Lambda execution roles
A key use of IAM roles in serverless applications is Lambda function execution roles. An execution role is attached to a Lambda function and grants the function the permissions necessary to execute correctly, including access to any other AWS resources that are required. For example, if the Lambda function uses the AWS SDK to make a DynamoDB request that inserts a record in a table, the execution role must include a policy with the dynamodb:PutItem action for the table resource.
The execution role is assumed by the Lambda service when performing control plane and data plane operations. The AWS Security Token Service (STS) is used to fetch short-lived, temporary security credentials which are made available via the function’s environment variables during invocation.
Each function in your application should have its own unique execution role with the minimum permissions required to perform its duty. In this way, single-purpose functions (introduced in Chapter 6) are also key to security: IAM permissions can be tightly scoped to the function and remain extremely restricted according to the limited functionality.
IAM guardrails
As you are no doubt beginning to notice, effective serverless security in the cloud is about basic security hygiene. Establishing guardrails for the use of AWS IAM is a core part of promoting a secure approach to everyday engineering activity. Here are some recommended guardrails:
Apply the principle of least privilege in policies.
IAM policies should only include the minimum set of permissions required for the associated resource to perform the necessary control or data plane operations. As a general rule, do not use wildcards (*) in your policy statements. Wildcards are the antithesis of least privilege, as they apply blanket permissions for actions and resources. Unless the action explicitly requires a wildcard, always be specific.
Avoid using managed IAM policies.
These are policies provided by AWS, and they’re often tempting shortcuts, especially when you’re just getting started or using a service for the first time. You can use these policies early in prototyping or development, but you should replace them with custom policies as soon as you understand the integration better. Because these policies are designed to be applied to generic scenarios, they are simply not restricted enough and will usually violate the principle of least privilege when applied to interactions within your application.
Prefer roles to users.
IAM users are issued with static, long-lived AWS access credentials (an access key ID and secret access key). These credentials can be used to directly access the application provider’s AWS account, including all the resources and data in that account. Depending on the associated IAM roles and policies, the authenticating user may even have the ability to create or destroy resources. Given the power they grant the holder, the use and distribution of static credentials must be limited to reduce the risk of unauthorized access. Where possible, restrict IAM users to an absolute minimum (or, even better, do not have any IAM users at all).
Prefer a role per resource.
Each resource in your application, such as an EventBridge rule, a Lambda function, and an SQS queue, should have its own unique role. Permissions for those roles should be fine-grained and least-privileged.
