AWS IAM

AWS Identity and Access Management (IAM) enables secure access control to AWS services and resources. IAM is foundational to AWS security—every AWS API call is authenticated and authorized through IAM.

Table of Contents Core Concepts Common Patterns CLI Reference Best Practices Troubleshooting References Core Concepts Principals

Entities that can make requests to AWS: IAM users, roles, federated users, and applications.

Policies

JSON documents defining permissions. Types:

Identity-based: Attached to users, groups, or roles Resource-based: Attached to resources (S3 buckets, SQS queues) Permission boundaries: Maximum permissions an identity can have Service control policies (SCPs): Organization-wide limits Roles

Identities with permissions that can be assumed by trusted entities. No permanent credentials—uses temporary security tokens.

Trust Relationships

Define which principals can assume a role. Configured via the role's trust policy.

Common Patterns Create a Service Role for Lambda

AWS CLI:

Create the trust policy

cat > trust-policy.json << 'EOF' { "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Principal": { "Service": "lambda.amazonaws.com" }, "Action": "sts:AssumeRole" } ] } EOF

Create the role

aws iam create-role \ --role-name MyLambdaRole \ --assume-role-policy-document file://trust-policy.json

Attach a managed policy

aws iam attach-role-policy \ --role-name MyLambdaRole \ --policy-arn arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole

boto3:

import boto3 import json

iam = boto3.client('iam')

trust_policy = { "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Principal": {"Service": "lambda.amazonaws.com"}, "Action": "sts:AssumeRole" } ] }

Create role

iam.create_role( RoleName='MyLambdaRole', AssumeRolePolicyDocument=json.dumps(trust_policy) )

Attach managed policy

iam.attach_role_policy( RoleName='MyLambdaRole', PolicyArn='arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole' )

Create Custom Policy with Least Privilege cat > policy.json << 'EOF' { "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Action": [ "dynamodb:GetItem", "dynamodb:PutItem", "dynamodb:Query" ], "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/MyTable" } ] } EOF

aws iam create-policy \ --policy-name MyDynamoDBPolicy \ --policy-document file://policy.json

Cross-Account Role Assumption

In Account B (trusted account), create role with trust for Account A

cat > cross-account-trust.json << 'EOF' { "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Principal": { "AWS": "arn:aws:iam::111111111111:root" }, "Action": "sts:AssumeRole", "Condition": { "StringEquals": { "sts:ExternalId": "unique-external-id" } } } ] } EOF

From Account A, assume the role

aws sts assume-role \ --role-arn arn:aws:iam::222222222222:role/CrossAccountRole \ --role-session-name MySession \ --external-id unique-external-id

CLI Reference Essential Commands Command Description aws iam create-role Create a new IAM role aws iam create-policy Create a customer managed policy aws iam attach-role-policy Attach a managed policy to a role aws iam put-role-policy Add an inline policy to a role aws iam get-role Get role details aws iam list-roles List all roles aws iam simulate-principal-policy Test policy permissions aws sts assume-role Assume a role and get temporary credentials aws sts get-caller-identity Get current identity Useful Flags --query: Filter output with JMESPath --output table: Human-readable output --no-cli-pager: Disable pager for scripting Best Practices Security Never use root account for daily tasks Enable MFA for all human users Use roles instead of long-term access keys Apply least privilege — grant only required permissions Use conditions to restrict access by IP, time, or MFA Rotate credentials regularly Use permission boundaries for delegated administration Policy Design Start with AWS managed policies, customize as needed Use policy variables (${aws:username}) for dynamic policies Prefer explicit denies for sensitive actions Group related permissions logically Monitoring Enable CloudTrail for API auditing Use IAM Access Analyzer to identify overly permissive policies Review credential reports regularly Set up alerts for root account usage Troubleshooting Access Denied Errors

Symptom: AccessDeniedException or UnauthorizedAccess

Debug steps:

Verify identity: aws sts get-caller-identity Check attached policies: aws iam list-attached-role-policies --role-name MyRole Simulate the action: aws iam simulate-principal-policy \ --policy-source-arn arn:aws:iam::123456789012:role/MyRole \ --action-names dynamodb:GetItem \ --resource-arns arn:aws:dynamodb:us-east-1:123456789012:table/MyTable

Check for explicit denies in SCPs or permission boundaries Verify resource-based policies allow the principal Role Cannot Be Assumed

Symptom: AccessDenied when calling AssumeRole

Causes:

Trust policy doesn't include the calling principal Missing sts:AssumeRole permission on the caller ExternalId mismatch (for cross-account roles) Session duration exceeds maximum

Fix: Review and update the role's trust relationship.

Policy Size Limits Managed policy: 6,144 characters Inline policy: 2,048 characters (user), 10,240 characters (role/group) Trust policy: 2,048 characters

Solution: Use multiple policies, reference resources by prefix/wildcard, or use tags-based access control.

References IAM User Guide IAM API Reference IAM CLI Reference Policy Reference boto3 IAM