair/*",
"arn:aws:ec2:us-east-1:$account-id:network-interface/*",
"arn:aws:ec2:us-east-1:$account-id:placement-group/*",
"arn:aws:ec2:us-east-1:$account-id:security-group/*",
"arn:aws:ec2:us-east-1::snapshot/*",
"arn:aws:ec2:us-east-1:$account-id:subnet/*",
"arn:aws:ec2:us-east-1:$account-id:volume/*"
],
"Condition": {
"StringEquals": {
"ec2:InstanceType": "t2.micro"
}
}
}
]
}

Summary

For every AWS service, different actions are available. Depending on the action you can use resource-level permissions and sometimes also conditions. The Complete AWS IAM Reference collects all that information and makes it accessible to you.

Tags: aws security iam
Michael Wittig

Michael Wittig

I’m the author of Amazon Web Services in Action. I work as a software engineer, and independent consultant focused on AWS and DevOps.

You can contact me via Email, Twitter, and LinkedIn.

Briefcase icon
Hire me
Cover of Rapid Docker on AWS

New book: Rapid Docker on AWS

A rapid way to get your web application up and running on AWS. Made for web developers and DevOps engineers who want to dockerize their web applications and run their containers on Amazon Web Services. Prior knowledge of Docker and AWS is not required.

Buy icon
Buy now
Marbot Logo

Incident Management for Slack

Team up to solve incidents with our chatbot marbot. Never miss a critical alert. Escalate alerts from your AWS infrastructure among your team members. Strong integrations with all parts of your AWS infrastructure: CloudWatch, Elastic Beanstalk, RDS, EC2, ...

Slack icon
Try for free
📚 Rapid Docker on AWS
A rapid way to get your web application up and running on AWS. Learn how to package your application into Docker containers. Learn more.