---
name: security-guidance
description: "Use when installing or tuning the PreToolUse security hook that warns before Claude writes command injection, XSS, SQL injection, unsafe deserialization, eval or GitHub Actions injection. Warns once per file and rule; ENABLE_SECURITY_REMINDER=0 turns it off."
---
# Security Guidance Hook
**A PreToolUse hook that blocks 12 common security anti-patterns before Claude Code writes them.**
This skill is a **hook**, not a slash command. Once installed, it runs automatically before every `Edit`, `Write`, or `MultiEdit` operation and warns + blocks if it detects a known dangerous pattern.
## What It Catches
The hook scans both:
- **The file path being edited** — flags GitHub Actions workflow files with risky `${{ }}` patterns
- **The content being written** — substring matches against 11 anti-patterns
| Pattern | Category | Risk |
|---|---|---|
| GitHub Actions workflow expressions | Path-based | Workflow command injection via untrusted inputs |
| `child_process.exec`, `exec(`, `execSync(` | Substring | Node.js command injection |
| `new Function` | Substring | JS code injection |
| `eval(` | Substring | JS code injection |
| `dangerouslySetInnerHTML` | Substring | React XSS |
| `document.write` | Substring | DOM XSS |
| `.innerHTML =` | Substring | DOM XSS |
| `pickle` | Substring | Python deserialization RCE |
| `os.system`, `from os import system` | Substring | Python command injection |
| `shell=True` (subprocess) | Substring | Python command injection |
| f-string SQL or `.format` SQL | Substring | SQL injection |
| `yaml.load(`, `yaml.unsafe_load` | Substring | YAML deserialization RCE |
## How It Works
1. Claude Code is about to run `Edit`, `Write`, or `MultiEdit`
2. PreToolUse hook fires → invokes `security_reminder_hook.py` with the tool input as JSON on stdin
3. The hook extracts file_path + content + checks against the pattern table
4. If a pattern matches AND this warning hasn't been shown for this file+rule in this session:
- Print the warning to stderr (Claude sees it)
- Exit code 2 → blocks the tool call
- Save the warning key to `~/.claude/security_warnings_state_<session>.json`
5. If a pattern matches BUT the warning was already shown this session:
- Allow the tool call (exit code 0) — Claude already saw the warning once
6. If no pattern matches:
- Allow the tool call (exit code 0)
## Installation
This plugin ships as a Claude Code plugin with `hooks.json` wiring:
```bash
# In Claude Code:
/plugin marketplace add alirezarezvani/claude-skills
/plugin install security-guidance@claude-code-skills
```
Once installed, no further configuration needed — the hook runs automatically.
## Configuration
Disable per-session via environment variable:
```bash
ENABLE_SECURITY_REMINDER=0 claude
# Hook is bypassed for this session
```
Use sparingly — the hook is most useful exactly when you're tempted to disable it (because you're under deadline pressure to ship something you know is sketchy).
## Per-File Override Pattern
If a specific file legitimately needs `eval()` or `pickle` (e.g., a sandboxed REPL, a deliberately unsafe parser for a fuzzer), document it in the file with a comment:
```python
# SAFETY: pickle is the required serialization format for this internal tool.
# This file does NOT accept untrusted input. See SECURITY.md for boundary analysis.
import pickle
```
The hook will still warn on first edit per session. After acknowledging, subsequent edits in the same session are allowed (session-state caching).
## Why The Patterns Are Substring-Based (Not AST-Based)
Trade-off: AST-based detection would be more precise (no false positives on string literals containing "eval("). Substring-based is:
- **Faster** — runs in ms, doesn't parse the file
- **Cross-language** — same hook works for JS/TS/Python/YAML/etc.
- **Conservative** — false positives are easy to dismiss (one keystroke); false negatives are dangerous
For 90%+ of cases, substring detection is sufficient. If you need stricter detection, layer in a proper SAST tool (semgrep, CodeQL) as a CI step.
## State Files
The hook caches "warning shown" state in `~/.claude/security_warnings_state_<session_id>.json`. These files:
- Are auto-cleaned after 30 days (10% chance per hook invocation)
- Are session-scoped (each Claude session gets its own)
- Contain a JSON list of `<file_path>-<rule_name>` keys
You can safely delete `~/.claude/security_warnings_state_*.json` files at any time — the hook regenerates them on next run.
## Debug Log
The hook writes to `~/.claude/security-warnings-log.txt` for debugging hook misfires:
```bash
tail -f ~/.claude/security-warnings-log.txt
# Shows JSON decode errors, state-file save failures, etc.
```
(Upstream version wrote to `/tmp/security-warnings-log.txt` — we moved it to `~/.claude/` for persistence across reboots.)
## Source + Attribution
This plugin is ported from David Dworken's MIT-licensed implementation in [`alirezarezvani/aeo-box`](https://github.com/alirezarezvani/aeo-box/tree/main/.claude/plugins/security-guidance).
**Verbatim:** the original 9 patterns (GitHub Actions, child_process.exec, new Function, eval, dangerouslySetInnerHTML, document.write, innerHTML, pickle, os.system) are preserved with their exact warning text.
**Modifications:**
- Added 3 patterns: `subprocess shell=True`, SQL injection via f-string or `.format`, `yaml.unsafe_load`
- Debug log moved from `/tmp/security-warnings-log.txt` → `~/.claude/security-warnings-log.txt`
- Restructured as a claude-skills plugin with `attribution` block in `.claude-plugin/authoring-notes.json` (originally in `plugin.json`; relocated when issue #954 showed Claude Code rejects manifests carrying extension keys)
## Anti-Patterns
### Disabling the hook by default
Defeats the purpose. If `ENABLE_SECURITY_REMINDER=0` becomes your default, you've trained yourself to ignore the safety net. Use it only for specific verified-safe operations.
### Modifying the pattern list without security review
Anyone can add a pattern. Removing one requires a security review — patterns exist because they map to real CVE classes.
### Treating session-state as immutable security policy
The cache prevents nag-spam but is per-session. Don't rely on "I dismissed this once" as long-term policy — use the per-file documentation pattern instead (comment justifying the use).
## Related Skills
- `engineering-team/skills/red-team` — adversarial pen-testing
- `engineering-team/skills/threat-detection` — threat modeling + detection design
- `engineering-team/skills/ai-security` — AI-specific security (prompt injection, etc.)
- `engineering/ship-gate` — pre-production audit (8-category, ~89 checks)
- `engineering/skill-security-auditor` — security scan for skill packages
## Trigger Phrases
- "add security hook"
- "block unsafe code before write"
- "detect command injection"
- "prevent SQL injection patterns"
- "warn on eval / pickle / os.system"
- "GitHub Actions security hook"
---
**Version:** 2.7.3
**Source:** Ported from [`alirezarezvani/aeo-box`](https://github.com/alirezarezvani/aeo-box) `.claude/plugins/security-guidance/` (originally by David Dworken at Anthropic, MIT)
**License:** MIT
## More in this skill
Load these only when the task needs them.
- [references/pretooluse_hook_canon.md](references/pretooluse_hook_canon.md) — PreToolUse Hook Discipline — When To Block, When To Warn