Cross-account IAM assume role policy without external id or MFA

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Metadata

Id: 09c35abf-5852-4622-ac7a-b987b331232e

Cloud Provider: aws

Framework: Terraform

Severity: High

Category: Access Control

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Description

When creating cross-account IAM roles, it’s crucial to implement additional security measures like External ID or Multi-Factor Authentication (MFA) to prevent unauthorized cross-account access. Without these safeguards, your resources become vulnerable to confused deputy attacks, where a malicious third party could trick your role into performing actions they shouldn’t be authorized to do. To secure your configuration, add a Condition block to your assume role policy that requires either an ExternalId,as shown in the example below, or MFA validation:

"Condition": {
  "StringEquals": {
    "sts:ExternalId": "98765"
  }
}

Compliant Code Examples

resource "aws_iam_role" "negative2" {
  name = "test_role"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": "sts:AssumeRole",
      "Principal": {
        "AWS": "arn:aws:iam::987654321145:root"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": "",
      "Condition": { 
         "Bool": { 
            "aws:MultiFactorAuthPresent": "true" 
          }
      }
    }
  ]
}
EOF

  tags = {
    tag-key = "tag-value"
  }
}

resource "aws_iam_role" "negative1" {
  name = "test_role"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": "sts:AssumeRole",
      "Principal": {
        "AWS": "arn:aws:iam::987654321145:root"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": "",
      "Condition": {
        "StringEquals": {
          "sts:ExternalId": "98765"
        }
      }
    }
  ]
}
EOF

  tags = {
    tag-key = "tag-value"
  }
}

Non-Compliant Code Examples

resource "aws_iam_role" "positive2" {
  name = "test_role"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": {
      "Action": "sts:AssumeRole",
      "Principal": {
        "AWS": "arn:aws:iam::987654321145:root"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": "",
      "Condition": { 
         "Bool": { 
            "aws:MultiFactorAuthPresent": "false" 
          }
      }
  }
}
EOF

  tags = {
    tag-key = "tag-value"
  }
}

resource "aws_iam_role" "positive3" {
  name = "test_role"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": {
      "Action": "sts:AssumeRole",
      "Principal": {
        "AWS": "arn:aws:iam::987654321145:root"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": "",
      "Condition": {
        "StringEquals": {
          "sts:ExternalId": ""
        }
      }
  }
}
EOF

  tags = {
    tag-key = "tag-value"
  }
}

resource "aws_iam_role" "positive1" {
  name = "test_role"

  assume_role_policy = <<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": "sts:AssumeRole",
      "Principal": {
        "AWS": "arn:aws:iam::987654321145:root"
      },
      "Effect": "Allow",
      "Resource": "*",
      "Sid": ""
    }
  ]
}
EOF

  tags = {
    tag-key = "tag-value"
  }
}