Securing AWS Lambda Execution Roles

When to Use

• When deploying new Lambda functions and defining their IAM execution roles
• When remediating overly permissive Lambda roles discovered during security audits
• When implementing least-privilege access patterns for serverless architectures
• When building reusable IAM templates for Lambda functions across teams
• When Security Hub or Prowler reports Lambda functions with excessive permissions

Do not use for securing Lambda function invocation (use resource-based policies and API Gateway authorizers), for Lambda code security (use SAST tools), or for Lambda network security (use VPC configuration and security groups).

Prerequisites

• IAM permissions for policy creation, role modification, and Access Analyzer operations
• AWS IAM Access Analyzer enabled in the account
• CloudTrail data events enabled for Lambda to capture actual API usage
• Existing Lambda functions to audit and scope permissions for
• Understanding of each function's required AWS service interactions

Workflow

Step 1: Audit Current Lambda Execution Role Permissions

Enumerate all Lambda functions and their associated IAM roles to identify over-privileged functions.

# List all Lambda functions with their execution roles
aws lambda list-functions \
  --query 'Functions[*].[FunctionName,Role]' --output table

# For each function, analyze attached policies
for func in $(aws lambda list-functions --query 'Functions[*].FunctionName' --output text); do
  role_arn=$(aws lambda get-function-configuration --function-name "$func" --query 'Role' --output text)
  role_name=$(echo "$role_arn" | awk -F'/' '{print $NF}')
  echo "=== $func -> $role_name ==="

  # Check for AWS managed policies (often too broad)
  aws iam list-attached-role-policies --role-name "$role_name" \
    --query 'AttachedPolicies[*].[PolicyName,PolicyArn]' --output table

  # Check inline policies
  for policy in $(aws iam list-role-policies --role-name "$role_name" --query 'PolicyNames' --output text); do
    echo "  Inline: $policy"
    aws iam get-role-policy --role-name "$role_name" --policy-name "$policy" \
      --query 'PolicyDocument' --output json
  done
done

Step 2: Analyze Actual API Usage with CloudTrail

Use CloudTrail and IAM Access Analyzer to determine which API actions the function actually uses.

# Query CloudTrail for actual API calls made by a Lambda execution role
aws cloudtrail lookup-events \
  --lookup-attributes AttributeKey=Username,AttributeValue=LAMBDA_ROLE_NAME \
  --start-time 2026-01-23T00:00:00Z \
  --end-time 2026-02-23T00:00:00Z \
  --query 'Events[*].[EventTime,EventName,EventSource]' \
  --output table | sort -k2 | uniq -f1

# Use IAM Access Analyzer policy generation (based on CloudTrail activity)
aws accessanalyzer start-policy-generation \
  --policy-generation-details '{
    "principalArn": "arn:aws:iam::ACCOUNT:role/lambda-execution-role",
    "cloudTrailDetails": {
      "trailArn": "arn:aws:cloudtrail:us-east-1:ACCOUNT:trail/management-trail",
      "startTime": "2026-01-23T00:00:00Z",
      "endTime": "2026-02-23T00:00:00Z"
    }
  }'

# Check the generated policy
aws accessanalyzer get-generated-policy \
  --job-id JOB_ID \
  --query 'generatedPolicyResult.generatedPolicies[*].policy'

Step 3: Create Least-Privilege Execution Policies

Build scoped IAM policies that grant only the specific actions and resources each function needs.

# Example: Scoped policy for a function that reads from S3 and writes to DynamoDB
cat > lambda-scoped-policy.json << 'EOF'
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "ReadInputBucket",
      "Effect": "Allow",
      "Action": [
        "s3:GetObject",
        "s3:ListBucket"
      ],
      "Resource": [
        "arn:aws:s3:::input-data-bucket",
        "arn:aws:s3:::input-data-bucket/*"
      ]
    },
    {
      "Sid": "WriteDynamoDB",
      "Effect": "Allow",
      "Action": [
        "dynamodb:PutItem",
        "dynamodb:UpdateItem",
        "dynamodb:BatchWriteItem"
      ],
      "Resource": "arn:aws:dynamodb:us-east-1:ACCOUNT:table/results-table"
    },
    {
      "Sid": "CloudWatchLogs",
      "Effect": "Allow",
      "Action": [
        "logs:CreateLogGroup",
        "logs:CreateLogStream",
        "logs:PutLogEvents"
      ],
      "Resource": "arn:aws:logs:us-east-1:ACCOUNT:log-group:/aws/lambda/my-function:*"
    }
  ]
}
EOF

# Create the policy
aws iam create-policy \

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