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

Related Skills & Commands

Framework-Specific References

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

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)