Navigating AWS IAM: Understanding Permission Precedence with Allow and Deny Groups

Navigating AWS IAM: Understanding Permission Precedence with Allow and Deny Groups

Introduction: In the intricate world of AWS Identity and Access Management (IAM), managing user permissions is a critical aspect of ensuring secure and efficient cloud operations. One of the key challenges is understanding how permissions are evaluated when users belong to multiple IAM groups with conflicting policies. In this blog post, we'll delve into the nuances of permission precedence, focusing on scenarios where both allow and deny permissions come into play.

IAM Groups: The Foundation of Access Control

IAM groups play a pivotal role in streamlining access management within AWS environments. By associating users with specific groups, administrators can efficiently assign and revoke permissions on a large scale. However, when users are members of multiple groups with varying access policies, it's crucial to comprehend how these permissions interact.

The Dilemma: Allow vs. Deny Permissions

Imagine you have two IAM groups – "S3 Allow" and "S3 Deny." The "S3 Allow" group grants broad permissions for Amazon S3, while the "S3 Deny" group explicitly denies all S3 actions. Now, if you attach both of these groups to a user, a conflict arises: Should the user have S3 permissions or not?

Understanding Permission Precedence

In AWS IAM, permission evaluation follows a specific hierarchy. Deny permissions take precedence over allow permissions. This means that even if a user is part of a group with S3 allow permissions, the deny permissions will override them.

The Rule of Precedence:

  • Explicit Deny > Explicit Allow

Practical Implications: The User's S3 Permissions

Given the aforementioned rule, when a user is a member of both "S3 Allow" and "S3 Deny" groups, the deny permissions will take precedence. Consequently, the user will not have any S3 permissions, despite being part of a group that allows them.

Best Practices for IAM Group Management

  1. Avoid Conflicting Policies: Strive to create IAM groups with clear and non-conflicting policies. Minimize the use of explicit deny statements to reduce complexity.

  2. Regular Audits: Conduct regular audits of IAM group memberships and policies to ensure alignment with organizational security requirements.

  3. Consistent Naming Conventions: Adopt a consistent naming convention for IAM groups to enhance clarity and streamline administration.


In the dynamic landscape of AWS IAM, understanding how permissions are evaluated is essential for maintaining a secure and well-managed cloud environment. The interplay between allow and deny permissions within IAM groups requires careful consideration to avoid unintended consequences. By adhering to best practices and comprehending the rule of precedence, administrators can effectively navigate the complexities of permission management in AWS IAM.

As organizations continue to leverage the power of AWS, mastering IAM principles becomes paramount for achieving a robust and resilient cloud infrastructure.