Welcome back to our AWS security journey! In the first two parts, we laid the foundation by exploring AWS security principles and the essentials of authorizing users with IAM. Now, it’s time to dive deeper into the world of IAM policies and permissions. This is where you really start to fine-tune who gets access to what, and how they can use it.
Understanding IAM Policies and Permissions
In AWS, policies are the key to controlling access. They’re like the rules of the road, dictating who can do what with your resources. These policies are typically stored as JSON documents and define what actions are allowed or denied. Let’s break down the two main types of IAM policies:
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Identity-Based Policies: These are attached directly to an IAM user, group, or role. They determine what actions that identity can perform on AWS resources. Think of this as giving someone a set of keys that open certain doors but not others.
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Resource-Based Policies: These are attached to the AWS resource itself (like an S3 bucket). They specify which identities (users, groups, or roles) are allowed to perform actions on that resource. Imagine putting a lock on a door that only certain keys can open, regardless of who holds them.
Fine-Tuning Permissions
When you use policies, you’re fine-tuning the permissions granted to various principals (like IAM users or roles). By following the principle of least privilege—which we’ve talked about before—you make sure each user has just enough access to do their job and nothing more.
How AWS Evaluates Permissions
Now, let’s talk about how AWS actually determines whether to allow or deny a request. It’s a straightforward process, but it’s crucial to understand:
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Implicit Deny: By default, all requests are denied. This is AWS's way of saying, "You can't do anything unless I explicitly say you can."
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Explicit Allow: If a policy explicitly allows an action, this allow overrides the default deny.
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Explicit Deny: If a policy explicitly denies an action, this deny overrides any explicit allows.
So, if a conflict arises—where one policy allows an action and another denies it—AWS will always go with the most restrictive option, which is to deny the action. This keeps your resources secure by preventing unintended access.
Identity-Based vs. Resource-Based Policies: Examples
To really get this, let’s walk through a couple of examples:
Example 1: Identity-Based Policy vs. Resource-Based Policy
Imagine Bob has an identity-based policy attached to his IAM user that allows him to perform GET, PUT, and LIST actions on S3 bucket X. However, bucket X also has a resource-based policy that allows GET and LIST but explicitly denies PUT.
So, can Bob PUT objects into bucket X? Nope. The resource-based policy’s explicit deny on PUT overrides the identity-based policy’s allow. Bob can only GET and LIST objects in bucket X.
Example 2: Evaluating Mixed Permissions
Let’s take a look at bucket Y. Bob’s identity-based policy allows him to LIST objects in bucket Y but doesn’t mention GET or PUT. Meanwhile, the resource-based policy for bucket Y allows GET and LIST but doesn’t specify PUT.
Can Bob GET objects from bucket Y? Yes, he can. The resource-based policy explicitly allows GET and LIST, so Bob can perform these actions even though his identity-based policy doesn’t explicitly allow them. But since neither policy mentions PUT, Bob can’t PUT objects into bucket Y.
Key Takeaways: Fine-Tuning Access with Policies
Here’s what you should remember about authorizing users with IAM policies:
- Policies Define Permissions: Whether attached to a user or a resource, policies define what actions are allowed or denied.
- Two Types of Policies: Identity-based policies are attached to IAM principals (users, groups, or roles), while resource-based policies are attached directly to AWS resources.
- Evaluation Logic: AWS denies all requests by default. An explicit allow can override this, but an explicit deny always trumps an allow.
Wrapping It Up
Mastering IAM policies and permissions is like having a finely tuned security system for your AWS environment. By understanding how identity-based and resource-based policies work, and how AWS evaluates them, you’re well on your way to securing your cloud like a pro.
Stay tuned for Part 4, where we’ll explore more advanced IAM practices, including policy simulation and troubleshooting! Until then, keep those policies tight, and your AWS environment secure.