AWS IAM Best Practices

AWS IAM Best Practices

Review and harden IAM policies following AWS security best practices and least privilege principles.

When to Use

Use this skill when you need to review IAM policies, implement least privilege access, or harden IAM security.

Core Principles

Least Privilege

• Grant minimum permissions needed
• Use managed policies when possible
• Avoid wildcard (*) permissions
• Regular access reviews

Defense in Depth

• Enable MFA for all users
• Use IAM roles instead of access keys
• Implement service control policies (SCPs)
• Enable CloudTrail for audit

Separation of Duties

• Separate admin and user roles
• Use different roles for different environments
• Implement approval workflows
• Regular permission audits

IAM Security Checks

Find Overly Permissive Policies

# List policies with full admin access
aws iam list-policies --scope Local \
  --query 'Policies[*].[PolicyName,Arn]' --output table | \
  grep -i admin

# Find policies with wildcard actions
aws iam list-policies --scope Local --query 'Policies[*].Arn' --output text | \
while read arn; do
  version=$(aws iam get-policy --policy-arn "$arn" \
    --query 'Policy.DefaultVersionId' --output text)
  doc=$(aws iam get-policy-version --policy-arn "$arn" \
    --version-id "$version" --query 'PolicyVersion.Document')
  if echo "$doc" | grep -q '"Action": "\*"'; then
    echo "Wildcard action in: $arn"
  fi
done

# Find inline policies (should use managed policies)
aws iam list-users --query 'Users[*].UserName' --output text | \
while read user; do
  policies=$(aws iam list-user-policies --user-name "$user" \
    --query 'PolicyNames' --output text)
  if [ -n "$policies" ]; then
    echo "Inline policies on user $user: $policies"
  fi
done

MFA Enforcement

# List users without MFA
aws iam get-credential-report --output text | \
  awk -F, 'NR>1 && $4=="false" {print $1}'

# Check if MFA is required in policies
aws iam list-policies --scope Local --query 'Policies[*].Arn' --output text | \
while read arn; do
  version=$(aws iam get-policy --policy-arn "$arn" \
    --query 'Policy.DefaultVersionId' --output text)
  doc=$(aws iam get-policy-version --policy-arn "$arn" \
    --version-id "$version" --query 'PolicyVersion.Document')
  if echo "$doc" | grep -q "aws:MultiFactorAuthPresent"; then
    echo "MFA enforced in: $arn"
  fi
done

# Enable MFA for a user (returns QR code)
aws iam create-virtual-mfa-device \
  --virtual-mfa-device-name user-mfa \
  --outfile /​tmp/​qr.png \
  --bootstrap-method QRCodePNG

Access Key Management

# Find old access keys (>90 days)
aws iam list-users --query 'Users[*].UserName' --output text | \
while read user; do
  aws iam list-access-keys --user-name "$user" \
    --query 'AccessKeyMetadata[*].[AccessKeyId,CreateDate,Status]' \
    --output text | \
  while read key_id create_date status; do
    age_days=$(( ($(date +%s) - $(date -d "$create_date" +%s)) / 86400 ))
    if [ $age_days -gt 90 ]; then
      echo "$user: Key $key_id is $age_days days old"
    fi
  done
done

# Rotate access key
OLD_KEY="AKIAIOSFODNN7EXAMPLE"
USER="myuser"

# Create new key
NEW_KEY=$(aws iam create-access-key --user-name "$USER")
echo "New key created. Update applications, then run:"
echo "aws iam delete-access-key --user-name $USER --access-key-id $OLD_KEY"

# Deactivate old key (test first)
aws iam update-access-key \
  --user-name "$USER" \
  --access-key-id "$OLD_KEY" \
  --status Inactive

Role and Policy Analysis

# List unused roles (no activity in 90 days)
aws iam list-roles --query 'Roles[*].[RoleName,RoleLastUsed.LastUsedDate]' \
  --output text | \
while read role last_used; do
  if [ "$last_used" = "None" ]; then
    echo "Never used: $role"
  fi
done

# Find roles with trust relationships to external accounts
aws iam list-roles --query 'Roles[*].RoleName' --output text | \
while read role; do
  trust=$(aws iam get-role --role-name "$role" \
    --query 'Role.AssumeRolePolicyDocument')
  if echo "$trust" | grep -q '"AWS":'; then
    echo "External trust: $role"
  fi
done

# Analyze policy permissions
aws iam simulate-principal-policy \
  --policy-source-arn arn:aws:iam::123456789012:user/​myuser \
  --action-names s3:GetObject s3:PutObject \
  --resource-arns arn:aws:s3:::mybucket/​*

IAM Policy Templates

Least Privilege S3 Access

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetObject",
        "s3:PutObject"
      ],
      "Resource": "arn:aws:s3:::my-bucket/​user-data/​${aws:username}/​*"
    },
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::my-bucket",
      "Condition": {
        "StringLike": {
          "s3:prefix": "user-data/​${aws:username}/​*"
        }
      }
    }
  ]
}

MFA-Required Policy

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Deny",
      "Action": "*",