harden-vps

harden-vps is a skill for Claude Code, Codex from danielvm-git/bigpowers. It costs 99 tokens per session (3,065 once invoked), scanned A, original, MIT.

A checklist and setup for securing a production Linux VPS, which is a rented server used to run an application. It covers the operating system, the application service, and the VPS provider’s backup features.

In plain words
What is it for?
Use it to configure Ubuntu’s firewall, fail2ban for blocking repeated SSH attacks, automatic updates, safer SSH settings, systemd service protections, monitoring alerts, backups, health checks, and snapshots.
Why use it?
It helps close common server security gaps, such as an open firewall, repeated SSH login attempts, missing automatic updates, and missing backups.

Skill for Claude CodeCodex

Written for no agent in particular: nothing here depends on one. Also seen: model in frontmatter; positional $N argument.

Good fit Use it to configure Ubuntu’s firewall, fail2ban for blocking repeated SSH attacks, automatic updates, safer SSH settings, systemd service protections, monitoring alerts, backups, health checks, and snapshots.

Compare 6 skills from other repositories ↓
Install with agentmods
npx agentmods add skills/danielvm-git/bigpowers/harden-vps
Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent
npx skills add danielvm-git/bigpowers --skill harden-vps
Clone the repo
git clone --depth 1 https://github.com/danielvm-git/bigpowers

Made for: Claude Code, Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for harden-vps

README.md
[![agentmods](https://agentmods.dev/badge/skills/danielvm-git/bigpowers/harden-vps/github.svg)](https://agentmods.dev/skills/danielvm-git/bigpowers/harden-vps)
Your own site
<a href="https://agentmods.dev/skills/danielvm-git/bigpowers/harden-vps"><img src="https://agentmods.dev/badge/skills/danielvm-git/bigpowers/harden-vps/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for harden-vps

Your own site · 80×15
<a href="https://agentmods.dev/skills/danielvm-git/bigpowers/harden-vps"><img src="https://agentmods.dev/badge/skills/danielvm-git/bigpowers/harden-vps.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 99 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 3,065 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 2 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits
  • NVIDIA SkillSpector warn 7 Sept 2026
SkillSpector: 9 findings, up to high

These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →

  • high Privilege Escalation · line 76
    Code accesses credential files (SSH keys, AWS credentials, etc.). This could indicate credential theft attempts.
    Fix: Remove references to credential paths. Use environment variables or secrets managers. For docs, use placeholder paths (e.g., /path/to/config). Never load .env or token files in production code paths.
  • high Privilege Escalation · line 82
    Code accesses credential files (SSH keys, AWS credentials, etc.). This could indicate credential theft attempts.
    Fix: Remove references to credential paths. Use environment variables or secrets managers. For docs, use placeholder paths (e.g., /path/to/config). Never load .env or token files in production code paths.
  • high Privilege Escalation · line 151
    Code accesses credential files (SSH keys, AWS credentials, etc.). This could indicate credential theft attempts.
    Fix: Remove references to credential paths. Use environment variables or secrets managers. For docs, use placeholder paths (e.g., /path/to/config). Never load .env or token files in production code paths.
  • high Privilege Escalation · line 208
    Code accesses credential files (SSH keys, AWS credentials, etc.). This could indicate credential theft attempts.
    Fix: Remove references to credential paths. Use environment variables or secrets managers. For docs, use placeholder paths (e.g., /path/to/config). Never load .env or token files in production code paths.
  • medium Excessive Agency · line 3
    Skill selects an external model or provider that may use a different account or billing plan than the operator expects. Undisclosed model switches can cause unexpected cost or quota consumption.
    Fix: Remove the model/provider override or disclose it prominently and require explicit operator approval before invoking an external coding CLI or billed model.
  • medium Data Exfiltration · line 71
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Rogue Agent · line 102
    Skill establishes unauthorized persistence across sessions via cron jobs, startup scripts, or state files. Session persistence allows an attacker to maintain access beyond the current interaction.
    Fix: Remove any persistence mechanisms (cron jobs, startup scripts, state files). Skills should not maintain state across sessions without explicit user consent.
  • medium Data Exfiltration · line 216
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 230
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
How audits are shown
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.00099 $0.03065
Opus 5.5 $0.00040 $0.01226
Sonnet 5.5 $0.00020 $0.00613
Haiku 4.5 $0.00010 $0.00307

Measured 1mo ago against content hash f7f584e16eaf, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

harden-vps scanned grade A with 2 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Sends data to an external URLlowData exfiltration

A POST to an outside endpoint may be telemetry or may be exfiltration; either way the mod talks to somewhere, and you should know where.

