What is sysadmin-security?

A security-audit and hardening guide for Debian and Ubuntu Linux computers and servers. Hardening means reducing common ways an unauthorised person could gain access.

Use it to inspect or secure Linux user access, SSH settings, firewall rules, open services, file permissions, software updates, and backup practices.

It helps identify weaknesses in accounts, SSH, firewalls, services, permissions, updates, and backups while explaining possible lock-out risks before changes.

sysadmin-security — Skill for Claude Code and Codex by HermeticOrmus

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 HermeticOrmus/linux-sysadmin-skills --skill sysadmin-security

Clone the repo

git clone --depth 1 https://github.com/HermeticOrmus/linux-sysadmin-skills

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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/hermeticormus/linux-sysadmin-skills/sysadmin-security/github.svg)](https://agentmods.dev/skills/hermeticormus/linux-sysadmin-skills/sysadmin-security)

Your own site

<a href="https://agentmods.dev/skills/hermeticormus/linux-sysadmin-skills/sysadmin-security"><img src="https://agentmods.dev/badge/skills/hermeticormus/linux-sysadmin-skills/sysadmin-security/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.

Your own site · 80×15

<a href="https://agentmods.dev/skills/hermeticormus/linux-sysadmin-skills/sysadmin-security"><img src="https://agentmods.dev/badge/skills/hermeticormus/linux-sysadmin-skills/sysadmin-security.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 57 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 614 The whole file, excluding the scripts and references it only reads on demand.

Security scan B 2 findings. A grade says what 26 rules found in the file — not that it is safe.

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.

Model	Per session	Once invoked
Fable 5.1	$0.00057	$0.00614
Opus 5.5	$0.00023	$0.00246
Sonnet 5.5	$0.00011	$0.00123
Haiku 4.5	$0.00006	$0.00061

Measured 1mo ago against content hash 192c77ec0980, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade B, and why

sysadmin-security scanned grade B 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.

Asks for rootlowPrivilege escalation

A mod that escalates privileges can change anything on the machine, not only the project.

- Verify sudo permissions.

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

Reaches for credential filesmediumPrivilege escalation

SSH keys, cloud credentials, git-credentials, .npmrc, /etc/shadow: reading these is how a config file becomes a credential leak.

- Review `~/.ssh/authorized_keys`.

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

skills/sysadmin-security/SKILL.md · 101 lines

How it starts

The opening of the file, as written. The whole thing — 101 lines — stays where its author put it; the contents beside it link to each section on GitHub.

System security audit

Audit and improve the security of a Debian/Ubuntu Linux machine. Work through the checklist below interactively. Inspection is read-only and runs freely; every change is explained first, including how it could lock you out, and waits for confirmation.

Security checklist

1. User and access control

Review user accounts: who should have access?
Check for unused accounts.
Verify sudo permissions.
Review SSH keys.
Check password policies.
Disable root login over SSH.

2. SSH hardening

Disable password authentication in favor of keys.
Change the default SSH port (optional).
Configure fail2ban.
Limit SSH access by user or IP.
Use SSH key passphrases.
Review ~/.ssh/authorized_keys.

3. Firewall

Enable UFW (Uncomplicated Firewall).
Configure allowed ports.
Block unnecessary services.
Review current rules.
Set a default-deny policy.

4. Updates and patches

Check for system updates with apt update.
Enable automatic security updates.
Review update history.
Check for end-of-life software.

5. Services and ports

List running services with systemctl.
Disable unnecessary services.
Review open ports with ss -tlnp.
Check listening services.
Verify service permissions.

6. File permissions

Check sensitive file permissions (/etc/passwd, /etc/shadow).
Review home directory permissions.
Check SUID/SGID files.
Verify critical system files.

7. Logs and monitoring

Review system logs with journalctl.
Check authentication logs.
Monitor failed login attempts.
Set up log rotation.
Consider log monitoring tools.

8. Application security

Review installed packages with apt list --installed.
Remove unnecessary software.
Check for vulnerable packages.
Update applications.
Verify sources (PPAs, repositories).

9. Network security

Review network interfaces.
Check the routing table.
Verify DNS settings.
Review /etc/hosts.
Check for suspicious connections.

Read the full file on GitHub · 101 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.

1mo ago First seen · 101 lines · 57 tokens per session scan B 192c77ec0980

Subscribe to this mod's changes

sysadmin-security is a skill published in the GitHub repository HermeticOrmus/linux-sysadmin-skills (6 stars, last pushed 4mo ago), licensed MIT. It adds 57 tokens to every session and 614 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it B with 2 findings (asks for root, reaches for credential files). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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