IAM Trust Policies

Abusable AWS IAM Trust Policies that can lead to compromise or privilege escalation

Abusing Vulnerable Trust Policies

• Poorly written IAM Trust Policies can lead to compromise

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AWS Service Trust Policy

Bad policy

• This policy allows the Lambda service in any AWS account to assume the role. An attacker only needs to know the ARN of the role

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
	  "AWS": "*",
	  "Service": "lambda.amazonaws.com"
	},
      "Action": "sts:AssumeRole"
    }
  ]
}

Better policy

• The role assumption is restricted to a particular lambda function within a particular AWS account but other conditions can be specified too

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Action": "sts:AssumeRole",
      "Condition": {
        "ArnLike": {
          "aws:SourceArn": "arn:aws:lambda:<Region>:<AwsAccountId>:function:<LambdaFunction>"
        }
      }
    }
  ]
}

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Resources

AWS global condition context keys - AWS Identity and Access ManagementAWS Identity and Access Management

IAM and AWS STS condition context keys - AWS Identity and Access ManagementAWS Identity and Access Management

IAM JSON policy elements reference - AWS Identity and Access ManagementAWS Identity and Access Management