Least Privilege Principle Applied to AWS

The principle of least privilege states that a resource should have permission to access or perform actions on other resources only when it is strictly necessary to complete its tasks. Implementing and adhering to this principle reduces the risk of exposure in case of credential leaks, as the attack surface is significantly minimized. It also limits the potential impact of malfunctions or vulnerabilities associated with that resource.

However, as systems grow in complexity, it becomes increasingly difficult to determine the exact permissions required and when they are needed. The broader the system’s scope and the greater the number of possible scenarios, the harder it is to define precise access boundaries. Besides, software engineers may accidentaly provide a broader access than it's needed due to a bad delimitation of scope in system components, use of overpriveliged users in production environment and anti-patterns.

Where the problem begins?

During the development of new services, developers often need to use databases, message brokers, and other components to perform various operations. In the early stages of development, everything typically runs in a controlled and predictable environment, and it's not exposed to the public. Therefore, using superusers (accounts created by default when installing these services) to access tables or publish messages is usually not a concern.

However, once the MVP is ready to be released and real users start accessing the platform, continuing to use those same superusers becomes a serious security risk. Hackers can exploit vulnerabilities to perform actions on behalf of the superuser and effectively gaining unrestricted control.

Even if credentials remain secure and attackers cannot access them directly, they might not need to. By injecting malicious commands (through SQL injection, cross-site scripting, and similar attacks), they can make the system executes dangerous operations as the superuser, causing potentially catastrophic damage.

How AWS manages this problem?

When an AWS account is first created, it automatically includes a special account known as the root user. This account functions as a superuser with full, unrestricted access to all resources in the account. It can be thought of as the kernel of an operating system capable of executing any action without limitation.

The root user cannot be deleted, but its use should be strictly minimized. AWS recommends securing it by applying the principle of least privilege and following several best practices:

• Use a strong password.
• Enable multi-factor authentication.
• Remove access keys from the root user to prevent programmatic access.
• Use the root user account only when performing tasks that require root user access. List of tasks that require root user access.

In practice, there are several ways to secure the root user. If you’re experimenting or learning in a controlled environment, keeping these four principles in mind is usually enough. However, in a company or production context, I strongly recommend reading the official AWS guide: Root user best practices for your AWS account.

The final point above introduces the need to create and manage individual users with restricted permissions to perform specific tasks. This is where AWS Identity and Access Management (IAM) comes into play.

AWS Identity and Access Management

AWS Identity and Access Management (IAM) is the service responsible for managing authentication and authorization of users within AWS. It allows you to create users with specific permissions, each using different methods of authentication.

For example, suppose that Application A needs to access Amazon S3 to read files. You can create a user with read-only permissions for S3 and generate access keys to enable programmatic access through the AWS SDK. Additionally, if a developer needs to access S3 via the AWS Management Console, you can create a username and password for that same user to sign in through the console.

To determine whether an action is allowed or denied, AWS uses policies. Policies are JSON documents that can be attached to IAM identities and define which actions are permitted on specific resources. When an identity attempts to access a resource, AWS evaluates these policies in real time. If no policy explicitly allows the requested action, the request is denied by default.

There are several types of policies, but let’s focus on two of the most important ones:

• Identity-based Policies: These are attached to a user, user group, or role. They belong to that identity and define what actions it can perform on which resources.

• Resource-based Policies: These are attached directly to a resource. They define who can access that resource and what operations are allowed. Resource-based policies can even grant access to users from other AWS accounts, enabling cross-account interactions.

Identity-based policies are particularly useful when you’re just learning or experimenting with AWS. If you want to explore this topic in more detail, I highly recommend reading the official documentation on Policy tyes.

These tools are powerful for managing and controlling access to AWS resources, but they must be configured correctly. For instance, if a root account is used by Application B solely to read S3 buckets, it’s not following the principle of least privilege, and therefore increases the risk surface of your environment.

How to apply it?

Protect the Root Account

Well, at first, all measuarements must be done to protect the root account. The password must be complex using special symbols, numbers, different characters and so on. Besides, if the password is being managed by a password manager, then the user must check if it's trustable and secure. Also, enabling multi-factor authentication is extremely required, not only one, like Microsoft Authentication, but multiple ones (hence the name, multi-factor).

It's also possible to determine multiple persons approval for root user sign-in. I recommend reading Root user best practices

Creating Identities

After setting up and securing the root account, it’s time to start creating users, user groups, or roles.

A user can be imagined as a single person or application. Each user has their own policies. If the company needs several users with the same permissions, defining policies for each one individually would be cumbersome.

Instead, you can create a user group and attach policies directly to that group. All users in the group inherit the group’s policies, which are then combined with any policies attached directly to the user. If a user no longer needs the permissions from a group, you can simply remove them from that group.

Roles, on the other hand, are not tied to a specific person or application. They can be assumed by anyone (or anything) that needs them. When a role is assumed, AWS generates temporary security credentials for the session. Roles are typically used to grant temporary or broader access to other identities or AWS services without sharing long-term credentials.

Applying the principle

Following the Principle of Least Privilege, each IAM identity should have only the permissions required to perform its specific tasks and nothing more. For example, if an IAM identity only needs to read files from an S3 bucket, it should be granted read-only permissions for that bucket and no access to other operations or services.

The AWS documentation provides detailed guidance on how to determine the necessary permissions for each identity. For more information, see Implementation Guidance.

It’s also important to regularly review and revoke unused access. Over time, projects evolve, team members change roles, and services become obsolete. Leaving unused permissions active increases the potential attack surface and violates the least privilege principle.

To strengthen this practice, AWS offers several tools and features:

• IAM Access Analyzer: Helps identify which resources are shared with external entities and suggests policies that grant only required permissions.

• Service Control Policies (SCPs): Allow administrators to set permission guardrails across multiple AWS accounts within an organization.

• IAM Policy Simulator: Lets you test and validate IAM policies before applying them, ensuring they grant only the intended permissions.

In short, applying the principle is a continuous process. Periodic reviews, automation, and AWS tools help ensure that permissions stay as minimal as possible without disrupting normal operations.

Conclusion

The Principle of Least Privilege is one of the most fundamental security practices in cloud environments. By ensuring that every identity, service, and resource has only the permissions it truly needs, organizations can drastically reduce the risk of breaches, data exposure, and misuse.

In AWS, this principle is implemented and enforced primarily through IAM by creating dedicated users, groups, and roles with tightly scoped policies, and by avoiding the use of powerful identities like the root user in everyday operations. Proper configuration, combined with continuous review and monitoring, turns IAM into a robust shield against internal mistakes and external attacks alike.

Security, in general, it’s a discipline. The sooner you apply least privilege across your AWS accounts and applications, the smaller your attack surface becomes, and the more confidence you’ll have in your system’s resilience.