S3 bucket access to any principal

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Metadata

Id: cloudformation-aws-s3-bucket-access-to-any-principal

Provider: AWS

Platform: CloudFormation

Severity: Critical

Category: Access Control

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Description

Allowing any principal in an S3 bucket policy grants public (or universally authorized) access to the bucket. This can lead to data exposure, unintended read/write access, and unauthorized modifications. This rule checks AWS::S3::Bucket resources that have an associated AWS::S3::BucketPolicy. The rule flags bucket policies where Properties.PolicyDocument.Statement includes Effect: "Allow" and a wildcard principal (for example, Principal: "*", or Principal: { AWS: "*" }). Bucket policy statements that explicitly list allowed principals (for example, an AWS account ARN or a specific IAM role) are acceptable. Statements with Principal as * (or equivalent) will be flagged as insecure.

Secure example with an explicit principal:

MyBucket:
  Type: AWS::S3::Bucket
  Properties:
    BucketName: my-bucket

MyBucketPolicy:
  Type: AWS::S3::BucketPolicy
  Properties:
    Bucket: !Ref MyBucket
    PolicyDocument:
      Version: "2012-10-17"
      Statement:
        - Effect: Allow
          Principal:
            AWS: "arn:aws:iam::123456789012:root"
          Action: "s3:GetObject"
          Resource: !Sub "${MyBucket.Arn}/*"

Compliant Code Examples

Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Properties:
      PublicAccessBlockConfiguration:
        BlockPublicAcls: false
        BlockPublicPolicy: false
        IgnorePublicAcls: true
        RestrictPublicBuckets: true
  BucketPolicy:
    Type: AWS::S3::BucketPolicy
    Properties:
      Bucket: !Ref Bucket
      PolicyDocument:
        Statement:
        - Effect: Allow
          Principal:
            AWS:
            - arn:aws:iam::111122223333:user/Alice
            - arn:aws:iam::111122223333:user/foo
          Action: s3:GetObject
          Resource: arn:aws:s3:::DOC-EXAMPLE-BUCKET/*
          Condition:
            StringLike:
              'aws:Referer':
                - 'http://www.example.com/*'
                - 'http://example.net/*'

Non-Compliant Code Examples

Resources:
  Bucket:
    Type: AWS::S3::Bucket
    Properties:
      PublicAccessBlockConfiguration:
        BlockPublicAcls: false
        BlockPublicPolicy: false
        IgnorePublicAcls: true
        RestrictPublicBuckets: false
  BucketPolicy:
    Type: AWS::S3::BucketPolicy
    Properties:
      Bucket: !Ref Bucket
      PolicyDocument:
        Statement:
        - Effect: Allow
          Principal:
            AWS:
            - "*"
          Action: s3:GetObject
          Resource: arn:aws:s3:::DOC-EXAMPLE-BUCKET/*
          Condition:
            StringLike:
              'aws:Referer':
                - 'http://www.example.com/*'
                - 'http://example.net/*'
  Bucket2:
    Type: AWS::S3::Bucket
    Properties:
      PublicAccessBlockConfiguration:
        BlockPublicAcls: false
        BlockPublicPolicy: false
        IgnorePublicAcls: false
        RestrictPublicBuckets: true
  BucketPolicy2:
    Type: AWS::S3::BucketPolicy
    Properties:
      Bucket: !Ref Bucket2
      PolicyDocument:
        Statement:
        - Effect: Allow
          Principal:
            AWS:
            - "*"
          Action: s3:GetObject
          Resource: arn:aws:s3:::DOC-EXAMPLE-BUCKET/*
          Condition:
            StringLike:
              'aws:Referer':
                - 'http://www.example.com/*'
                - 'http://example.net/*'