AWS Deep Dive

author: Nathan Acks
date: 2022-09-19

The AWS deep dive reading notes return!

AWS IAM Policies in a Nutshell

IAM policy statements can include an Sid value (different than the policy-level Id). This is only required by a few services (SQS and SNS). It seems like it’s probably safe to just set both the Id and the Sid to some UUIDv4 value (though the two should probably use different UUIDs).

(The reason there’s both an Sid and an Id is that the policy Statement can point either to a single object or an array of objects. Sid is redundant in the first case, but uniquely identifies each policy statement object in the second case.)

Many (most?) IAM policy properties accept wild-carding. An Action of s3:Get* is equivalent to writing s3:GetObject, s3:GetBucket, …

(The “?” string can also be used as a single-character wildcard.)

There’s also “policy variables” which can be used anywhere a wildcard can be. These are things like aws:username, and when used they’re referenced as ${aws:username}.

Statement properties (Principal, Action, and Resource) generally accept either a string or an array of strings.

The IAM policy statement Principal isn’t needed for policies attached to users, groups, and roles. In all of these cases, the Principal defaults to the object the policy is attached to (makes sense). Thus, Principal is primarily used only for policies attached to resources like S3 buckets and SQS queues.

(Policies attached to resources sometimes don’t need to include a Resource object in the policy statement. But sometimes they do. For example, “permissions policies” attached to roles - see below - don’t need a Resource, but policies attached to S3 buckets do need a Resource.)

Group ARNs are not allowed to be principals.

Roles require two policies to be attached - a “permissions policy” (which specifies what the role can do) and a “trust policy” (which specifies who can assume the role). In a sense, a role functions as both an actor (via the “permissions policy”) and a resource (via the “trust policy”).

Trust policies are where the sts:AssumeRole action gets used.

The ARN format:

arn:aws:[service]:[region]:[account]:[resourceType]/[resourcePath]

The “service”, “region”, and “account” portions of an ARN can be collapsed if not relevant. Thus arn:aws:s3:::bucket-name/* is equivalent to arn:aws:s3:*:*:bucket-name/*.

The statement Condition property is an object that contains one or more conditions of the following form:

"conditionOperator": {
	"conditionKey": "conditionValue"
}

For example:

"DateGreaterThan": {
	"aws:CurrentTime": "2022-09-19T00:00:00Z"
}

Many (all?) available condition keys are also policy variables.

Annoyingly, not every action supports every condition key.