The Principle of Least Privilege (PoLP) is a fundamental security concept that requires systems to operate with the minimum permissions necessary. In serverless architectures like AWS Lambda, implementing least privilege access is critical for reducing attack surfaces and preventing lateral movement during security incidents. This guide provides actionable strategies for implementing PoLP in your serverless applications.

Simple Analogy

Implementing least privilege in serverless is like giving hotel room keys that only work for specific rooms and floors. A guest (Lambda function) with a “gym access” key can’t enter other guests’ rooms (other resources), and the housekeeper (another function) has a master key that only works during cleaning hours (time-bound permissions).

Why Least Privilege Matters in Serverless

Reduced Attack Surface

Each permission granted to a function represents a potential attack vector. By minimizing permissions:

  • Limit damage from compromised functions
  • Prevent lateral movement between resources
  • Contain impact of credential leaks

Compliance Requirements

Least privilege is mandated by security frameworks:

  • GDPR Article 25 (Data Protection by Design)
  • HIPAA Security Rule §164.308(a)(3)(ii)(B)
  • PCI DSS Requirement 7.2.1

Operational Efficiency

Properly scoped permissions:

  • Simplify auditing and compliance reporting
  • Reduce “permission drift” over time
  • Improve deployment reliability

Common Mistakes to Avoid

❌ Over-Permissioned Roles

Using wildcard permissions or administrator access:

// Bad: Wildcard permission
{
  “Effect”: “Allow”,
  “Action”: “s3:*”,
  “Resource”: “*”
}

❌ Reusing IAM Roles

Using the same execution role for multiple functions with different purposes

❌ Ignoring Resource Constraints

Not limiting permissions to specific resources or conditions

Least Privilege Implementation Guide

Step 1: Permission Granularity

Start with the most restrictive permissions and expand as needed:

Function PurposeBad PermissionGood Permission
Read from S3 buckets3:*s3:GetObject
Write to DynamoDB tabledynamodb:*dynamodb:PutItem
Send SNS notificationsns:*sns:Publish

Step 2: Resource Constraints

Limit permissions to specific resources:

// Good: Restricted to specific bucket
{
  “Effect”: “Allow”,
  “Action”: “s3:GetObject”,
  “Resource”: “arn:aws:s3:::my-app-data/*”
}

Step 3: Condition-Based Policies

Add contextual restrictions to permissions:

// Require MFA for sensitive operations
“Condition”: {
  “BoolIfExists”: {“aws:MultiFactorAuthPresent”: true}
}

// Restrict by source IP
“Condition”: {
  “IpAddress”: {“aws:SourceIp”: [“192.0.2.0/24”]}
}

Advanced Techniques

✅ Temporary Credentials

Use AWS STS for short-lived permissions:

  • AssumeRole for cross-account access
  • GetSessionToken for MFA-protected access
  • Maximum session duration: 1 hour

✅ Permission Boundaries

Set maximum permission limits for IAM entities:

// Set permission boundary in AWS SAM
MyLambdaFunction:
  Type: AWS::Serverless::Function
  Properties:
    PermissionsBoundary: arn:aws:iam::123456789012:policy/DevBoundary

✅ Automated Policy Tightening

Tools for continuous permission refinement:

  • AWS IAM Access Analyzer
  • CloudTracker (Open Source)
  • Repokid (Netflix)
  • PMapper (Policy Mapper)

Expand Your Serverless Security Knowledge

Real-World Implementation Example

Scenario: User profile service with DynamoDB and S3

Least Privilege Implementation:

  1. Separate functions for read/write operations
  2. Read function: dynamodb:GetItem on specific table
  3. Write function: dynamodb:PutItem with MFA condition
  4. Avatar upload function: s3:PutObject to specific bucket prefix
  5. All functions: CloudWatch log permissions only

Simple Analogy

Implementing least privilege is like building a secure office building where:

  • Janitors (utility functions) have keys to utility closets but not executive offices
  • Accountants (data functions) can access financial systems but not HR records
  • Security guards (monitoring functions) can view camera feeds but not sensitive documents

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