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

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:

1. Verify identity: aws sts get-caller-identity
2. Check attached policies: aws iam list-attached-role-policies --role-name MyRole
3. 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

4. Check for explicit denies in SCPs or permission boundaries
5. 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