AWS IAM

Manage identity and access in AWS with least-privilege policies, roles, federation, and permission boundaries.

When to Use This Skill

• Creating roles for EC2 instances, Lambda functions, or ECS tasks
• Writing custom IAM policies with least-privilege access
• Setting up OIDC federation for GitHub Actions or other CI/CD systems
• Implementing permission boundaries for delegated administration
• Auditing access with IAM Access Analyzer and credential reports
• Configuring cross-account access with assume-role patterns
• Enforcing MFA and session policies

Prerequisites

• AWS CLI v2 installed and configured
• IAM permissions: iam:* (or scoped to specific actions for least privilege)
• For OIDC: ability to create identity providers (iam:CreateOpenIDConnectProvider)
• AWS Organizations access for Service Control Policies (SCPs)

IAM Policy Structure

Every IAM policy follows the same JSON structure. Always specify the minimum actions and resources required.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowS3ReadWrite",
      "Effect": "Allow",
      "Action": [
        "s3:GetObject",
        "s3:PutObject",
        "s3:ListBucket"
      ],
      "Resource": [
        "arn:aws:s3:::my-app-bucket",
        "arn:aws:s3:::my-app-bucket/*"
      ],
      "Condition": {
        "StringEquals": {
          "s3:x-amz-server-side-encryption": "aws:kms"
        }
      }
    },
    {
      "Sid": "DenyUnencryptedUploads",
      "Effect": "Deny",
      "Action": "s3:PutObject",
      "Resource": "arn:aws:s3:::my-app-bucket/*",
      "Condition": {
        "StringNotEquals": {
          "s3:x-amz-server-side-encryption": "aws:kms"
        }
      }
    }
  ]
}

Create and Manage Roles

# Create an EC2 instance role with trust policy
aws iam create-role \
  --role-name EC2AppRole \
  --assume-role-policy-document '{
    "Version": "2012-10-17",
    "Statement": [{
      "Effect": "Allow",
      "Principal": {"Service": "ec2.amazonaws.com"},
      "Action": "sts:AssumeRole"
    }]
  }' \
  --tags '[{"Key":"Team","Value":"platform"},{"Key":"Environment","Value":"production"}]'

# Create and attach an inline policy
aws iam put-role-policy \
  --role-name EC2AppRole \
  --policy-name s3-access \
  --policy-document '{
    "Version": "2012-10-17",
    "Statement": [{
      "Effect": "Allow",
      "Action": ["s3:GetObject", "s3:PutObject"],
      "Resource": "arn:aws:s3:::my-app-bucket/*"
    }]
  }'

# Attach a managed policy
aws iam attach-role-policy \
  --role-name EC2AppRole \
  --policy-arn arn:aws:iam::aws:policy/CloudWatchAgentServerPolicy

# Create instance profile and associate the role
aws iam create-instance-profile --instance-profile-name EC2AppProfile
aws iam add-role-to-instance-profile \
  --instance-profile-name EC2AppProfile \
  --role-name EC2AppRole

# Create a Lambda execution role
aws iam create-role \
  --role-name LambdaExecRole \
  --assume-role-policy-document '{
    "Version": "2012-10-17",
    "Statement": [{
      "Effect": "Allow",
      "Principal": {"Service": "lambda.amazonaws.com"},
      "Action": "sts:AssumeRole"
    }]
  }'

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

Cross-Account Access

# In Account B: create role that Account A can assume
aws iam create-role \
  --role-name CrossAccountReadRole \
  --assume-role-policy-document '{
    "Version": "2012-10-17",
    "Statement": [{
      "Effect": "Allow",
      "Principal": {"AWS": "arn:aws:iam::111111111111:root"},
      "Action": "sts:AssumeRole",
      "Condition": {
        "StringEquals": {"sts:ExternalId": "unique-external-id-12345"}
      }
    }]
  }'

# In Account A: assume the role
aws sts assume-role \
  --role-arn arn:aws:iam::222222222222:role/CrossAccountReadRole \
  --role-session-name cross-account-session \
  --external-id unique-external-id-12345

# Use the temporary credentials
export AWS_ACCESS_KEY_ID="ASIAXXX"
export AWS_SECRET_ACCESS_KEY="xxx"
export AWS_SESSION_TOKEN="xxx"

