Building a Low-Cost CTFd Instancing System for Cybersecurity Training

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Introduction

Capture The Flag (CTF) competitions are essential for cybersecurity training, but scaling per-player instances can be costly. Jacob E., a cybersecurity expert, developed CTFd-Ployer, a lightweight solution enabling on-demand Docker instances for CTF challenges without expensive Kubernetes setups. This article explores the technical implementation, key commands, and how to deploy this system efficiently.

Learning Objectives

  • Understand how CTFd-Ployer reduces costs for per-player CTF instances.
  • Learn to deploy and configure Docker-based challenge instances with SSL.
  • Implement dynamic flag generation and Discord alerts for CTF competitions.

1. Setting Up CTFd-Ployer on GCP/AWS

Verified Command:

git clone https://github.com/jselliott/CTFd-ployer.git 
cd CTFd-ployer 
docker-compose up -d

Step-by-Step Guide:

  1. Clone the repository to your cloud instance (GCP/AWS).
  2. Use `docker-compose` to spin up the CTFd and Ployer services.
  3. Configure `config.yml` to define challenge containers, timeouts, and subdomains.

2. Configuring Dynamic Subdomains with SSL

Verified Command:

certbot certonly --manual --preferred-challenges=dns -d .ctf.example.com

Step-by-Step Guide:

  1. Use Certbot to request a wildcard SSL certificate for player subdomains.

2. Add DNS TXT records for domain validation.

  1. Integrate the certificate with the CTFd-Ployer reverse proxy (NGINX).

3. Deploying Docker Challenge Instances

Verified Command:

 Example CTFd-Ployer challenge config 
challenges: 
- name: "Web Exploit" 
image: "ctf/web-challenge:latest" 
timeout: 3600 
ports: 
- "80:80"

Step-by-Step Guide:

  1. Define challenges in `config.yml` with Docker images and ports.
  2. Players trigger instances via CTFd, which spawns isolated containers.

3. Randomized subdomains (e.g., `player123.ctf.example.com`) are assigned dynamically.

4. Dynamic Flag Generation and Anti-Cheat

Verified Command:

 Example flag generation script 
import os 
flag = "FLAG_" + os.urandom(8).hex()

Step-by-Step Guide:

  1. Flags are generated per-instance and tied to player sessions.
  2. Use Discord webhooks to alert admins of flag-sharing attempts.
  3. Configure CTFd to validate flags against the Ployer API.

5. Cost Optimization for CTF Hosting

Verified Command:

gcloud compute instances create ctfd-host --machine-type=e2-small --preemptible

Step-by-Step Guide:

  1. Use preemptible VMs on GCP or Spot Instances on AWS to reduce costs.
  2. Monitor usage with `docker stats` to avoid over-provisioning.

3. Jacob’s solution hosted 1,300 players for $15/day.

What Undercode Say

Key Takeaways:

  1. Lightweight Alternatives Matter: Avoid over-engineering with Kubernetes for small-scale CTFs. Docker + reverse proxy is often sufficient.
  2. Cost Control: Preemptible instances and efficient Docker usage cut cloud expenses significantly.
  3. Extensibility: The system supports plugins (e.g., Discord alerts) and custom challenge types.

Analysis:

Jacob’s project democratizes CTF hosting for smaller teams, addressing a critical gap in cybersecurity training accessibility. Future enhancements could include auto-scaling and multi-cloud support. As CTFs grow in popularity, such tools will become vital for affordable, scalable cyber education.

Prediction:

Low-cost instancing systems like CTFd-Ployer will empower more organizations to host advanced CTFs, accelerating skill development in red-teaming, cloud security, and exploit mitigation. Expect broader adoption in academia and bootcamps by 2025.

Resources:

IT/Security Reporter URL:

Reported By: Jselli6387 Github – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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