# Client Side Security

> Apply when reviewing or writing browser-facing code that handles user input, renders HTML, sets cookies, or loads third-party scripts. Covers XSS, CSRF, CSP, clickjacking, XS-Leaks, and HTTP security headers.

- Skill: `backspace-shmackspace/client-side-security` (Agent Skill)
- Install (CLI): `npx skillmds@latest add backspace-shmackspace/client-side-security`
- Raw SKILL.md: https://api.skillmd.com/api/skills/backspace-shmackspace/client-side-security/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Web & Frontend
- Author: backspace-shmackspace (https://skillmd.com/u/backspace-shmackspace)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/backspace-shmackspace/client-side-security

---


# Client-Side Web Security

Protect browser clients against code injection, request forgery, UI redress, cross-site leaks, and unsafe third-party scripts with layered, context-aware controls.

## XSS Prevention (Context-Aware)

- **HTML context**: prefer `textContent`. If HTML is required, sanitize with a vetted library (e.g., DOMPurify) and strict allow-lists.
- **Attribute context**: always quote attributes and encode values.
- **JavaScript context**: do not build JS from untrusted strings; avoid inline event handlers; use `addEventListener`.
- **URL context**: validate protocol/domain and encode; block `javascript:` and data URLs where inappropriate.
- **Redirects/forwards**: never use user input directly for destinations; use server-side mapping (ID to URL) or validate against trusted domain allow-lists.
- **CSS context**: allow-list values; never inject raw style text from users.

Example sanitization:

```javascript
const clean = DOMPurify.sanitize(userHtml, {
  ALLOWED_TAGS: ['b','i','p','a','ul','li'],
  ALLOWED_ATTR: ['href','target','rel'],
  ALLOW_DATA_ATTR: false
});
```

## DOM-based XSS and Dangerous Sinks

- Prohibit `innerHTML`, `outerHTML`, `document.write` with untrusted data.
- Prohibit `eval`, `new Function`, string-based `setTimeout`/`setInterval`.
- Validate and encode data before assigning to `location` or event handler properties.
- Use strict mode and explicit variable declarations to prevent global namespace pollution from DOM clobbering.
- Adopt Trusted Types and enforce strict CSP to prevent DOM sink exploitation.

Trusted Types + CSP:

```http
Content-Security-Policy: script-src 'self' 'nonce-{random}'; object-src 'none'; base-uri 'self'; require-trusted-types-for 'script'
```

## Content Security Policy (CSP)

- Prefer nonce-based or hash-based CSP over domain allow-lists.
- Start with Report-Only mode; collect violations; then enforce.
- Baseline to aim for: `default-src 'self'; style-src 'self' 'unsafe-inline'; frame-ancestors 'self'; form-action 'self'; object-src 'none'; base-uri 'none'; upgrade-insecure-requests`.

## CSRF Defense

- Fix XSS first; then layer CSRF defenses.
- Use framework-native CSRF protections and synchronizer tokens on all state-changing requests.
- Cookie settings: `SameSite=Lax` or `Strict`; sessions `Secure` and `HttpOnly`; use `__Host-` prefix when possible.
- Validate Origin/Referer; require custom headers for API mutations in SPA token models.
- Never use GET for state changes; validate tokens on POST/PUT/DELETE/PATCH only. Enforce HTTPS for all token transmission.

## Clickjacking Defense

- Primary: `Content-Security-Policy: frame-ancestors 'none'` or a specific allow-list.
- Fallback for legacy browsers: `X-Frame-Options: DENY` or `SAMEORIGIN`.
- Consider UX confirmations for sensitive actions when framing is required.

## Cross-Site Leaks (XS-Leaks) Controls

- Use `SameSite` cookies appropriately; prefer `Strict` for sensitive actions.
- Adopt Fetch Metadata protections to block suspicious cross-site requests.
- Isolate browsing contexts: COOP/COEP and CORP where applicable.
- Disable caching and add user-unique tokens for sensitive responses to prevent cache probing.

## Third-Party JavaScript

- Minimize and isolate: prefer sandboxed iframes with `sandbox` and postMessage origin checks.
- Use Subresource Integrity (SRI) for external scripts and monitor for changes.
- Provide a first-party, sanitized data layer; deny direct DOM access from tags where possible.
- Govern via tag manager controls and vendor contracts; keep libraries updated.

SRI example:

```html
<script src="https://cdn.vendor.com/app.js"
  integrity="sha384-..." crossorigin="anonymous"></script>
```

## HTML5, CORS, WebSockets, Storage

- **postMessage**: always specify exact target origin; verify `event.origin` on receive.
- **CORS**: avoid `*`; allow-list origins; validate preflights; do not rely on CORS for authorization.
- **WebSockets**: require `wss://`, origin checks, auth, message size limits, and safe JSON parsing.
- **Client storage**: never store secrets in `localStorage`/`sessionStorage`; prefer HttpOnly cookies; if unavoidable, isolate via Web Workers.
- **Links**: add `rel="noopener noreferrer"` to external `target=_blank` links.

## HTTP Security Headers

- **HSTS**: enforce HTTPS everywhere.
- **X-Content-Type-Options**: `nosniff`.
- **Referrer-Policy** and **Permissions-Policy**: restrict sensitive signals and capabilities.

## AJAX and Safe DOM APIs

- Avoid dynamic code execution; use function callbacks, not strings.
- Build JSON with `JSON.stringify`; never via string concatenation.
- Prefer creating elements and setting `textContent`/safe attributes over raw HTML insertion.

## Implementation Checklist

- [ ] Contextual encoding/sanitization for every sink; no dangerous APIs without guards
- [ ] Strict CSP with nonces and Trusted Types; violations monitored
- [ ] CSRF tokens on all state-changing requests; secure cookie attributes
- [ ] Frame protections set; XS-Leak mitigations enabled (Fetch Metadata, COOP/COEP/CORP)
- [ ] Third-party JS isolated with SRI and sandbox; vetted data layer only
- [ ] HTML5/CORS/WebSocket usage hardened; no secrets in web storage
- [ ] Security headers enabled and validated

## Test Plan

- Automated checks for dangerous DOM/API patterns.
- E2E tests for CSRF and clickjacking; CSP report monitoring.
- Manual probes for XS-Leaks (frame count, timing, cache) and open redirect behavior.