TOKEN=$(curl -s -d "client_id=$CONTABO_CLIENT_ID" \

Downgraded: this mod is about security review, or the phrase is quoted, so it is likely naming the pattern rather than instructing it.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

curl -sL "$(curl -sL https://api.github.com/repos/contabo/cntb/releases/latest \
.cline/skills/harden-vps/SKILL.md · 292 lines

How it starts

The opening of the file, as written. The whole thing — 292 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Harden VPS

Three-layer production hardening for a self-hosted app on any VPS. Each layer independently verifiable. Apply OS first (firewall blocks attacks immediately), then your application (alerts + backups), then your VPS provider (snapshots). See REFERENCE.md for full script bodies, systemd unit template, and gotchas.

Quick start

SSH as root into the VPS. Find credentials in the your VPS provider Customer Control Panel.

HARD GATE — Run ufw status first. No firewall = layer 1 takes priority over everything.

Layer 1 — Ubuntu OS

# UFW
ufw default deny incoming && ufw default allow outgoing
ufw allow 22/tcp && ufw allow 80/tcp && ufw allow 443/tcp && ufw enable
# → verify: ufw status | grep -q active

# fail2ban
apt install -y fail2ban
# Configure /etc/fail2ban/jail.local: sshd, maxretry=3, bantime=3600, findtime=600
systemctl restart fail2ban
# → verify: fail2ban-client status sshd

# unattended-upgrades
apt install -y unattended-upgrades && dpkg-reconfigure -plow unattended-upgrades
# → verify: systemctl is-active unattended-upgrades | grep -q active

# SSH: PermitRootLogin no, PasswordAuthentication no, PubkeyAuthentication yes
# → verify: sshd -T | grep -E 'permitrootlogin no|passwordauthentication no'

# Deploy healthcheck.sh → /opt/your-app/scripts/healthcheck.sh
# Crontab: */5 * * * * /opt/your-app/scripts/healthcheck.sh

Layer 2 — applicationlication

# systemd: User=your-app, NoNewPrivileges=yes, ProtectSystem=full,
#   ProtectKernelTunables=yes, ProtectKernelModules=yes,
#   ProtectControlGroups=yes, RestrictAddressFamilies=AF_INET AF_INET6,
#   RestrictRealtime=yes, PrivateTmp=yes, LimitNOFILE=65536
# → verify: systemctl show your-app -p NoNewPrivileges -p ProtectSystem -p User

# Alerts (your application requires auth; insert via SQLite)
sqlite3 /opt/your-app/data/your-app.db "
INSERT INTO monitoring_alerts (id,name,metric,threshold,operator,enabled,duration_seconds)
VALUES ('a1','Disk >80%','disk_used_percent',80,'gt',1,300);
INSERT INTO monitoring_alerts (id,name,metric,threshold,operator,enabled,duration_seconds)
VALUES ('a2','CPU >90%','cpu_percent',90,'gt',1,60);
INSERT INTO monitoring_alerts (id,name,metric,threshold,operator,enabled,duration_seconds)
VALUES ('a3','RAM >85%','mem_used_percent',85,'gt',1,120);
"
systemctl restart your-app

# Backup crontab (root):
# 0 2 * * * cp /opt/your-app/data/your-app.db /backup/your-app-$(date +\%Y\%m\%d).db
# 0 3 * * * find /backup/ -name "your-app-*.db" -mtime +90 -delete

Read the full file on GitHub · 292 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 1mo ago First seen · 292 lines · 99 tokens per session scan A f7f584e16eaf

Subscribe to this mod's changes

harden-vps is a skill published in the GitHub repository danielvm-git/bigpowers (273 stars, last pushed 1mo ago), licensed MIT. It adds 99 tokens to every session and 3,065 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 2 findings (sends data to an external url, makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

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