OIDC Federation for GitHub Actions

# Create the GitHub OIDC identity provider
aws iam create-open-id-connect-provider \
  --url https://token.actions.githubusercontent.com \
  --client-id-list sts.amazonaws.com \
  --thumbprint-list "6938fd4d98bab03faadb97b34396831e3780aea1"

# Create a role for GitHub Actions with repo-scoped trust
aws iam create-role \
  --role-name GitHubActionsDeployRole \
  --assume-role-policy-document '{
    "Version": "2012-10-17",
    "Statement": [{
      "Effect": "Allow",
      "Principal": {
        "Federated": "arn:aws:iam::123456789012:oidc-provider/token.actions.githubusercontent.com"
      },
      "Action": "sts:AssumeRoleWithWebIdentity",
      "Condition": {
        "StringEquals": {
          "token.actions.githubusercontent.com:aud": "sts.amazonaws.com"
        },
        "StringLike": {
          "token.actions.githubusercontent.com:sub": "repo:my-org/my-repo:ref:refs/heads/main"
        }
      }
    }]
  }'

# Attach deployment permissions to the role
aws iam attach-role-policy \
  --role-name GitHubActionsDeployRole \
  --policy-arn arn:aws:iam::123456789012:policy/DeploymentPolicy

GitHub Actions workflow usage:

# .github/workflows/deploy.yml
permissions:
  id-token: write
  contents: read

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: aws-actions/configure-aws-credentials@v4
        with:
          role-to-assume: arn:aws:iam::123456789012:role/GitHubActionsDeployRole
          aws-region: us-east-1
      - run: aws sts get-caller-identity

Permission Boundaries

# Create a permission boundary policy
aws iam create-policy \
  --policy-name DeveloperBoundary \
  --policy-document '{
    "Version": "2012-10-17",
    "Statement": [
      {
        "Sid": "AllowedServices",
        "Effect": "Allow",
        "Action": [
          "s3:*",
          "lambda:*",
          "dynamodb:*",
          "sqs:*",
          "sns:*",
          "logs:*",
          "cloudwatch:*",
          "ecr:*",
          "ecs:*"
        ],
        "Resource": "*"
      },
      {
        "Sid": "DenyIAMChanges",
        "Effect": "Deny",
        "Action": [
          "iam:CreateUser",
          "iam:DeleteUser",
          "iam:CreateRole",
          "iam:DeleteRole",
          "iam:AttachRolePolicy",
          "iam:PutRolePermissionsBoundary",
          "iam:DeleteRolePermissionsBoundary"
        ],
        "Resource": "*"
      },
      {
        "Sid": "DenyOutsideRegion",
        "Effect": "Deny",
        "Action": "*",
        "Resource": "*",
        "Condition": {
          "StringNotEquals": {
            "aws:RequestedRegion": ["us-east-1", "us-west-2"]
          },
          "ForAnyValue:StringNotLike": {
            "aws:PrincipalArn": "arn:aws:iam::*:role/admin-*"
          }
        }
      }
    ]
  }'

# Create a role with the permission boundary
aws iam create-role \
  --role-name DeveloperRole \
  --assume-role-policy-document file://trust-policy.json \
  --permissions-boundary "arn:aws:iam::123456789012:policy/DeveloperBoundary"

IAM Access Analyzer and Auditing

# Create an IAM Access Analyzer
aws accessanalyzer create-analyzer \
  --analyzer-name account-analyzer \
  --type ACCOUNT

# List findings (externally accessible resources)
aws accessanalyzer list-findings \
  --analyzer-arn arn:aws:access-analyzer:us-east-1:123456789012:analyzer/account-analyzer

# Generate credential report
aws iam generate-credential-report
aws iam get-credential-report --output text --query Content | base64 -d > credential-report.csv

# Find users with console access but no MFA
aws iam list-users --query "Users[].UserName" --output text | while read user; do
  mfa=$(aws iam list-mfa-devices --user-name "$user" --query "MFADevices" --output text)
  if [ -z "$mfa" ]; then
    echo "NO MFA: $user"
  fi
done

# List all policies attached to a role
aws iam list-attached-role-policies --role-name EC2AppRole
aws iam list-role-policies --role-name EC2AppRole

# Get the last-accessed services for a role
aws iam generate-service-last-accessed-details --arn arn:aws:iam::123456789012:role/EC2AppRole
# Then retrieve results with the returned JobId
aws iam get-service-last-accessed-details --job-id "job-id-from-above"

