Cis AWS Foundations 2.21

Ensure AWS resource policies do not allow unrestricted access using "Principal": "*"

Description

Ensure AWS resource-based policies, such as Amazon S3 bucket policies, Amazon SQS queue policies, Amazon SNS topic policies, and AWS Lambda resource policies, do not grant unrestricted access using "Principal": "*" with "Effect": "Allow" unless the policy includes restrictive conditions that limit access to specific trusted identities, accounts, services, or network boundaries.

Rationale

Resource-based policies are evaluated alongside identity-based IAM policies during authorization decisions. When a policy statement specifies "Principal": "*" with "Effect": "Allow", it grants the specified permissions to any AWS principal unless additional conditions restrict the request. This may unintentionally allow access from users, roles, or services in any AWS account. Such broad access significantly increases the risk of unauthorized data access, resource abuse, or data exfiltration.

Impact

Unrestricted resource-based policies may expose data or services to unauthorized access, potentially leading to data breaches, service misuse, or unintended public exposure.

Audit Procedure

1. Identify resources that support resource-based policies within the AWS account, such as S3 buckets, SQS queues, SNS topics, and Lambda functions
2. Retrieve the resource policies for each resource. Example CLI commands:

SQS Queue Policies

aws sqs get-queue-attributes \
  --queue-url https://sqs.region.amazonaws.com/account/QUEUE \
  --attribute-names Policy

S3 Bucket Policies

aws s3api get-bucket-policy \
  --bucket YOUR-BUCKET-NAME

SNS Topic Policies

aws sns get-topic-attributes \
  --topic-arn TOPIC-ARN \
  --query "Attributes.Policy" \
  --output text

3. Inspect the retrieved policies and identify statements containing:

• "Effect": "Allow" AND "Principal": "*"

  OR

• "Principal": {"AWS": "*"}

4. Evaluate whether the statement includes restrictive conditions such as:

• aws:SourceArn
• aws:SourceAccount
• aws:PrincipalArn
• Other service-specific condition keys

5. Determine audit status:

• Compliant: Wildcard principals are present only when restrictive conditions limit access to trusted principals or services
• Non-Compliant: Wildcard principals are used without sufficient restrictions

Expected Result

All resource-based policies that use "Principal": "*" should include restrictive conditions (such as aws:SourceArn, aws:SourceAccount, or aws:PrincipalArn) that limit access to specific trusted entities. Policies without such conditions are non-compliant.

Remediation

If a resource policy contains "Principal": "*" with "Effect": "Allow" and lacks sufficient restrictions, modify the policy to limit access.

OPTION 1 - Restrict the Principal

Replace the wildcard principal ("Principal": "*") with a specific account, role, user, or service.

Example: Non-Compliant Policy

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowPublicAccess",
      "Effect": "Allow",
      "Principal": "*",
      "Action": "sqs:SendMessage",
      "Resource": "arn:aws:sqs:us-east-1:123456789012:my-queue"
    }
  ]
}

Steps:

1. Retrieve the current policy:

aws sqs get-queue-attributes \
  --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/my-queue \
  --attribute-names Policy \
  --query 'Attributes.Policy'

2. Update the policy with a specific principal:

aws sqs set-queue-attributes \
  --queue-url https://sqs.us-east-1.amazonaws.com/123456789012/my-queue \
  --attributes '{
    "Policy": "{\"Version\":\"2012-10-17\",\"Statement\":[{\"Sid\":\"AllowSpecificAccount\",\"Effect\":\"Allow\",\"Principal\":{\"AWS\":\"arn:aws:iam::345678901234:root\"},\"Action\":\"sqs:SendMessage\",\"Resource\":\"arn:aws:sqs:us-east-1:123456789012:my-queue\"}]}"
  }'

Resulting Compliant Policy:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowSpecificAccount",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::345678901234:root"
      },
      "Action": "sqs:SendMessage",
      "Resource": "arn:aws:sqs:us-east-1:123456789012:my-queue"
    }
  ]
}

OPTION 2 - Restrict Using Conditions

If a wildcard principal is required, add restrictive conditions.

Example compliant policy:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowServiceIntegration",
      "Effect": "Allow",
      "Principal": "*",
      "Action": "sqs:SendMessage",
      "Resource": "arn:aws:sqs:us-east-1:123456789012:my-queue",
      "Condition": {
        "StringEquals": {
          "aws:SourceAccount": "345678901234"
        }
      }
    }
  ]
}

Default Value

By default, AWS does not prevent the use of "Principal": "*" in resource-based policies. Policies may allow unrestricted access unless explicitly restricted through policy definitions or organizational controls. It is the responsibility of the customer to ensure that resource policies are properly scoped and do not grant unintended public or cross-account access.

References

• AWS IAM Policy Evaluation Logic
• AWS S3 Bucket Policy Documentation
• AWS SQS Queue Policy Documentation
• AWS SNS Topic Policy Documentation

CIS Controls

Profile

Level 1 | Manual