Which Way to Write IAM Policy Documents in Terraform

There are many ways to write IAM policy documents in terraform. In this article, we’ll cover each of them and explain why we use it or why we don’t.

For each pattern, we’ll create an example policy using the last statement of this AWS example. It’s a good test case because it references both an S3 bucket name and an IAM user name, which we’ll handle differently.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "s3:*",
            "Resource": [
                "arn:aws:s3:::bucket-name/home/${aws:username}",
                "arn:aws:s3:::bucket-name/home/${aws:username}/*"
            ]
        }
    ]
}

Table of Contents

Inline jsonencode() Function

This is what we use. You’ll also see it in HashiCorp examples.

resource "aws_s3_bucket" "test" {
  bucket_prefix = "test"
  acl           = "private"
}

resource "aws_iam_policy" "jsonencode" {
  name = "jsonencode"
  path = "/"

  policy = jsonencode({
    Version = "2012-10-17"
    Statement = [
      {
        Action = [
          "s3:*",
        ]
        Effect = "Allow"
        Resource = [
          "${aws_s3_bucket.test.arn}/home/$${aws:username}",
          "${aws_s3_bucket.test.arn}/home/$${aws:username}/*"
        ]
      },
    ]
  })
}
  • ${aws_s3_bucket.test.arn} interpolates the ARN of the bucket we’re granting access to.
  • $${aws:username} escapes interpolation to render a literal ${aws:username} string. ${aws:username} is an AWS IAM policy variable. IAM’s policy variable syntax collides with terraform’s string interpolation syntax. We have to escape it, otherwise terraform expects a variable named aws:username.
  • If you need it, the policy JSON can be referenced with aws_iam_policy.jsonencode.policy (not shown here).

Why we like this pattern:

  • It declares everything in one resource.
  • The policy is written in HCL. Terraform handles the conversion to JSON.
  • There are no extra lines or files like there are in the following patterns. It only requires the lines to declare the resource and the lines that will go into the policy.

aws_iam_policy_document Data Source

The next-best option is the aws_iam_policy_document data source. It’s 95% as good as jsonencode().

resource "aws_s3_bucket" "test" {
  bucket_prefix = "test"
  acl           = "private"
}

data "aws_iam_policy_document" "test" {
  statement {
    actions = [
      "s3:*",
    ]
    resources = [
      "${aws_s3_bucket.test.arn}/home/&{aws:username}",
      "${aws_s3_bucket.test.arn}/home/&{aws:username}/*",
    ]
  }
}

resource "aws_iam_policy" "aws_iam_policy_document" {
  name = "aws_iam_policy_document"
  path = "/"

  policy = data.aws_iam_policy_document.test.json
}
  • The bucket interpolation works the same as in the jsonencode() pattern above.
  • &{aws:username} is an alternate way to escape interpolation that’s specific to this resource. See note in the resource docs. Like above, it renders a literal ${aws:username} string. You can still use $${} interpolation in these resources. The &{} syntax is just another option.

Why we think this is only 95% as good as jsonencode():

  • It requires two resources instead of one.
  • It requires several more lines of code.
  • The different options for escaping interpolation can get mixed together in one declaration, which makes for messy code.
  • The alternate interpolation escape syntax is specific to this resource. If it’s used as a reference when writing other code, it can cause surprises.

These aren’t big problems. We’ve used this resource plenty of times without issues. It’s a fine way to render policies, we just think the jsonencode() pattern is a little cleaner.

Template File

Instead of writing the policy directly in one of your .tf files, you can put them in .tpl template files and render them later with templatefile(). If you don’t need any variables, you could use file() instead of templatefile().

First, you need a template. We’ll call ours test_policy_jsonencode.tpl.

