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Step 1: Understand Requirements & Security Principles

Requirements:

Customer-facing payment application (PCI DSS compliance applies)

Hosted on third-party cloud (e.g., AWS)

Must segment public-facing and internal resources

Needs to be scalable and resilient

Must have strong security controls

Step 2: Design the High-Level Network Layout

Core Components:

VPC (Virtual Private Cloud): Isolates your environment from other tenants in the cloud.

Subnets:

Public subnet: For resources that must communicate with the internet.

Private subnet: For internal resources, NOT directly exposed to the internet.

Step 3: Place Resources in Appropriate Subnets

Public Subnet:

Internet-facing Load Balancer (LB): Distributes traffic to application servers.

Web Application Firewall (WAF): Protects against web exploits.

Autoscaling Instances: EC2 (or VM) servers running your web front-end, automatically scaling as traffic grows.

Private Subnet:

Application servers: Back-end logic, not exposed to internet directly.

Database: Sensitive data storage, only accessible by application servers.

Internal Load Balancer: Manages traffic among app servers.

WAF: Can be used internally as well for defense-in-depth.

Step 4: Add Connectivity and Security Controls

Internet Gateway: Allows resources in public subnet to communicate with the internet.

NAT Gateway: Allows outbound internet traffic from private subnet without exposing private IPs.

Security Groups: Firewalls at the instance level; allow only necessary traffic (e.g., LB to web server, web server to DB).

Network ACLs: Subnet-level firewalls for additional control.

Step 5: Network Diagram Explanation (Based on Your Images)

Public Subnet (Top Layer)

Load Balancer

Accepts HTTPS traffic from customers.

Sends only necessary HTTP/HTTPS to web servers in public subnet.

WAF (Web Application Firewall)

Sits in front of Load Balancer.

Filters malicious requests (SQLi, XSS, etc.).

Autoscaling Group

Multiple web servers for redundancy and scalability.

Placed in public subnet to respond to traffic spikes.

Private Subnet (Bottom Layer)

Application Servers

Receive requests from public subnet’s load balancer.

Not directly exposed to the internet.

Database

Only accessible from application servers, never public.

Security groups restrict all inbound traffic except from app servers.

Internal Load Balancer

Balances requests to application servers.

Step 6: Flow of Data (Step-by-Step)

Client -> Internet Gateway -> WAF -> Load Balancer (Public Subnet):Customers initiate connections to your app over the internet.

Load Balancer -> Autoscaling Web Servers (Public Subnet):Load balancer routes requests to available web servers.

Web Servers -> Application Logic (Private Subnet):Web servers pass necessary requests to the internal application servers.

App Servers -> Database (Private Subnet):Application servers query/update customer payment data in the database.

Outbound (NAT Gateway):App servers may need to access updates or external APIs—use NAT Gateway for secure outbound connections.

Step 7: Security Best Practices

Security Groups: Only allow necessary ports (e.g., 443 for HTTPS to LB, 3306 for MySQL between app server and DB).

Network ACLs: Add another layer of subnet-level restrictions.

Encryption: Use HTTPS for all external connections, encrypt data at rest and in transit (TLS, disk encryption).

IAM Roles/Policies: Principle of least privilege for accessing resources.

Monitoring/Logging: Enable VPC flow logs, cloud service logs, and application logging.

Patch Management: Automate patching for OS and applications.

Backups: Regular, secure backups of critical data.

Step 8: Compliance Considerations

For payment applications (PCI DSS):

Isolate cardholder data environment (CDE).

Strong access controls (multi-factor authentication, role separation).

Regular vulnerability assessments and penetration testing.

Retain logs for auditing.

Step 9: Draw the Architecture (Summary)

Internet Gateway: Allows inbound/outbound internet access.

Public Subnet: WAF, Load Balancer, Autoscaling group.

Private Subnet: App servers, DB, internal LB.

NAT Gateway: Outbound access for private resources.

Security Groups/ACLs: Control all traffic flows.

Monitoring/Logging: Enabled at all levels.

Bonus: Sample Security Group Rules

Web Server (Public Subnet):

Inbound: 443 (HTTPS) from Internet

Outbound: 80/443 to App Servers

App Server (Private Subnet):

Inbound: 80/443 from Web Servers

Outbound: 3306 (MySQL) to Database

Database (Private Subnet):

Inbound: 3306 from App Servers

Outbound: None (unless replication required)

References to Security+ Domains

1.0 General Security Concepts: Principle of least privilege, defense in depth.

2.0 Threats, Vulnerabilities, Mitigations: WAF, segmentation, patching.

3.0 Security Architecture: Network segmentation, secure design.

4.0 Security Operations: Monitoring, logging, response.

5.0 Security Program Management: Compliance, policy.