Optimizing IAM Policies by Removing Unnecessary Permissions with Last Access Data

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In the realm of AWS security, a foundational best practice is to design AWS Identity and Access Management (IAM) policies that reflect the principle of least privilege. This means providing only the necessary permissions required for specific tasks. However, discerning the exact permissions needed by users or applications can be quite challenging. To address this, AWS has introduced service last accessed data in the IAM console, which reveals when an IAM entity—be it a user, group, or role—last engaged with a particular AWS service. Understanding this data allows administrators to streamline permissions, effectively tightening IAM policies while minimizing effort.

This article will first explain the core concepts behind service last accessed data. Subsequently, we will explore a practical scenario to illustrate how this information can facilitate the removal of superfluous permissions from IAM policies.

Access Advisor Overview

When examining a user, group, role, or managed policy in the IAM console, you will now find a new tab called Access Advisor. This section features a table displaying service last accessed data, which includes:

  • Permissions granted by the policy if reviewing a managed policy.
  • Permissions allocated to the IAM entity if reviewing a user, group, or role.
  • The timestamp indicating when each service was last accessed.

The interpretation of the Last Accessed column varies based on whether you are analyzing a managed policy, user, group, or role. For a managed policy, it indicates the last time any user, group, or role associated with that policy accessed a specific service. In contrast, for users or roles, it reflects when that individual last authenticated against a service, while for groups, it shows the last access time of any user within the group.

Importantly, “Last Accessed” pertains to authentication times, and not authorization. For instance, if a user possesses read-only permissions for Amazon S3 and attempts to delete an S3 bucket, the action will be denied, yet the attempt will still be recorded in the service last accessed data. For further details, refer to the comprehensive service last accessed data documentation.

It is also essential to note that the services listed in the Access Advisor tab represent the current permissions state, not historical data. If a policy previously allowed access to multiple services but now restricts it to only S3, the service last accessed table will only display S3. For auditing historical access, AWS CloudTrail logs should be consulted.

Practical Use Case: Refining Application Permissions

Consider Alice, a DevOps administrator tasked with managing her team’s AWS infrastructure. Her team has developed an application running on Amazon EC2, which communicates with various AWS services. Unfamiliar with IAM, Alice opts for the PowerUserAccess AWS managed policy, granting extensive read/write access across all services except IAM. While this approach resolves her immediate needs, it is not advisable as a long-term solution.

Using the service last accessed data, Alice can refine the role’s permissions, stripping away access to services that are not utilized. After some time, she navigates to the IAM console, locates the PowerUserAccess policy linked to her IAM role, and selects the Access Advisor tab.

Upon reviewing the last accessed timestamps, she discovers that the application only interacts with Amazon DynamoDB, S3, Amazon Simple Notification Service (SNS), Amazon Simple Queue Service (SQS), and AWS CloudWatch. Armed with this insight, Alice can detach the PowerUserAccess policy and create a custom policy that grants only the necessary permissions, thereby reducing the security risks in alignment with the principle of least privilege.

This example illustrates how service last accessed data can be employed in a policy-focused manner to address common access control challenges. In a future post, we will delve into utilizing this data from a principal-centric perspective, examining the Access Advisor tab through the lens of users, groups, or roles.

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