hunt-xss skill

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

Hunting skill for xss vulnerabilities. Built from 174 public bug bounty reports. Use when hunting xss on any target. For markup injection that reflects raw HTML but does NOT execute JavaScript (no `<script>`/event-handler execution), see hunt-html-injection — escalate here once script execution is possible.

F30/100content scan

Is the hunt-xss skill safe?

A critical finding: do not install it without reading the flagged line. We read 1 file in the folder on 2026-09-28.

criticalSKILL.md:322
Decodes hidden content and executes it: the real instructions are not readable in the file.
```
<svg onload=eval(atob('YWxlcnQoMSk='))>
```

Install the hunt-xss 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. Read the findings above first.

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-xss ~/.claude/skills/hunt-xss

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

Autonomous Testing Priority

Verify reflection before claiming XSS — encoding is everything.

Your payload must appear in the response body with angle brackets UNESCAPED. is XSS. <script> is safe encoding — not vulnerable.

Use a UNIQUE NUMERIC CANARY in your proof payload — e.g. alert(91234) or ">. Pick a distinctive 4+ digit number, not alert(1). Practice pages are full of example payloads like alert(1)/alert('XSS') in their hint text; a unique number is how you tell YOUR reflected payload apart from the page's decoy examples. Proof = your alert() shows up in the response with raw, unescaped angle brackets.

Try these contexts in order:

Inline script injection (works when HTML context allows new tags):

<script>alert(CANARY)</script>

Use whatever canary string your proof contract specifies. Confirmed when alert(CANARY) appears literally (not HTML-encoded) in the response.

Attribute event injection (when < is filtered but attributes are injectable):

" onmouseover="alert(CANARY)
   " onerror="alert(CANARY)
   " onload="alert(CANARY)

URL/href context:

javascript:alert(CANARY)

Distinguishing success from failure:

Vulnerable: response contains alert( unescaped — browser would execute it
Filtered/safe: response contains <script> or <script> — properly encoded
Blocked: response is an error, or the reflected value is absent entirely

For stored XSS: inject into a field that other pages display (comments, usernames, ticket titles). Then fetch the rendering page and check for unescaped payload. The payload executes when any user views that page — higher severity than reflected.

Crown Jewel Targets

XSS is high-value when it combines privileged context + persistent delivery + scope escalation. The highest payouts come from:

Admin panels and authenticated dashboards (e.g., /admin, /settings) — attacker can hijack sessions with elevated privileges, exfiltrate tokens, or pivot to account takeover
Payment/financial flows (paypal.com, checkout pages, currency converters) — XSS here enables credential harvesting and financial fraud at scale
Stored XSS in collaborative features (wikis, markdown renderers, issue trackers, RDoc, labels, tags) — one payload infects every viewer, multiplying impact
SSO/signin pages (e.g., paypal.com/signin) — XSS here is critical because it can steal auth tokens across the entire platform
Shared SaaS tenant surfaces (.myshopify.com, api.collabs.) — XSS in one tenant's context can bleed across tenant boundaries
Help/documentation sites (help.shopify.com) — lower severity individually, but often have looser sanitization and trusted user perception
SVG/file upload endpoints — frequently bypasses CSP and sanitization simultaneously

Asset types that pay most: Main product domains > Admin subdomains > API endpoints > Marketing/help sites

OOB-Or-It-Didn't-Happen Gate (Blind / Stored XSS)

For blind and stored XSS — claims require an out-of-band confirmation, the same as blind SSRF. The OOB receiver fires when the payload actually executes in a browser somewhere (an admin reviewing logs, a SOC analyst opening a ticket, an email rendering a stored payload).

What is NOT confirmation

ASP.NET request validator rejected your < and returned a different status code → not XSS, that's WAF noise.
Your payload appears in the response body URL-encoded or HTML-encoded → not XSS, that's correct output encoding.
The form action attribute contains your payload string as %22onclick%3D… → not XSS, the browser does NOT decode URL encoding inside HTML attribute values; the %22 stays as literal %22 in the DOM.
Your tag appears in the response as <script> → not XSS, that's escaping.

