There is one slide in the “Security Engineering on AWS” course about Permissions Boundaries. The text is as follows:
A permissions boundary is an advanced feature in which you limit the maximum permissions that a principal can have. These boundaries can be applied to AWS Organizations organizations or to IAM users or roles.
As the IAM administrator, you can define one or more permissions boundaries using managed policies and allow your employee to create a principal with this boundary. The employee can then attach a permissions policy to this principal. However, the effective permissions of the principal are the intersection of the permissions boundary and permissions policy. As a result, the new principal cannot exceed the boundary that you defined.
When you use a policy to set the permissions boundary for a user, it limits the user’s permissions but does not provide permissions on its own.
It might be a challenge to understand the use case and how it is used in practice. There is a helpful blog post here.
Say you have a development team with full access to certain services, but no access to IAM.
Say you want the developer to assign a role to EC2, and you want them to create the role, a policy, attach the policy to the role, and attach the role to an EC2 instance. That is a lot of permissions, and instead we want to keep to the principal of least privilege.
The example in the blog post is lengthy to set up, so I simplify a bit here, and omit the JSON for brevity (see the referenced blog post above for the detailed steps)
The admin does the following tasks:
First the admin creates a boundary policy “DynamoDB_Boundary_Frankfurt“. The policy will allow put, update, delete on any table in the Frankfurt region.. Employees will be required to set this policy as the permission boundary for the roles they create.
Create “Employee_Policy”. This policy will allow the employee to create roles and policies, attach policies to roles, and attach roles to resources. However the admin wants to retain control over the naming conventions of the roles and policies for ease of auditing. The roles and policies must have the prefix MyTestApp. Also, there will be a condition that the above permissions boundary policy must be attached to the role, otherwise the role creation will fail.
Create a role “MyEmployeeRole” and attach the “Employee_Policy”.
Create a policy “Pass_Role_Policy” to allow the employee to pass the roles they create to services such as EC2.
Attach the policy to “MyEmployee_Role”
The Employee does the following tasks:
Create a role “MyTestAppRole”. The employee must provide the permissions boundary “DynamoDB_Boundary_Frankfurt”, otherwise the role creation will fail. The policy will allow EC2 to assume the role.
Create a policy “MyTestApp_DDB_Permissions” allowing all DynamoDB actions on a specific table MyTestApp_DDB_Table.
Attach the policy to “MyTestAppRole”
To summarise, the administrator created a permission boundary allowing DynamoDB put, update, delete on all resources in Frankfurt.
The employee created a policy which allowed all actions on a specific table with no region restriction.
The effective permission is to allow put, update, and delete on the specific table MyTestApp_DDB_Table in the Frankfurt region.