Troubleshooting: Error Creating IAM Role MalformedPolicyDocument in Terraform

If you’ve encountered the “error creating IAM role malformedpolicydocument has prohibited field resource terraform,” you’re not alone. Dealing with IAM role policy documents can be complex and critical for maintaining security in your AWS environment. Understanding the root cause of this error and knowing how to tackle it is crucial.

In this article, we’ll delve into the specifics of this issue, providing you with actionable steps to troubleshoot and resolve it effectively.

Troubleshooting IAM Role Creation Error

The error message you encountered while creating an IAM role with Terraform indicates that there is an issue with the policy document. Specifically, it seems that the policy document contains a prohibited field related to the resource configuration.

Here are some steps to troubleshoot and resolve this issue:

1. Check the Policy Document Syntax:

   • Ensure that the policy document is correctly formatted. Make sure that all opening and closing brackets {} are balanced, and that there are no syntax errors.
   • Verify that the resource section is properly defined. It should specify the AWS resource type (e.g., aws_s3_bucket, aws_iam_user, etc.) and the resource name.

2. Review the IAM Role Configuration:

   • Double-check your IAM role configuration in your Terraform code. Look for any discrepancies or typos.
   • Confirm that the assume_role_policy attribute is correctly defined. This policy document specifies which entities are allowed to assume the role.

3. Example IAM Role Configuration:
   Below is an example of how to create an IAM role with Terraform. You can adapt this to your specific use case:

   resource "aws_iam_role" "my_role" {
     name = "my-iam-role"
   
     assume_role_policy = jsonencode({
       Version = "2012-10-17",
       Statement = [
         {
           Action = "sts:AssumeRole",
           Effect = "Allow",
           Principal = {
             Service = "ec2.amazonaws.com"
           }
         }
       ]
     })
   }
   

   In this example:

   • The role name is set to "my-iam-role".
   • The assume_role_policy allows EC2 instances to assume this role.

4. Reapply Terraform:
   After making the necessary adjustments, reapply your Terraform configuration using the following command:

   terraform apply
   

Resolving Malformed Policy Document Error

The error message you’re encountering—“Malformed Policy Document: Has prohibited field Resource”—is related to an issue with your IAM (Identity and Access Management) policy document in AWS. Let’s break down the problem and explore how to resolve it.

1. Error Explanation:

   • The error occurs when the IAM policy document you’ve defined contains an invalid or disallowed field named “Resource.”
   • The assume_role_policy attribute in an IAM role resource should specify only the AssumeRole action, which allows entities (such as EC2 instances) to assume the role. It should not include other permissions or resources.
   • The Resource field is typically used in attached policies, not in the assume role policy.

2. Solution:

   • To fix this, split your policy into separate documents:
     • Create one policy document for the AssumeRole action (allowing entities to assume the role).
     • Create another policy document for additional permissions (e.g., attaching other actions to the role).
   • Here’s an example of how to structure your Terraform code:

# Allow EC2 instances to assume the role
data "aws_iam_policy_document" "asg_assume_role_policy" {
  statement {
    actions = ["sts:AssumeRole"]
    effect  = "Allow"
    principals {
      type        = "Service"
      identifiers = ["ec2.amazonaws.com"]
    }
  }
}

# Create the policy allowing other actions for EC2 instances
data "aws_iam_policy_document" "asg_domain_join_policy" {
  statement {
    actions   = [
      "ssm:DescribeAssociation",
      "ssm:GetDocument",
      "ssm:ListAssociations",
      "ssm:UpdateAssociationStatus",
      "ssm:UpdateInstanceInformation",
      "ssm:CreateAssociation",
    ]
    effect    = "Allow"
    resources = ["*"]  # Replace with specific resource ARNs if needed
  }
}

resource "aws_iam_role" "ad_join_role" {
  name               = "asg-domain-join-policy"
  assume_role_policy = data.aws_iam_policy_document.asg_assume_role_policy.json

  # Attach the policy
  inline_policy {
    policy = data.aws_iam_policy_document.asg_domain_join_policy.json
  }
}

3. Notes:
   • In the example above:
     • The asg_assume_role_policy document allows EC2 instances to assume the role.
     • The asg_domain_join_policy document specifies additional actions for EC2 instances.
     • The second policy is attached as an inline policy to the role.
   • Ensure that the Resource field is correctly used in the appropriate context (e.g., attached policies).

