Remediating S3 Bucket Misconfiguration

When to Use

• When AWS Config or Security Hub reports S3 buckets with public access or missing encryption
• When a security scan reveals S3 bucket policies granting access to Principal "*" (everyone)
• When preparing for a data protection audit requiring evidence of storage security controls
• When responding to a data exposure incident involving publicly accessible S3 objects
• When establishing preventive controls for new S3 bucket creation across an AWS Organization

Do not use for Azure Blob Storage or GCP Cloud Storage misconfigurations, for S3 data classification (see implementing-cloud-dlp-policy), or for S3 access pattern analysis unrelated to security.

Prerequisites

• AWS account with S3 administrative permissions (s3:, s3-outposts:)
• AWS Config enabled to evaluate S3 resource compliance
• AWS CloudTrail logging S3 data events for access auditing
• Macie enabled for sensitive data discovery in S3 buckets

Workflow

Step 1: Identify All Public and Misconfigured Buckets

Use multiple detection methods to identify S3 buckets with public access. Rely on AWS Config rules, S3 Access Analyzer, and Macie rather than manual inspection.

# Enable S3 Access Analyzer for external access detection
aws accessanalyzer create-analyzer \
  --analyzer-name s3-analyzer \
  --type ACCOUNT

# List all S3 buckets with public access indicators
aws s3api list-buckets --query 'Buckets[*].Name' --output text | while read bucket; do
  public_status=$(aws s3api get-public-access-block --bucket "$bucket" 2>/dev/null)
  if [ $? -ne 0 ]; then
    echo "NO PUBLIC ACCESS BLOCK: $bucket"
  fi
done

# Check bucket policies for public access grants
aws s3api list-buckets --query 'Buckets[*].Name' --output text | while read bucket; do
  policy=$(aws s3api get-bucket-policy --bucket "$bucket" 2>/dev/null)
  if echo "$policy" | grep -q '"Principal":"*"' 2>/dev/null; then
    echo "PUBLIC POLICY DETECTED: $bucket"
  fi
done

# Use AWS Config to find non-compliant buckets
aws configservice get-compliance-details-by-config-rule \
  --config-rule-name s3-bucket-public-read-prohibited \
  --compliance-types NON_COMPLIANT \
  --query 'EvaluationResults[*].EvaluationResultIdentifier.EvaluationResultQualifier.ResourceId'

Step 2: Enable S3 Block Public Access at Account Level

Apply the four Block Public Access settings at the AWS account level as a safety net. This prevents any bucket in the account from being made public, regardless of individual bucket policies or ACLs.

# Enable account-level Block Public Access (all four settings)
aws s3control put-public-access-block \
  --account-id 123456789012 \
  --public-access-block-configuration '{
    "BlockPublicAcls": true,
    "IgnorePublicAcls": true,
    "BlockPublicPolicy": true,
    "RestrictPublicBuckets": true
  }'

# Verify account-level settings
aws s3control get-public-access-block --account-id 123456789012

# Enable at bucket level for defense in depth
aws s3api put-public-access-block \
  --bucket production-data-bucket \
  --public-access-block-configuration '{
    "BlockPublicAcls": true,
    "IgnorePublicAcls": true,
    "BlockPublicPolicy": true,
    "RestrictPublicBuckets": true
  }'

Step 3: Audit and Remediate Bucket Policies and ACLs

Review all bucket policies for overly permissive Principal statements and remove legacy ACLs. Enforce bucket ownership controls to disable ACLs entirely.

# Remove a public bucket policy
aws s3api delete-bucket-policy --bucket exposed-bucket

# Replace with a restrictive policy
aws s3api put-bucket-policy --bucket exposed-bucket --policy '{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "DenyUnencryptedTransport",
      "Effect": "Deny",
      "Principal": "*",
      "Action": "s3:*",
      "Resource": [
        "arn:aws:s3:::exposed-bucket",
        "arn:aws:s3:::exposed-bucket/*"
      ],
      "Condition": {
        "Bool": {"aws:SecureTransport": "false"}
      }
    },
    {
      "Sid": "AllowOnlyVPCEndpoint",
      "Effect": "Deny",
      "Principal": "*",
      "Action": "s3:*",
      "Resource": [
        "arn:aws:s3:::exposed-bucket",
        "arn:aws:s3:::exposed-bucket/*"
      ],
      "Condition": {
        "StringNotEquals": {"aws:SourceVpce": "vpce-0abc123def456"}
      }
    }
  ]
}'

# Enforce bucket owner for all objects (disable ACLs)
aws s3api put-bucket-ownership-controls --bucket exposed-bucket \

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