AWS IAM Skill | Agent Skills

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.

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.

Agent Skills: AWS IAM

Skill Files