${jsonencode(
  {
    Version = "2012-10-17",
    Statement = [
      {
        Effect = "Allow",
        Action = "s3:*",
        Resource = [
          "${bucket}/home/$${aws:username}",
          "${bucket}/home/$${aws:username}/*"
        ]
      }
    ]
  }
)}

Then, you can render the template into your resources.

resource "aws_s3_bucket" "test" {
  bucket_prefix = "test"
  acl           = "private"
}

resource "aws_iam_policy" "template_file_jsonencode" {
  name = "template_file_jsonencode"
  path = "/"

  policy = templatefile(
    "${path.module}/test_policy_jsonencode.tpl",
    { bucket = aws_s3_bucket.test.arn }
  )
}
  • The interpolation and escape syntax is the same as in the jsonencode() example above.
  • The jsonencode() call wrapped around the contents of the .tpl file allows us to write HCL instead of JSON.
  • You could write a .tpl file containing raw json instead of using jsonencode() around HCL, but then you’d be mixing another language into your module. We recommend standardizing on HCL and letting terraform convert to JSON.
  • templatefile() requires you to explicitly pass every variable you want to interpolate in the .tpl file, like bucket in this example.

Why we don’t use this pattern:

  • It splits the policy declaration across two files. We find this makes modules harder to read.
  • It requires two variable references for every interpolation. One to pass it through to the template, and another to resolve it into the policy. These are tedious to maintain.

In the past, we used these for long policies to help keep our .tf files short. Today, we use the jsonencode() pattern and declare long aws_iam_policy resources in dedicated .tf files. That keeps the policy separate but avoids the overhead of passing through variables.

Heredoc Multi-Line String

You can use heredoc multi-line strings to construct JSON. The HashiCorp docs specifically say not to do this. Because they do, we won’t include an example of using them to construct policy JSON. If you have policies rendered in blocks like this:

<<EOT
{
    "Version": "2012-10-17",
    ...
}
EOT

We recommend replacing them with the jsonencode() pattern.

Happy automating!

Operating Ops

Need more than just this article? We’re available to consult.

You might also want to check out these related articles:

Allowing AWS IAM Users to Manage their Passwords, Keys, and MFA

We do these three things for IAM users that belong to humans:

  • Set a console access password and rotate it regularly. We don’t manage resources in the console, but its graphical UI is handy for inspection and diagnostics.
  • Create access keys and rotate them regularly. We use these with aws-vault to run things like terraform.
  • Enable a virtual Multi-Factor Authentication (MFA) device. AWS accounts are valuable resources. It’s worthwhile to protect them with a second factor of authentication.

There’s much more to managing IAM users, like setting password policies and enforcing key rotation. These are just three good practices we follow.

Users with the AdministratorAccess policy can do all three, but that’s a lot of access. Often, we don’t need that much. Maybe we’re just doing investigation and ReadOnlyAccess is enough. Maybe users have limited permissions and instead switch into roles with elevated privileges (more on this in a future article). In cases like those, we need a policy that allows users to manage their own authentication. Here’s what we use.

This article is about enabling human operators to responsibly manage their accounts. Service accounts used by automation and security policy enforcement are both topics for future articles.

Table of Contents

Console Access Policy Statements

This one is easy. The AWS docs have a limited policy that works.

{
    "Sid": "GetAccountPasswordPolicy",
    "Effect": "Allow",
    "Action": "iam:GetAccountPasswordPolicy",
    "Resource": "*"
},
{
    "Sid": "ChangeSelfPassword",
    "Effect": "Allow",
    "Action": "iam:ChangePassword",
    "Resource": "arn:aws:iam::[account id without hyphens]:user/${aws:username}"
}

Access Key Policy Statements

This one is also easy. The AWS docs have a limited policy that works. We made a small tweak.

{
    "Sid": "ManageSelfKeys",
    "Effect": "Allow",
    "Action": [
        "iam:UpdateAccessKey",
        "iam:ListAccessKeys",
        "iam:GetUser",
        "iam:GetAccessKeyLastUsed",
        "iam:DeleteAccessKey",
        "iam:CreateAccessKey"
    ],
    "Resource": "arn:aws:iam::[account id without hyphens]:user/${aws:username}"
}
  • The AWS policy uses * in the account ID component of the ARN. We like to set the account ID so we’re granting the most specific access we can. Security scanning tools also often check for * characters, and removing them reduces the number of flags.
  • Like above, ${aws:username} is an IAM policy variable. See links there for how to handle this in terraform.
  • We changed the sid from “ManageOwn” to “ManageSelf” so it doesn’t sound like it allows taking ownership of keys for other users.

