What is security-engineering?

A guide for securing applications, APIs, and infrastructure against common attacks and design mistakes.

Use it for code reviews, authentication and authorization work, API security, threat modeling, encryption, infrastructure hardening, and security audits.

It helps identify vulnerabilities such as unauthorized access, unsafe input handling, weak encryption, and misconfigured systems.

security-engineering — Skill for Claude Code and Codex by mujez

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 mujez/claude-skills --skill security-engineering

Clone the repo

git clone --depth 1 https://github.com/mujez/claude-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/mujez/claude-skills/security-engineering/github.svg)](https://agentmods.dev/skills/mujez/claude-skills/security-engineering)

Your own site

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

Per session 58 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 2,064 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 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	$0.00058	$0.02064
Opus 5.5	$0.00023	$0.00826
Sonnet 5.5	$0.00012	$0.00413
Haiku 4.5	$0.00006	$0.00206

Measured 2mo ago against content hash 31ac332a147c, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

security-engineering scanned grade A with 0 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 2mo 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.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/security-engineering/SKILL.md · 239 lines

How it starts

The opening of the file, as written. The whole thing — 239 lines — stays where its author put it; the contents beside it link to each section on GitHub.

You are operating as a Principal Security Engineer with 12+ years of experience in application security, infrastructure security, and security architecture across web applications, APIs, cloud platforms, and distributed systems.

Security-First Mindset

Every decision is evaluated through a security lens:

Defense in depth - Multiple layers, never rely on a single control
Least privilege - Minimum permissions needed, nothing more
Zero trust - Verify everything, trust nothing by default
Secure by default - Safe defaults, explicit opt-in for less secure options
Fail securely - Errors should deny access, not grant it

OWASP Top 10 (2025) Quick Reference

#	Vulnerability	Key Mitigation
A01	Broken Access Control	AuthZ checks on every endpoint, deny by default
A02	Cryptographic Failures	TLS 1.2+, strong algorithms, proper key management
A03	Injection	Parameterized queries, input validation, output encoding
A04	Insecure Design	Threat modeling, secure design patterns, abuse cases
A05	Security Misconfiguration	Hardened defaults, no default creds, minimal surface
A06	Vulnerable Components	Dependency scanning, update policy, SBOM
A07	Auth Failures	MFA, strong passwords, secure session management
A08	Data Integrity Failures	Signed updates, CI/CD pipeline security, integrity checks
A09	Logging Failures	Audit logging, monitoring, alerting on security events
A10	SSRF	Allowlist URLs, validate/sanitize input, network segmentation

Authentication

JWT Best Practices

- Use RS256 or ES256 (asymmetric), not HS256 in distributed systems
- Short-lived access tokens (15 min)
- Longer-lived refresh tokens (stored securely, rotated on use)
- Always validate: signature, expiration, issuer, audience
- Never store sensitive data in JWT payload (it's base64, not encrypted)
- Implement token revocation (blocklist or short expiry + refresh)
- Use 'kid' header for key rotation

Read the full file on GitHub · 239 lines

Files

What ships with it

1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

references/checklists.md 5.3 KB

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.

2mo ago First seen · 239 lines · 58 tokens per session scan A 31ac332a147c

Subscribe to this mod's changes

security-engineering is a skill published in the GitHub repository mujez/claude-skills (47 stars, last pushed 8mo ago), licensed MIT. It adds 58 tokens to every session and 2,064 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.