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When your AWS Lambda function operates within a Virtual Private Cloud (VPC), it gains access to private resources like databases. This also means you need to secure its network communication.

Network security is critical to prevent unauthorized access and data breaches for your serverless applications.

Security Groups (SGs) act as virtual firewalls for your Lambda's network interfaces within a VPC.

• They control inbound and outbound traffic at the instance (ENI) level.
• SGs are stateful: if you allow outbound traffic, the return inbound traffic is automatically allowed.
• You attach SGs directly to your Lambda function's ENIs when it's configured in a VPC.

When setting up Security Group rules, always follow the principle of least privilege.

• Inbound Rules: Specify which traffic is allowed into your Lambda (e.g., from a database).
• Outbound Rules: Specify which traffic your Lambda is allowed to send out (e.g., to S3 or a database).

Only open ports and allow traffic from trusted sources or to necessary destinations.

Network Access Control Lists (NACLs) provide an additional layer of security at the subnet level within your VPC.

• NACLs are stateless: you must explicitly allow both inbound and outbound return traffic.
• They can include both allow and deny rules, processed in order by rule number.
• NACLs apply to all resources within a subnet, including your Lambda's ENIs.

Both Security Groups and NACLs filter network traffic, but they operate at different levels:

• Security Groups: Instance-level (ENI), stateful, allow rules only.
• NACLs: Subnet-level, stateless, allow & deny rules, processed by rule number.

Typically, you use SGs for granular control over specific resources and NACLs as a broader, coarser security layer for subnets.

When your Lambda function is in a VPC, its traffic to public AWS services (like S3, DynamoDB, CloudWatch) would normally exit the VPC via a NAT Gateway or Internet Gateway.

This can introduce security risks and extra costs. VPC Endpoints allow private, secure communication to these services within your VPC.

Interface Endpoints (powered by AWS PrivateLink) provide a private connection to services using an Elastic Network Interface (ENI) in your VPC.

• They allow your Lambda to access services like S3, DynamoDB, and CloudWatch Logs without traversing the public internet.
• Traffic stays entirely within the AWS network, enhancing security and potentially reducing latency.

VPC Endpoints themselves can be secured:

• Endpoint Policies: You can attach IAM resource policies directly to an endpoint to control which principals can use it and what actions they can perform.
• Security Groups: For Interface Endpoints, you can associate a Security Group with the endpoint's ENIs. This SG controls traffic to and from the endpoint.

This ensures only authorized Lambda functions or resources can use the private connection.

Imagine a Lambda function in a VPC that needs to upload files to an S3 bucket.

To do this securely and privately, you would:

• Configure the Lambda in a private subnet with specific Security Group rules.
• Set up an S3 Interface Endpoint in your VPC.
• Apply an Endpoint Policy to the S3 Endpoint and attach a Security Group to its ENIs.

This ensures S3 traffic never leaves the AWS private network.

This Python Lambda function uploads a simple text file to an S3 bucket. When deployed within a VPC, its network access to S3 would be governed by the Security Groups, Network ACLs, and VPC S3 Endpoint configured in your VPC.

Which of the following statements about network security for Lambda functions in a VPC are TRUE?

You've learned how to secure your Lambda functions within a VPC!

• Security Groups offer stateful, instance-level traffic control.
• Network ACLs provide stateless, subnet-level filtering.
• VPC Endpoints enable private access to other AWS services like S3 and DynamoDB.

Always apply the principle of least privilege to minimize your attack surface and keep your serverless applications secure.