Individual skill

hunt-html-injection

Hunt HTML Injection — user-supplied input is rendered as raw HTML in the response without sanitisation, allowing an attacker to inject arbitrary HTML tags (but not necessarily JavaScript). Lower severity than XSS but enables phishing, UI manipulation, and credential harvesting via injected forms. Use when testing text-display surfaces (search results, profile fields, comments, error messages, feedback forms). For markup that executes JavaScript, escalate to hunt-xss.

knowledge/Claude-BugHunter/skills/hunt-html-injection/SKILL.md

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Last checked: Nov 4, 2026

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License: Apache-2.0

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## What is HTML Injection

HTML Injection occurs when user input is inserted into a page's HTML without escaping, so injected tags are rendered by the browser as markup rather than displayed as literal text. Unlike XSS, the injected content does not require JavaScript execution — injecting `<b>`, `<h1>`, `<a>`, `<img>`, or `<form>` tags is sufficient.

**To PROVE impact unambiguously, escalate to an active vector carrying a unique numeric canary** — e.g. `"><img src=x onerror=alert(91234)>` or `<svg onload=alert(91234)>`. A distinctive 4+ digit number (not `alert(1)`) distinguishes YOUR reflected injection from the example payloads practice pages embed in their own hint text. Proof = the raw, unescaped vector with your canary appears in the response.

**Impact:**
- Phishing via injected `<form>` or `<a href="attacker.com">` tags
- UI defacement — `<h1>HACKED</h1>` renders visually on the page
- Credential harvesting via injected login forms
- Redirect via `<meta http-equiv="refresh">`
- Stepping stone to XSS (may be blocked by WAF on `<script>` but not `<img onerror>`)

## Attack Surface