MFA Device Policy Statements

This one was trickier. We based our policy on an example from the AWS docs, but we made several changes.

{
    "Sid": "ManageSelfMFAUserResources",
    "Effect": "Allow",
    "Action": [
        "iam:ResyncMFADevice",
        "iam:ListMFADevices",
        "iam:EnableMFADevice",
        "iam:DeactivateMFADevice"
    ],
    "Resource": "arn:aws:iam::[account id without hyphens]:user/${aws:username}"
},
{
    "Sid": "ManageSelfMFAResources",
    "Effect": "Allow",
    "Action": [
        "iam:DeleteVirtualMFADevice",
        "iam:CreateVirtualMFADevice"
    ],
    "Resource": "arn:aws:iam::[account id without hyphens]:mfa/${aws:username}"
}
  • Like we talked about above, our goal is to enable users to follow good practices. We selected statements that enable but not ones that require.
  • The AWS example included arn:aws:iam::*:mfa/* in the resources for iam:ListMFADevices. According to the the AWS docs for the IAM service’s actions, this permission only supports user in the resources list. We removed the mfa resource.
  • Also according to the the AWS docs for the IAM service’s actions, iam:DeleteVirtualMFADevice and iam:CreateVirtualMFADevice support different resources from iam:ResyncMFADevice and iam:EnableMFADevice. We split them into separate statements that limit each one to their supported resources. This probably doesn’t change access level, but our routine is to limit resource lists as much as possible. That helps make it clear to future readers what the policy enables.
  • Like above, ${aws:username} is an IAM policy variable. See links there for how to handle this in terraform.
  • We continued our convention from above of naming sids for “self” to indicate they’re limited to the user who has the policy.

Complete Policy Document

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "GetAccountPasswordPolicy",
            "Effect": "Allow",
            "Action": "iam:GetAccountPasswordPolicy",
            "Resource": "*"
        },
        {
            "Sid": "ChangeSelfPassword",
            "Effect": "Allow",
            "Action": "iam:ChangePassword",
            "Resource": "arn:aws:iam::[account id without hyphens]:user/${aws:username}"
        },
        {
            "Sid": "ManageSelfKeys",
            "Effect": "Allow",
            "Action": [
                "iam:UpdateAccessKey",
                "iam:ListAccessKeys",
                "iam:GetUser",
                "iam:GetAccessKeyLastUsed",
                "iam:DeleteAccessKey",
                "iam:CreateAccessKey"
            ],
            "Resource": "arn:aws:iam::[account id without hyphens]:user/${aws:username}"
        },
        {
            "Sid": "ManageSelfMFAUserResources",
            "Effect": "Allow",
            "Action": [
                "iam:ResyncMFADevice",
                "iam:ListMFADevices",
                "iam:EnableMFADevice",
                "iam:DeactivateMFADevice"
            ],
            "Resource": "arn:aws:iam::[account id without hyphens]:user/${aws:username}"
        },
        {
            "Sid": "ManageSelfMFAResources",
            "Effect": "Allow",
            "Action": [
                "iam:DeleteVirtualMFADevice",
                "iam:CreateVirtualMFADevice"
            ],
            "Resource": "arn:aws:iam::[account id without hyphens]:mfa/${aws:username}"
        }
    ]
}

User Guide

  1. Replace [account id without hyphens] with the ID for your account in the policy above.
  2. Attach the policy to users (we like to do this through groups).
  3. Tell users to edit their authentication from My Security Credentials in the user dropdown. This policy won’t let them access their user through the IAM console. My Security Credentials may not appear in the dropdown if the user has switched into a role.

Happy automating!

Need more than just this article? We’re available to consult.

You might also want to check out these related articles: