Part 2 -Securing AWS Lambda Using Resource-Based Policies and Least Privilege Principle

Introduction

AWS Lambda functions allow developers to execute code without provisioning or managing servers. Like other AWS services, Lambda functions have their own resource-based permissions, which can be defined using resource-based policies. Ensuring that these functions have the minimum necessary permissions is a critical part of maintaining security in your AWS environment. This is where the principle of least privilege comes into play.

The principle of least privilege (PoLP) is a computer security concept in which a user is given the minimum levels of access necessary to complete his/her job functions. This principle is applied to ensure that users do not have excess permissions that could leave the system vulnerable to exploits.

In the context of AWS Lambda, the principle of least privilege means giving a function only the permissions it needs to perform its intended tasks, no more, no less.

What are Resource-Based Policies?

In AWS, a resource-based policy is a policy that you attach to a resource, such as an AWS Lambda function, instead of a principal, like a user, group, or role. In this policy, you specify the principals that are allowed or denied access to the resource.

A resource-based policy is useful when you want to allow users from other AWS accounts to access your resources, or when you want to grant cross-account access without needing to manage individual IAM users or roles in the other accounts.

Writing a Resource-Based Policy for Lambda

Here's a sample resource-based policy for a Lambda function:

{
  "Version": "2012-10-17",
  "Id": "default",
  "Statement": [
    {
      "Sid": "AllowExecutionFromIAMUser",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::123456789012:user/Alice"
      },
      "Action": "lambda:InvokeFunction",
      "Resource": "arn:aws:lambda:us-west-2:123456789012:function:my-function"
    }
  ]
}

In this example, the policy allows the IAM user "Alice" in account 123456789012 to invoke the Lambda function named "my-function". The policy is attached directly to the Lambda function. The Action element specifies the action that is allowed (in this case, invoking the function), and the Resource element specifies the function to which the policy is attached.

Ensuring Least Privilege

To follow the principle of least privilege, you should ensure that your Lambda functions have only the permissions necessary to perform their tasks. For example, if a function only needs to read items from a specific DynamoDB table, do not grant it permissions to write to the table or access other tables. Similarly, if a function only needs to be invoked by a specific user or service, do not allow it to be invoked by others.

When creating a resource-based policy for a Lambda function, ensure that the Principal element includes only the users or services that need to invoke the function. Do not include principals that do not need to invoke the function.

In the policy example above, if user "Alice" is the only user who needs to invoke the function, then the policy follows the principle of least privilege. However, if there are other users who also need to invoke the function, you would need to add them to the Principal element, like so:

{
  "Principal": {
    "AWS": [
      "arn:aws:iam::123456789012:user/Alice",
      "arn:aws:iam::123456789012:user/Bob"
    ]
  },
  ...
}

Remember, always review your resource-based policies regularly and remove any unnecessary permissions or principals to maintain a secure environment.

Conclusion

Securing your AWS Lambda functions using resource-based policies and the principle of least privilege is a key part of AWS security best practices. By carefully managing who can invoke your Lambda functions and what actions they can take, you can reduce the risk of unauthorized access or inadvertent modifications, ensuring the integrity of your serverless applications.