# General Frontend Security

> Framework-agnostic frontend security guide based on OWASP Secure Coding Practices: XSS prevention, CSRF protection, Content Security Policy, secure cookie configuration, client-side auth patterns, input validation, secure storage, and security headers. Applies to any web frontend, not only Nuxt. Activates on XSS, CSRF, CSP, clickjacking, security headers, "Sicherheitslücke", or a frontend security audit. NOT for backend/NestJS security (use generating-nest-servers). NOT for Nuxt-specific implementation (use developing-lt-frontend).

- Skill: `lennetech/general-frontend-security` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add lennetech/general-frontend-security`
- Raw SKILL.md: https://api.skillmd.com/api/skills/lennetech/general-frontend-security/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: lenneTech (https://skillmd.com/u/lennetech)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/lennetech/general-frontend-security

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# General Frontend Security

Framework-agnostic security practices for web applications based on OWASP guidelines.

## When to Use This Skill

- Reviewing frontend code for security vulnerabilities
- Implementing client-side authentication flows
- Setting up secure cookie handling
- Configuring Content Security Policy
- Auditing third-party dependencies
- General frontend security questions

## Skill Boundaries

| User Intent | Correct Skill |
|------------|---------------|
| "XSS prevention best practices" | **THIS SKILL** |
| "Security audit of frontend" | **THIS SKILL** |
| "Configure CSP headers" | **THIS SKILL** |
| "Build a secure login page in Nuxt" | developing-lt-frontend |
| "Fix @Restricted decorator in NestJS" | generating-nest-servers |
| "Run npm audit fix" | maintaining-npm-packages |

## Related Skills & Commands

| Command | Purpose |
|---------|---------|
| `/lt-dev:review` | General security review of branch diff (framework-agnostic) |
| `/lt-dev:backend:sec-review` | Security review of backend code changes (auth, decorators, models) |
| `/lt-dev:backend:sec-audit` | Full OWASP security audit (dependencies, config, code) |

## Framework-Specific References

| Framework | Reference File |
|-----------|---------------|
| Nuxt/Vue | See `developing-lt-frontend` skill (reference/security.md) |
| Angular | [angular-security.md](${CLAUDE_SKILL_DIR}/angular-security.md) |

## Key Principles

1. **Never trust client-side validation** - Server must always verify
2. **Store tokens securely** - Memory for access tokens, httpOnly cookies for refresh tokens
3. **Prevent XSS** - Never use `innerHTML` with user input; use `textContent` or DOMPurify
4. **Protect against CSRF** - Use CSRF tokens for state-changing requests + `SameSite` cookies
5. **Configure CSP** - Restrict script/style sources, use nonces, block framing
6. **Minimize dependencies** - Fewer deps = smaller attack surface; always run `pnpm audit`
7. **Load nothing from a foreign host at runtime** - No CDN scripts, fonts, icons or worker files. A third party that serves an asset on every page load can serve different code tomorrow, sees every visitor, and takes the feature down with it when it is unreachable. Ship assets in the build (`import url from "pkg/file?url"`). See the `developing-lt-frontend` skill for the failure this rule comes from.

**Complete OWASP reference with code examples: [owasp-reference.md](${CLAUDE_SKILL_DIR}/owasp-reference.md)**

## Security Checklist

### Development

- [ ] No sensitive data in client-side code
- [ ] Environment variables separated (public vs private)
- [ ] Input validation on all user inputs
- [ ] XSS prevention (no innerHTML with user data)
- [ ] CSRF tokens for state-changing requests

### Authentication

- [ ] Tokens stored securely (memory + httpOnly cookies)
- [ ] Token refresh mechanism implemented
- [ ] Proper logout (clear all client state)
- [ ] Session timeout configured

### Configuration

- [ ] No third-party hosts in the built app (check the network tab; `grep -rnoE "https?://" app/`)
- [ ] HTTPS enforced
- [ ] CSP headers configured
- [ ] Security headers set (X-Frame-Options, etc.)
- [ ] Cookies configured with secure flags
- [ ] CORS properly restricted

### Dependencies

- [ ] pnpm audit clean (or accepted risks)
- [ ] pnpm-lock.yaml committed
- [ ] SRI for external resources
- [ ] Regular dependency updates

### Build & Deploy

- [ ] Debug mode disabled
- [ ] Console logs removed
- [ ] Source maps disabled or restricted
- [ ] Error messages generic (no stack traces)