Check for Untrusted Cross-Account IAM Roles

01 Sign in to the AWS Management Console.

02 Navigate to Amazon IAM console at https://console.aws.amazon.com/iam/.

03 In the navigation panel, under Access management, choose Roles.

04 Click on the name of the IAM role that you want to examine.

05 Select the Trust relationships tab and identify the IDs of the entities that can assume this role, listed in the Trusted entities section.

06 Sign in to your Trend Micro Cloud One™ – Conformity account, access Cross-Account IAM Roles conformity rule settings, and compare the account ID(s) identified at the previous step against each AWS account ID defined in the rule configuration section. If one or more AWS account IDs are not included in the list of trusted account IDs defined in the conformity rule settings, the selected Amazon IAM role can be assumed by untrusted AWS entities, therefore the cross-account access is not secured.

07 Repeat steps no. 4 – 6 for each IAM role available within your AWS cloud account.

01 Run list-roles command (OSX/Linux/UNIX) with custom query filters to list the names of all the IAM roles available in your AWS cloud account:

aws iam list-roles
  --output table
  --query 'Roles[*].RoleName'

02 The command output should return a table with the requested IAM role identifiers:

---------------------------
|        ListRoles        |
+-------------------------+
|  cc-external-mgmt-role  |
|  cc-lambda-admin-role   |
|  cc-prod-manager-role   |
+-------------------------+

03 Run get-role command (OSX/Linux/UNIX) using the name of the Amazon IAM role that you want to examine as the identifier parameter, to describe the policy that grants another entity the permission to assume the selected role:

aws iam get-role
  --role-name cc-external-mgmt-role
  --query 'Role.AssumeRolePolicyDocument'

04 The command output should return the trust policy defined for the selected IAM role:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": "sts:AssumeRole",
            "Effect": "Allow",
            "Condition": {
                "StringEquals": {
                    "sts:ExternalId": "<external_id>"
                }
            },
            "Principal": {
                "AWS": "arn:aws:iam::123456789012:root"
            }
        }
    ]
}

05 Sign in to your Trend Micro Cloud One™ – Conformity account, access Cross-Account IAM Roles conformity rule settings, and compare the account ID returned by the get-role command output at the previous step against each AWS account ID defined in the rule configuration section. If one or more AWS account IDs are not included in the list of trusted account IDs defined in the conformity rule settings, the selected Amazon IAM role can be assumed by untrusted AWS entities, therefore the cross-account access is not secured.

06 Repeat steps no. 3 – 5 for each IAM role available within your AWS cloud account.

01 CloudFormation template (JSON). Replace <trusted-account-id> with the ID of the trusted AWS account that can assume your IAM role:

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Description": "Authorize Trusted AWS Account to Assume IAM Role",
  "Resources": {
    "CrossAccountIAMRole": {
      "Type": "AWS::IAM::Role",
      "Properties": {
        "RoleName": "cc-external-mgmt-role",
        "Description": "Full Access to Amazon EC2",
        "AssumeRolePolicyDocument": {
          "Version": "2012-10-17",
          "Statement": [
            {
              "Effect": "Allow",
              "Principal": {
                "AWS": "arn:aws:iam::<trusted-account-id>:root"
              },
              "Condition": {
                 "StringEquals": {
                    "sts:ExternalId": "<external_id>"
                 }
              },
              "Action": [
                "sts:AssumeRole"
              ]
            }
          ]
        },
        "ManagedPolicyArns": [
          "arn:aws:iam::aws:policy/AmazonEC2FullAccess"
        ],
        "Path": "/"
      }
    }
  }
}

02 CloudFormation template (YAML). Replace <trusted-account-id> with the ID of the trusted AWS account that can assume your IAM role:

AWSTemplateFormatVersion: '2010-09-09'
Description: Authorize Trusted AWS Account to Assume IAM Role
Resources:
  CrossAccountIAMRole:
    Type: AWS::IAM::Role
    Properties:
      RoleName: cc-external-mgmt-role
      Description: Full Access to Amazon EC2
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
          - Effect: Allow
            Principal:
              AWS: arn:aws:iam::<trusted-account-id>:root
            Condition:
              StringEquals:
                sts:ExternalId: <external_id>
            Action:
              - sts:AssumeRole
      ManagedPolicyArns:
        - arn:aws:iam::aws:policy/AmazonEC2FullAccess
      Path: /

01 Terraform configuration file (.tf). Replace <trusted-account-id> with the ID of the trusted AWS account that can assume your IAM role:

terraform {
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = "~> 3.27"
    }
  }

  required_version = ">= 0.14.9"
}

provider "aws" {
  profile = "default"
  region  = "us-east-1"
}

resource "aws_iam_role" "iam-role" {
  name = "cc-external-mgmt-role"
  path = "/"
  managed_policy_arns = [ "arn:aws:iam::aws:policy/AmazonEC2FullAccess" ]

  assume_role_policy = <<EOF
{
    "Version": "2012-10-17",
    "Statement": [
        {
          "Effect": "Allow",
          "Principal": {
            "AWS": "arn:aws:iam::<trusted-account-id>:root"
          },
          "Condition": {
             "StringEquals": {
                "sts:ExternalId": "<external_id>"
             }
          },
          "Action": [
            "sts:AssumeRole"
          ]
        }
    ]
}
EOF
}

01 Sign in to your Trend Micro Cloud One™ – Conformity account, access Cross-Account IAM Roles conformity rule settings, and copy the ID of the AWS account authorized to assume and use your IAM role(s).

02 Sign in to the AWS Management Console.

03 Navigate to Amazon IAM console at https://console.aws.amazon.com/iam/.

04 In the navigation panel, under Access management, choose Roles.

05 Click on the name of the cross-account IAM role that you want to reconfigure.

06 Select the Trust relationships tab and choose Edit trust relationship to update the trust policy defined for the selected role.

07 On the Edit Trust Relationship page, in the Policy Document box, replace the 12-digit AWS account number included in the "Principal" element value, with the trusted AWS account ID copied at step no. 1. If required, repeat this step for other entities that are authorized to assume the selected role. Click Update Trust Policy to apply the configuration changes.

08 Repeat steps no. 5 – 7 for each cross-account IAM role that you want to reconfigure.

01 Sign in to your Trend Micro Cloud One™ – Conformity account, access Cross-Account IAM Roles conformity rule settings, and copy the ID of the AWS account authorized to assume and use your IAM role(s).

02 Modify the trust relationship policy defined for the IAM role that you want to reconfigure to authorize only trusted AWS accounts to assume the selected IAM role. Replace the 12-digit AWS account number included in the "Principal" element value, with the trusted AWS account ID copied at step no. 1, then save the policy document to a JSON file named trusted-cross-account-access.json, e.g.:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": "sts:AssumeRole",
            "Effect": "Allow",
            "Condition": {
                "StringEquals": {
                    "sts:ExternalId": "<external_id>"
                }
            },
            "Principal": {
                "AWS": "arn:aws:iam::123412341234:root"
            }
        }
    ]
}

03 Run update-assume-role-policy command (OSX/Linux/UNIX) to replace the trust policy defined for the selected Amazon IAM role with the one modified at the previous step, i.e. trusted-cross-account-access.json (the command does not produce an output):

aws iam update-assume-role-policy
  --role-name cc-external-mgmt-role
  --policy-document file://trusted-cross-account-access.json

04 Repeat steps no. 1 – 3 for each cross-account IAM role that you want to reconfigure.