hunt-html-injection skill

by elementalsouls·elementalsouls/Claude-BugHunter·4.7k stars·MIT

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.

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Is the hunt-html-injection skill safe?

Clean: nothing in its files matched our rules. We read 1 file in the folder on 2026-09-28.

No findings.

Install the hunt-html-injection skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description.

git clone --depth 1 https://github.com/elementalsouls/Claude-BugHunter.git /tmp/Claude-BugHunter
mkdir -p ~/.claude/skills
cp -r /tmp/Claude-BugHunter/skills/hunt-html-injection ~/.claude/skills/hunt-html-injection

available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

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 , , , , or tags is sufficient.

To PROVE impact unambiguously, escalate to an active vector carrying a unique numeric canary — e.g. "> or . 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 or tags
UI defacement — HACKED renders visually on the page
Credential harvesting via injected login forms
Redirect via
Stepping stone to XSS (may be blocked by WAF on but not )
Dangling-markup exfiltration — even with and event handlers filtered, an unterminated tag can capture page content that follows it. Inject ); the browser treats everything up to the next ' as the URL, leaking any CSRF token, secret, or PII rendered after your injection point to your server. Works where full XSS is blocked but raw < is reflected.
Email/notification-context injection — a field reflected unescaped into a transactional email (signup confirmation, admin alert, support-chat transcript) renders injected //dangling markup in the recipient's inbox — an audience the web UI can't reach, and often the only place HTML is rendered unfiltered. Inject into name/subject/comment, then read the raw email source. Disclosed class: , .

Attack Surface

Any input that is reflected or stored and then displayed in an HTML context:

Search boxes (?q=)
Comments, feedback, reviews
Profile fields (name, bio, username)
Error messages (?error=, ?message=)
Subject / body of contact forms
Admin-visible fields (ticket titles, usernames in logs)

Autonomous Testing Priority

Inject a recognisable HTML tag with a unique canary string. Unescaped angle brackets in the response = confirmed injection.

Pattern 1 — Basic HTML tag injection:

<b>CANARY</b>
"><b>CANARY</b>

Use a unique string as CANARY (something distinct to this test run). Proof: the response contains CANARY with literal < angle brackets — not <b>CANARY. A properly encoded app would escape < to <.

Try multiple tag types when is filtered:

CANARY — heading tag (often less filtered)
— attribute context
click — link injection (phishing proof)

For stored injection: inject into the storage endpoint, then GET the page where the value is displayed and check for unescaped tags.

Escalate immediately: if injection works, try alert(1) — the same unsanitised input may allow full XSS.

Proof

Confirmed when your injected tag appears in the response body with literal < angle brackets (not HTML-encoded). A safe app renders <b>CANARY</b>; a vulnerable app renders CANARY.

Distinguishing HTML Injection from XSS

HTML injection: text renders as text in the browser — no JS execution needed.
XSS: alert(1) executes JavaScript.

Some WAFs block but pass or — start with non-script tags, then escalate.

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