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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.

skills/client-side-security/SKILL.md

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Aug 24, 2026
License
MIT

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# 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: