Understanding IAM Policy Evaluation Logic


Welcome back! In this chapter, we’re diving into the intricacies of IAM (Identity and Access Management) policy evaluation logic in AWS. This is a crucial concept to grasp for effective access control and security management within your AWS environment. Let’s delve into the details of policy evaluation and understand how AWS determines whether to allow or deny actions on resources.

Policy Evaluation Logic: A Closer Look

At its core, IAM policy evaluation logic determines whether an action on a resource should be allowed or denied. The decision-making process involves a series of steps and considerations:

Implicit Deny: The Default State

Every IAM identity starts with an implicit deny action. In other words, by default, an identity does not have access to any resource within your AWS environment. This default setting ensures that you have complete control over granting permissions.

Evaluation Workflow

When determining final permissions for an identity, AWS evaluates various types of policies in a specific order. Here are the types of policies AWS considers:

  1. Organization Service Control Policies
  2. Resource-Based Policies
  3. IAM Permission Boundaries
  4. Session Policies
  5. Identity-Based Policies

Allow and Deny Decisions

During evaluation, AWS goes through a process of deciding whether to allow or deny an action on a resource. Let’s outline the key steps:

  1. Implicit Deny: As mentioned earlier, the evaluation starts with an implicit deny. If no permissions are granted, this deny action remains in effect.
  2. Allow and Deny in Policies: AWS checks if there are any explicit “allow” or “deny” permissions in the policies. If an explicit “deny” permission is found, it takes precedence and results in an immediate “deny” action, regardless of other permissions.
  3. Multiple Policies: If there are multiple policies of different types, AWS evaluates each policy sequentially. An “allow” permission in one policy can counteract a “deny” permission in another. However, explicit “deny” always takes precedence over “allow.”
  4. Final Decision: After evaluating all applicable policies, AWS makes the final decision. If there is an explicit “allow” permission and no explicit “deny” permission, the action is allowed. On the other hand, if there is an explicit “deny” permission, the action is denied.

Example: Working with IAM Policy Evaluation Logic

Let’s consider an example to illustrate IAM policy evaluation logic. Imagine we create a policy that allows access to all resources within Amazon S3 but explicitly denies access to a specific S3 bucket.

  1. We create a policy that initially allows all S3 actions for all resources.
  2. We then add another statement to explicitly deny the action of reading an object from a particular S3 bucket.
  3. The evaluation logic follows these steps:
    • Since there’s an implicit deny by default, any action not explicitly allowed will be denied.
    • The first statement allows all S3 actions, but the second statement explicitly denies the “read object” action for the specified bucket.
  4. When we test the policy, we find that access to objects within the specified bucket is denied due to the explicit deny statement.

Using the Policy Simulator

AWS provides a valuable tool known as the Policy Simulator. This simulator allows you to test the effect of policies on specific identities and actions without actually making changes. You can input the user, service, and action you want to simulate, and the simulator will provide you with the expected result, either “allowed” or “denied.”

As a veteran user of this tool, I can attest to its indispensability in navigating IAM challenges, especially when grappling with those confounding “access denied” obstacles. The brilliance lies in its departure from the clean slate approach. On the left, you’re greeted with a familiar scene of users, groups, and roles. This is your playground, enabling you to tinker with IAM entities you’ve meticulously crafted. Meanwhile, on the right—the Policy Simulator section—you’ll find the real action. This is where you interact with a spectrum of AWS services, simulating various API actions.

For instance, imagine you need to test a user’s AWS Amplify permission. With a few clicks, you’re there: selecting the AWS Amplify service, pinpointing actions, and initiating a simulation. It’s a hands-on demonstration of the tool’s prowess. Take a tour with a user named Daniel. A glance at his IAM policies reveals the intricacies. Delve into policies like “Amazon S3 Full Access” or the “Student Policy,” each with their nuances of allow and deny.

One of the simulator’s treasures lies in clarity. Clicking on a denial result reveals the precise statement behind it, cutting through ambiguity in troubleshooting. The simulator is relentless in tracking even implicit denials. It knows that if you’re not explicitly permitted to act, you’re implicitly denied. It’s a lifeline for understanding IAM’s intricacies.

Let’s dive into the hands-on aspect. Suppose you’re curious about Daniel’s capabilities. Pick “Create Bucket” as an action and initiate the simulation. Voila! The simulator swiftly reports a denial, directly attributed to the “Student Policy.” This is insight made actionable.

What about those permissions you expect to work? “Delete Object” and “Get Object” for instance. Given the comprehensive “S3 Full Access,” you anticipate success. Running the simulation validates your expectations, showcasing “allowed” outcomes.

But what about uncharted territory? Imagine querying DynamoDB without explicit permissions. The simulator detects the implicit denial and clarifies IAM’s unyielding principle: no explicit permission equals automatic denial.

Did you know you can even craft your own temporary policies for testing? The simulator’s sandbox grants you a safe haven to experiment, all without impacting your existing setup.


Understanding IAM policy evaluation logic is fundamental to managing access and security within your AWS environment. By following the evaluation workflow and grasping the concept of implicit deny, allow, and deny actions in policies, you can confidently manage and implement effective access controls. The Policy Simulator serves as a handy tool to predict how your policies will impact users and resources.

With this chapter, you’ve gained insights into a critical aspect of AWS security. As you continue your journey in AWS, remember that mastering policy evaluation logic is key to ensuring the right level of access for your users and resources.

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