AWS IAM Assume Role: How to Access Other AWS Accounts Securely

Managing multiple AWS accounts can become messy very quickly, especially in small environments where one person or a small team handles development, testing, and production. Many beginners make the mistake of creating separate IAM users in every account and logging in with different credentials each time. That approach is not only inconvenient, but also poor from a security and management perspective.

A much better way is to use AWS IAM Assume Role.

In this post, you will learn what Assume Role is, why it matters, and how it helps you access other AWS accounts securely without maintaining separate long-term credentials in every account.

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What is AWS IAM Assume Role?

Assume Role in AWS allows a user, application, or service to temporarily take on a different set of permissions by assuming an IAM role.

Instead of creating a separate IAM user in another AWS account, you create a role in the target account and allow trusted users from another account to assume that role. Once assumed, AWS provides temporary security credentials that grant access based on the permissions attached to that role.

This is one of the most important AWS IAM concepts because it improves both security and operational simplicity.

Why use Assume Role in small environments?

In large enterprises, multi-account strategies are common, but even in small environments, Assume Role is extremely useful.

For example, you might have:

• One AWS account for development
• One AWS account for staging
• One AWS account for production

If you create separate IAM users in each account, you end up managing multiple usernames, passwords, MFA settings, and permission policies. This becomes hard to maintain and increases the risk of misconfiguration.

With Assume Role, you can:

• Log in to one AWS account
• Switch into another account securely
• Avoid creating unnecessary long-term users
• Centralize access management
• Reduce security risks

Real-world example

Let’s say you are a DevOps engineer working in Account A, but you need to access resources in Account B.

Instead of creating a permanent IAM user for yourself in Account B, you can do the following:

1. Create a role in Account B
2. Define a trust policy that allows users from Account A to assume that role
3. Attach the required permission policy to the role
4. Assume the role from Account A whenever access is needed

This gives you temporary access to Account B without storing separate credentials for that account.

How Assume Role works

At a high level, AWS IAM Assume Role works like this:

1. A role is created in the target account

This role contains the permissions needed to access resources in that account.

2. A trust relationship is defined

The trust policy tells AWS who is allowed to assume the role. This could be:

• An IAM user
• Another IAM role
• An AWS service
• A federated identity

3. The user or service assumes the role

When the role is assumed, AWS Security Token Service (STS) issues temporary credentials.

4. Temporary access is granted

These credentials are valid for a limited time and provide access only according to the permissions attached to the role.

This temporary credential model is much safer than using permanent access keys everywhere.

Important components of Assume Role

To understand Assume Role properly, you should know these key parts.

IAM Role

A role is an AWS identity with a set of permissions. Unlike an IAM user, a role does not have permanent credentials.

Trust Policy

The trust policy defines who can assume the role.

Example idea:

• Account A users are trusted to assume a role in Account B

Permission Policy

The permission policy defines what actions the assumed role can perform after access is granted.

Example idea:

• Read from S3
• Access EC2 instances
• View CloudWatch logs

STS Temporary Credentials

When the role is assumed, AWS issues temporary credentials using AWS Security Token Service (STS).

These credentials expire automatically, which is one reason Assume Role is considered more secure.

Benefits of using Assume Role

Better security

You avoid creating long-term credentials in multiple accounts. Temporary access reduces the attack surface.

Easier management

You do not need to maintain separate IAM users for each AWS account.

Cleaner multi-account access

Switching between accounts becomes easier and more scalable.

Supports least privilege

You can create roles with only the exact permissions needed for specific tasks.

Useful for automation

Applications, Lambda functions, EC2 instances, and CI/CD pipelines can assume roles securely.

Assume Role vs IAM User

A lot of beginners ask why they should use Assume Role instead of just creating another IAM user.

Here is the difference:

IAM User

• Permanent identity
• Long-term credentials
• Best for direct user access in limited cases
• Harder to manage across multiple accounts

IAM Role

• No permanent credentials
• Temporary security credentials
• Better for cross-account access
• More secure and flexible

In simple terms, if you need secure access across AWS accounts, roles are usually the better choice.

Common use cases of Assume Role

Assume Role is used in many real-world AWS environments.

Cross-account administration

An admin in one account can manage resources in another account.

DevOps and CI/CD pipelines

A pipeline in one account can deploy resources into another account.

Accessing production safely

Developers can get controlled temporary access to production resources when needed.

AWS services accessing other resources

Services like EC2, Lambda, and ECS can assume roles to access S3, DynamoDB, CloudWatch, and more.

Small team account management

A small company with separate billing, dev, and prod accounts can simplify access using Assume Role.

High-level steps to configure cross-account Assume Role

Here is the general flow for setting up Assume Role between two AWS accounts.

In the target account

• Create an IAM role
• Choose trusted entity type
• Define trust policy to allow the source account
• Attach required permissions to the role

In the source account

• Use an IAM user or existing role with permission to assume the target role
• Switch role from AWS Console or use AWS CLI

Result

You gain temporary access to the target account based on the attached policies.

Example scenario

Imagine this setup:

• Account A = Management account
• Account B = Production account

You log in to Account A using your regular IAM identity. Then you assume a role in Account B called something like ProdAdminRole.

Once you switch to that role, you can perform allowed actions in the production account without having a separate IAM user in Account B.

That is the real power of Assume Role.

Security best practices for Assume Role

To use IAM Assume Role properly, follow these best practices:

Use least privilege

Give the role only the permissions it truly needs.

Use MFA where possible

Require multi-factor authentication for sensitive role assumptions.

Avoid wildcard permissions

Do not use overly broad policies unless absolutely necessary.

Monitor with CloudTrail

Track role assumptions and activity for auditing and security visibility.

Use meaningful role names

Names like ReadOnlyAuditRole or ProdDeploymentRole make access easier to understand.

Limit who can assume the role

Keep the trust policy as strict as possible.

Console switching vs CLI assume-role

You can use Assume Role in two common ways.

AWS Management Console

You can use the Switch Role feature to access another AWS account through the browser.

AWS CLI

You can use the assume-role API via AWS CLI to get temporary credentials for automation or command-line work.

This is very useful for scripts, deployment pipelines, and administrative tasks.

Why this matters for beginners

If you are learning AWS, IAM Assume Role is one of those concepts that may seem advanced at first, but it is actually fundamental.

Once you understand it, many other AWS identity and access topics become easier:

• Cross-account access
• Temporary credentials
• Identity federation
• Service roles
• Multi-account AWS architecture

For DevOps engineers and cloud professionals, Assume Role is not optional knowledge. It is a practical skill used in real environments.

Final thoughts

AWS IAM Assume Role is one of the best ways to access other AWS accounts securely and efficiently. It removes the need for multiple permanent users, supports temporary access, and fits perfectly into modern AWS security practices.

Even in small environments, it makes account access cleaner, safer, and easier to manage.

If you are building AWS skills seriously, make sure you understand how roles, trust policies, permission policies, and temporary credentials work together. This concept appears again and again in real-world AWS projects.