: Stack Overflow: Malformed Policy Document Error
: Stack Overflow: Terraform IAM Role Error
: AWS Knowledge Center: Resolving MalformedPolicyDocument Errors

Troubleshooting IAM Role Policies

Troubleshooting IAM role policy documents can be crucial for ensuring proper access control and security in your AWS environment. Let’s explore some steps you can take to troubleshoot IAM role policies:

1. Validate Your Policies:

   • Use the IAM Policy Simulator to test and troubleshoot identity-based policies and permissions boundaries. You can simulate different scenarios to verify if your policies allow or deny specific actions for specific resources.
   • Ensure that your policy syntax adheres to the rules of JavaScript Object Notation (JSON). IAM policies are stored as JSON documents, but you don’t need to understand the syntax to create or manage them.

2. Check for Errors and Warnings:

   • When creating or editing a policy, use the JSON Policy Editor in the IAM console. It provides policy validation and identifies issues such as syntax errors, security warnings, and best practice violations.
   • Review the findings related to security, errors, warnings, and suggestions provided by IAM Access Analyzer during policy validation.

3. Follow Best Practices:

   • Apply the Principle of Least Privilege: Assign only the necessary permissions to roles and users. Avoid overly permissive policies.
   • Use Managed Policies: Start with AWS managed policies and move toward least-privilege permissions.
   • Regularly review and remove unused users, roles, permissions, and credentials.

Resolving MalformedPolicyDocument Error

When encountering the “MalformedPolicyDocument” error while creating an IAM role in Terraform, it’s essential to understand the issue and how to resolve it. Let’s break it down step by step:

1. Deciphering the Error:

   • The error message indicates that the IAM policy document associated with the role doesn’t comply with the expected format or structure.
   • Specifically, it mentions a “prohibited field Resource.”

2. Understanding IAM Policy Documents:

   • An IAM policy document defines permissions for AWS resources and API actions.
   • The “Resource” field specifies the objects to which the policy statement applies.
   • However, not all permissions require a resource specification.

3. Common Mistake:

   • Often, users include the “Resource” field unnecessarily, even when the action doesn’t require it.
   • For example, if you’re allowing or denying an action that applies to all resources (e.g., "*"), specifying a resource can trigger this error.

4. Correcting the Policy Document:

   • Remove the “Resource” field from the policy statement for actions that don’t need it.
   • Here’s an example of a corrected policy:

resource "aws_iam_role_policy" "example" {
  name  = "example"
  role  = aws_iam_role.example.id

  policy = <

5. Additional Tips:
   • Ensure that the “Sid” values are unique within your policy.
   • Remember that the “assume_role_policy” is for trust policies and doesn’t have a “Resource” field.

Best Practices for IAM Policy Documents

When it comes to IAM (Identity and Access Management) role policy documents, adhering to best practices ensures robust security and efficient management. Let’s dive into some key recommendations:

1. Use a Standard Format and Naming Convention:

   • Consistency matters. Employ a uniform format (such as JSON or YAML) for your policy documents.
   • Choose descriptive names that convey the purpose, scope, and access levels of each policy.

2. Follow a Policy Lifecycle and Version Control:

   • Regularly review and update policies to align with business needs and compliance standards.
   • Define clear roles and responsibilities for policy creation, review, approval, and updates.
   • Utilize version control systems (like Git or SVN) to track changes and maintain an audit trail.

3. Use a Framework or Standard for Reference:

   • Leverage established frameworks (e.g., NIST, ISO) to guide policy development.
   • Reference industry standards (e.g., SAML, OAuth) for policy syntax and semantics.
   • Draw inspiration from reputable sources (cloud providers, security organizations) for templates and examples.

4. Document Your Policy Rationale and Assumptions:

   • Explain why specific permissions are granted or restricted.
   • Document any assumptions made during policy design.
   • Provide context for future reference and clarity.

5. Communicate Your Policies Clearly and Regularly:

   • Share policies with relevant stakeholders (users, administrators, auditors).
   • Ensure everyone understands their roles and responsibilities.
   • Regularly review and communicate policy changes.

For more detailed information, you can refer to the LinkedIn article on IAM policies.

In conclusion, navigating the intricacies of IAM role policy documents is essential for ensuring proper access control and security within your AWS infrastructure. By following best practices, validating your policies, and staying informed about common pitfalls like the “error creating IAM role malformedpolicydocument has prohibited field resource terraform,” you can bolster your organization’s security posture. Remember to leverage tools like the IAM Policy Simulator, adhere to the Principle of Least Privilege, and document your policy rationale to maintain a robust and compliant IAM framework.

With diligence and attention to detail, you can successfully overcome IAM role-related challenges and fortify your AWS environment against potential security threats.