IAM policies (JSON structure) in AWS - Step-by-Step Execution

Process Flow - IAM policies (JSON structure)

Start: Define Policy

↓

Specify Version

↓

Add Statement Array

↓

For Each Statement

↓

Define Effect (Allow/Deny)

↓

Specify Actions

↓

Specify Resources

↓

Optional Conditions

↓

End: Complete Policy JSON

IAM policy JSON starts with a version, then includes one or more statements. Each statement defines if it allows or denies actions on resources, optionally with conditions.

Execution Sample

AWS

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::example_bucket"
    }
  ]
}

This policy allows listing the contents of a specific S3 bucket.

Process Table

Step	JSON Key Processed	Action	Value Set	Notes
1	"Version"	Set policy version	"2012-10-17"	Defines policy language version
2	"Statement" array start	Begin statements list	Array start	Holds one or more statements
3	Statement[0] "Effect"	Set effect	"Allow"	Allows the specified actions
4	Statement[0] "Action"	Set action	"s3:ListBucket"	Specifies allowed action
5	Statement[0] "Resource"	Set resource	"arn:aws:s3:::example_bucket"	Limits action to this bucket
6	End of Statement array	Close statements list	Array end	No more statements
7	End of JSON	Complete policy	Full JSON ready	Policy is valid and deployable

💡 All required keys processed, policy JSON structure complete and valid

Status Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	After Step 5	Final
Version	undefined	"2012-10-17"	"2012-10-17"	"2012-10-17"	"2012-10-17"	"2012-10-17"
Statement	undefined	[]	[{"Effect": "Allow"}]	[{"Effect": "Allow", "Action": "s3:ListBucket"}]	[{"Effect": "Allow", "Action": "s3:ListBucket", "Resource": "arn:aws:s3:::example_bucket"}]	[{"Effect": "Allow", "Action": "s3:ListBucket", "Resource": "arn:aws:s3:::example_bucket"}]

Key Moments - 3 Insights

Why do we need the "Version" key in the policy JSON?

The "Version" key tells AWS which policy language version to use. Without it, AWS might not understand the policy correctly. See execution_table step 1.

Can a statement have multiple actions or resources?

Yes, actions and resources can be single strings or arrays of strings. This example uses single strings for simplicity. See execution_table steps 4 and 5.

What happens if "Effect" is set to "Deny" instead of "Allow"?

Setting "Effect" to "Deny" explicitly blocks the actions on the resources, overriding any allows. This changes the policy behavior but not the JSON structure. See execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of "Effect" at step 3?

A "Allow"

B "Deny"

C "ListBucket"

D "2012-10-17"

💡 Hint

Check the 'Value Set' column in execution_table row for step 3.

At which step is the resource ARN set in the policy JSON?

A Step 2

B Step 5

C Step 4

D Step 6

💡 Hint

Look for the row where 'Resource' key is processed in execution_table.

If we add another action to the "Action" key as an array, how would the variable 'Statement' change after step 4?

A Action would be a string with one action

B Action would be removed

C Action would be an array with multiple actions

D Statement would be empty

💡 Hint

Refer to variable_tracker for how 'Statement' holds actions.

Concept Snapshot

IAM Policy JSON Structure:
- Start with "Version" key (e.g., "2012-10-17")
- "Statement" is an array of permission blocks
- Each statement has "Effect" (Allow or Deny), "Action" (one or many), and "Resource"
- Optional "Condition" can restrict when policy applies
- Valid JSON structure is required for AWS to accept the policy

Full Transcript

IAM policies in AWS are JSON documents that define permissions. They start with a "Version" key to specify the policy language version. Then, they include a "Statement" array where each statement defines permissions. Each statement must have an "Effect" key that is either "Allow" or "Deny", an "Action" key listing the AWS actions allowed or denied, and a "Resource" key specifying which AWS resources the actions apply to. Optionally, statements can include "Condition" keys to further restrict permissions. The JSON must be valid and follow this structure for AWS to accept and enforce the policy.

Practice

1. What is the main purpose of an IAM policy in AWS?

easy

A. To create virtual machines

B. To define permissions for users and resources

C. To monitor network traffic

D. To store data in the cloud

Solution

1. Step 1: Understand IAM policy role

   An IAM policy is a JSON document that specifies permissions for AWS users, groups, or roles.

2. Step 2: Identify main function

   Its main function is to control what actions are allowed or denied on AWS resources.

