S3 bucket access to any principal

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Metadata

Id: 7af43613-6bb9-4a0e-8c4d-1314b799425e

Cloud Provider: AWS

Platform: Terraform

Severity: Critical

Category: Access Control

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Description

When an S3 bucket policy allows access to all AWS principals (*), it creates a significant security vulnerability by potentially exposing sensitive data to anyone on the internet. Malicious actors could access, modify, or delete your data, leading to data breaches, regulatory violations, and reputational damage. To secure your S3 bucket, avoid using * in the Principal field with an Allow effect. Instead, explicitly specify authorized principals or use a Deny effect, as shown below:

"Statement": [
  {
    "Sid": "IPAllow",
    "Effect": "Deny",
    "Action": "s3:*",
    "Resource": "arn:aws:s3:::my_tf_test_bucket/*",
    "Condition": {
       "IpAddress": {"aws:SourceIp": "8.8.8.8/32"}
    }
  }
]

Compliant Code Examples

module "s3_bucket" {
  source = "terraform-aws-modules/s3-bucket/aws"
  version = "3.7.0"

  bucket = "my-s3-bucket"
  acl    = "private"

  versioning = {
    enabled = true
  }

  policy = <<POLICY
{
  "Version": "2012-10-17",
  "Id": "MYBUCKETPOLICY",
  "Statement": [
    {
      "Sid": "IPAllow",
      "Effect": "Deny",
      "Action": "s3:*",
      "Resource": "arn:aws:s3:::my_tf_test_bucket/*",
      "Condition": {
         "IpAddress": {"aws:SourceIp": "8.8.8.8/32"}
      }
    }
  ]
}
POLICY
}

resource "aws_s3_bucket_policy" "negative1" {
  bucket = aws_s3_bucket.b.id

  policy = <<POLICY
{
  "Version": "2012-10-17",
  "Id": "MYBUCKETPOLICY",
  "Statement": [
    {
      "Sid": "IPAllow",
      "Effect": "Deny",
      "Action": "s3:*",
      "Resource": "arn:aws:s3:::my_tf_test_bucket/*",
      "Condition": {
         "IpAddress": {"aws:SourceIp": "8.8.8.8/32"}
      }
    }
  ]
}
POLICY
}

Non-Compliant Code Examples

module "s3_bucket" {
  source = "terraform-aws-modules/s3-bucket/aws"
  version = "3.7.0"

  bucket = "my-s3-bucket"
  acl    = "private"

  versioning = {
    enabled = true
  }

  policy = <<POLICY
{
  "Version": "2012-10-17",
  "Id": "MYBUCKETPOLICY",
  "Statement": [
    {
      "Sid": "IPAllow",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      },
      "Action": "s3:*",
      "Resource": "arn:aws:s3:::my_tf_test_bucket/*",
      "Condition": {
         "IpAddress": {"aws:SourceIp": "8.8.8.8/32"}
      }
    }
  ]
}
POLICY
}

resource "aws_s3_bucket" "this" {
  bucket = "my_tf_test_bucket"
  tags = {
    Name = "My bucket"
  }
}

resource "aws_s3_bucket_policy" "this" {
  bucket = aws_s3_bucket.this.id
  policy = jsonencode({
    Version = "2012-10-17"
    Statement = [{
      # When used directly as a Cloudfront origin.
      Effect = "Allow"
      Action = "s3:GetObject"
      Principal = {
        Service = "cloudfront.amazonaws.com"
      }
      Resource = [
        "${aws_s3_bucket.this.arn}/*",
      ]
      Condition = {
        StringEquals = {
          "AWS:SourceArn" = aws_cloudfront_distribution.this.arn
        }
      }
      },
      {
        # Admin access for policy updates, etc.
        Effect = "Allow"
        Action = "s3:*"
        Principal = {
          AWS = [
            data.aws_caller_identity.current.id,
          ]
        }
        Resource = [
          "${aws_s3_bucket.this.arn}/*",
        ]
      },
      {
        # Delegate access to the access point.
        Effect = "Allow"
        Action = "*"
        Principal = {
          AWS = [
            "*"
          ]
        }
        Resource = [
          aws_s3_bucket.this.arn,
          "${aws_s3_bucket.this.arn}/*",
        ]
        Condition = {
          "StringEquals" = {
            "s3:DataAccessPointAccount" = data.aws_caller_identity.current.account_id
          }
        }
    }]
  })
}

resource "aws_s3_bucket_policy" "positive1" {
  bucket = aws_s3_bucket.b.id

  policy = <<POLICY
{
  "Version": "2012-10-17",
  "Id": "MYBUCKETPOLICY",
  "Statement": [
    {
      "Sid": "IPAllow",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      },
      "Action": "s3:*",
      "Resource": "arn:aws:s3:::my_tf_test_bucket/*",
      "Condition": {
         "IpAddress": {"aws:SourceIp": "8.8.8.8/32"}
      }
    }
  ]
}
POLICY
}