# Simulate a policy to test access
aws iam simulate-principal-policy \
  --policy-source-arn arn:aws:iam::123456789012:role/EC2AppRole \
  --action-names s3:GetObject s3:PutObject \
  --resource-arns arn:aws:s3:::my-app-bucket/data.json

Terraform IAM Role with OIDC

# OIDC provider for GitHub Actions
resource "aws_iam_openid_connect_provider" "github" {
  url             = "https://token.actions.githubusercontent.com"
  client_id_list  = ["sts.amazonaws.com"]
  thumbprint_list = ["6938fd4d98bab03faadb97b34396831e3780aea1"]
}

# Role for GitHub Actions
resource "aws_iam_role" "github_actions" {
  name = "GitHubActionsDeployRole"

  assume_role_policy = jsonencode({
    Version = "2012-10-17"
    Statement = [{
      Effect = "Allow"
      Principal = {
        Federated = aws_iam_openid_connect_provider.github.arn
      }
      Action = "sts:AssumeRoleWithWebIdentity"
      Condition = {
        StringEquals = {
          "token.actions.githubusercontent.com:aud" = "sts.amazonaws.com"
        }
        StringLike = {
          "token.actions.githubusercontent.com:sub" = "repo:my-org/my-repo:*"
        }
      }
    }]
  })

  permissions_boundary = aws_iam_policy.boundary.arn
}

resource "aws_iam_role_policy_attachment" "deploy" {
  role       = aws_iam_role.github_actions.name
  policy_arn = aws_iam_policy.deployment.arn
}

# Permission boundary
resource "aws_iam_policy" "boundary" {
  name = "DeveloperBoundary"
  policy = jsonencode({
    Version = "2012-10-17"
    Statement = [
      {
        Sid      = "AllowedServices"
        Effect   = "Allow"
        Action   = ["s3:*", "lambda:*", "dynamodb:*", "ecs:*", "logs:*"]
        Resource = "*"
      },
      {
        Sid      = "DenyIAMEscalation"
        Effect   = "Deny"
        Action   = ["iam:CreateUser", "iam:CreateRole", "iam:AttachRolePolicy"]
        Resource = "*"
      }
    ]
  })
}

Service Control Policies (Organizations)

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "DenyRootAccount",
      "Effect": "Deny",
      "Action": "*",
      "Resource": "*",
      "Condition": {
        "StringLike": {
          "aws:PrincipalArn": "arn:aws:iam::*:root"
        }
      }
    },
    {
      "Sid": "RequireIMDSv2",
      "Effect": "Deny",
      "Action": "ec2:RunInstances",
      "Resource": "arn:aws:ec2:*:*:instance/*",
      "Condition": {
        "StringNotEquals": {
          "ec2:MetadataHttpTokens": "required"
        }
      }
    },
    {
      "Sid": "DenyRegionsOutsideUS",
      "Effect": "Deny",
      "Action": "*",
      "Resource": "*",
      "Condition": {
        "StringNotEquals": {
          "aws:RequestedRegion": ["us-east-1", "us-west-2"]
        },
        "ForAnyValue:StringNotLike": {
          "aws:PrincipalArn": ["arn:aws:iam::*:role/OrganizationAdmin"]
        }
      }
    }
  ]
}

Troubleshooting

Problem	Cause	Fix
Access Denied on API call	Missing or incorrect policy	Use simulate-principal-policy to test; check resource ARN format
Role cannot be assumed	Trust policy does not include the caller	Verify Principal in trust policy matches caller ARN
OIDC federation fails	Thumbprint or audience mismatch	Verify OIDC provider URL, client ID list, and condition keys
Permission boundary blocks action	Boundary does not include the action	Add the action to the boundary; effective = identity AND boundary
Credential report shows stale keys	Keys not rotated in 90+ days	Rotate keys; disable unused access keys
Service-linked role creation fails	Organization SCP blocks iam:CreateServiceLinkedRole	Add exception in SCP for the specific service
Cross-account assume role fails	Missing ExternalId or wrong account	Verify ExternalId matches; check account number in Principal
MFA condition not enforced	Condition key not in policy	Add aws:MultiFactorAuthPresent condition

Related Skills

• terraform-aws - IaC deployment of IAM resources
• aws-ec2 - Instance profiles and roles
• aws-lambda - Lambda execution roles
• aws-ecs-fargate - ECS task and execution roles
• access-review - Access auditing and governance