3. Final Answer:

   To define permissions for users and resources -> Option B

4. Quick Check:

   IAM policy = permissions definition [OK]

Hint: IAM policies control access permissions in AWS [OK]

Common Mistakes:

• Confusing IAM policies with data storage
• Thinking IAM policies monitor network traffic
• Assuming IAM policies create virtual machines

2. Which of the following is the correct JSON key to specify the effect of a statement in an IAM policy?

easy

A. "Permission"

B. "Action"

C. "Resource"

D. "Effect"

Solution

1. Step 1: Recall IAM policy statement keys

   IAM policy statements include keys like Effect, Action, Resource, and optionally Condition.

2. Step 2: Identify key for permission type

   The key that specifies whether to allow or deny is "Effect".

3. Final Answer:

   "Effect" -> Option D

4. Quick Check:

   Effect key = permission type [OK]

Hint: Effect key sets allow or deny in IAM policy [OK]

Common Mistakes:

• Using "Permission" instead of "Effect"
• Confusing "Action" with permission type
• Mistaking "Resource" for effect

3. Given this IAM policy statement snippet:

{
  "Effect": "Allow",
  "Action": "s3:ListBucket",
  "Resource": "arn:aws:s3:::example-bucket"
}

What permission does this statement grant?

medium

A. Allows listing the bucket itself

B. Allows listing objects inside the bucket

C. Allows deleting the bucket

D. Allows uploading objects to the bucket

Solution

1. Step 1: Understand the Action "s3:ListBucket"

   This action allows listing the bucket itself and its metadata, not the objects inside.

2. Step 2: Match Resource and Action

   The resource is the bucket ARN, so permission is to list the bucket (its properties), not the objects inside the bucket.

3. Final Answer:

   Allows listing the bucket itself -> Option A

4. Quick Check:

   s3:ListBucket = list bucket (not objects) [OK]

Hint: s3:ListBucket lists the bucket, not objects inside [OK]

Common Mistakes:

• Confusing ListBucket with listing objects inside the bucket
• Assuming permission to delete or upload
• Ignoring the resource ARN level

4. Identify the error in this IAM policy statement:

{
  "Effect": "Allow",
  "Action": ["ec2:StartInstances", "ec2:StopInstances"],
  "Resource": "*",
  "Condition": {
    "StringEquals": {
      "ec2:Region": "us-west-2"
    }
  }
}

medium

A. The Condition key is not valid for EC2 actions

B. The Condition key should be inside the Action key

C. The policy is valid and has no errors

D. The Resource value "*" is not allowed for these actions

Solution

1. Step 1: Check Condition usage with EC2 actions

   EC2 supports conditions like StringEquals on ec2:Region to restrict actions by region.

2. Step 2: Verify Resource and structure

   Resource "*" is valid for EC2 start/stop actions because they apply to instances across resources.

3. Final Answer:

   The policy is valid and has no errors -> Option C

4. Quick Check:

   Condition on ec2:Region with Resource "*" is valid [OK]

Hint: Conditions can restrict actions by region or other keys [OK]

Common Mistakes:

• Thinking Condition is invalid for EC2
• Assuming Resource "*" is always wrong
• Misplacing Condition inside Action

5. You want to create an IAM policy that allows a user to read objects only from a specific S3 bucket named "my-data-bucket" but denies deleting any objects. Which policy statement correctly achieves this?

hard

A. {
"Effect": "Allow",
"Action": ["s3:GetObject"],
"Resource": "arn:aws:s3:::my-data-bucket/*"
}

B. {
"Effect": "Allow",
"Action": ["s3:GetObject", "s3:DeleteObject"],
"Resource": "arn:aws:s3:::my-data-bucket/*"
}

C. {
"Effect": "Deny",
"Action": "s3:DeleteObject",
"Resource": "arn:aws:s3:::my-data-bucket/*"
}

D. {
"Effect": "Allow",
"Action": "s3:*",
"Resource": "arn:aws:s3:::my-data-bucket"
}

Solution

1. Step 1: Identify required permissions

   The user needs permission to read objects only, which is "s3:GetObject" on the bucket's objects.

2. Step 2: Check for delete denial

   Not including "s3:DeleteObject" means no delete permission is granted. Explicit deny is not required if no allow exists.

3. Step 3: Validate resource ARN

   The resource must include "/*" to specify objects inside the bucket, not the bucket itself.

4. Final Answer:

   Allow s3:GetObject on objects in my-data-bucket only -> Option A

5. Quick Check:

   Allow read only, no delete = { "Effect": "Allow", "Action": ["s3:GetObject"], "Resource": "arn:aws:s3:::my-data-bucket/*" } [OK]

Hint: Allow only needed actions; omit delete to deny it [OK]

Common Mistakes:

• Allowing delete by mistake
• Using bucket ARN without /* for objects
• Using wildcard s3:* granting too many permissions