What IS confirmation

A request to your unique Collaborator subdomain (e.g., bxss-err-..oastify.com) arrives in the OOB listener after your payload was stored / reflected / queued.
For stored XSS: the request arrives hours or days later when an admin views the affected resource. Plant payloads early in the engagement and keep the listener open.
The User-Agent of the firing request is a browser (Mozilla/Chrome), not the server's own backend HTTP client.

Where to plant blind-XSS beacons

Any field whose value might be viewed in an admin UI / log viewer / email / report later:

Error messages (?ErrorMessage=./x')>)
Auth-flow source params (?Source=, ?ReturnUrl=)
Login form username field (admin may view audit logs of failed logins)
User-Agent header (some SOC consoles render UA as HTML)
Referer header (some analytics dashboards render Referer as HTML)
Email addresses on registration / contact forms
File-upload filenames

Always sub-tag the Collaborator subdomain by sink so callbacks identify which field fired.

Lesson from a authorized engagement: 10 blind-XSS Collaborator beacons planted across ErrorMessage, Source, the Authentication.asmx username field, User-Agent header, Referer header, and request paths. Zero callbacks over a 10-minute polling window. Conclusion: the SharePoint SOC views logs / errors in tooling that does not render HTML, AND the ASP.NET request validator blocks < in query strings before the payload reaches storage. Stored-XSS claim correctly retracted.

Attack Surface Signals

URL Patterns:

/admin*
/settings*
/wiki*
/reports*
?utm_source=
?redirect=
?q=
?search=
?callback=
?return_url=
/render*
/preview*
/documentation*

Response Headers (weak defense signals):

Content-Type: text/html (without nosniff)
Content-Security-Policy: (absent or using unsafe-inline)
Content-Type: image/svg+xml (CSP often not applied)
X-XSS-Protection: 0

JS Patterns in source that signal DOM XSS:

document.write(
innerHTML =
location.hash
location.search
location.href
document.referrer
eval(
setTimeout(string,
setInterval(string,
$.html(
$(location

Tech Stack Signals:

Rails applications using html_safe, raw, translate, Action Text, or ActionView sanitize helpers
GitLab/GitHub markdown pipelines (Banzai, Kramdown, RDoc, Kroki)
Applications allowing SVG uploads or rendering
Sites using style tag in allowlists
Kroki/Mermaid/PlantUML diagram rendering endpoints
Cache layers in front of authenticated pages (cache poisoning vector)

Step-by-Step Hunting Methodology

Map all reflection points — Spider the target and identify every place user input appears in HTML output. Prioritize: URL parameters, form fields, HTTP headers (User-Agent, Referer), file upload names/contents, and API response fields rendered in UI.

Classify by type — Determine if each reflection is Reflected (URL param → response), Stored (database → later rendering), or DOM-based (JS reads URL/storage → DOM sink). Each requires different payload delivery.

Probe sanitizer behavior — Send harmless canary strings first: aaa"bbb'ccc<ddd to determine which characters are escaped. Observe if output is in HTML context, attribute context, JS context, or URL context.

Marker Discipline: When choosing canary strings, they MUST be unique random alphanumeric strings (8+ chars, no English words, no protocol keywords). Bad markers: test, marker, evil, attacker, payload, javascript, script. Good markers: cpmark987abc, x4hd2k9pq, ZZMARKERZZ. Before claiming reflection, search the baseline (no-marker) response for the marker — if it appears naturally in the page (e.g., the word javascript is in every page's help-link hrefs), it's a false-positive trap and you need a different marker. This single check catches 80% of false-positive reflection reports.

Test allowlisted tag combinations — If a sanitizer is in use, probe for dangerous tag combos: +, +, , with expressions.

Hunt SVG and file upload vectors — Upload SVG files containing tags. Check Content-Type response header. Test if CSP applies to SVG responses separately.

Test markdown/documentation renderers — In wiki, README, or doc fields, try: text), inline HTML injection, Kroki/Mermaid payloads, RDoc link:javascript: syntax.

Check redirect parameters — Test ?redirect=javascript:alert(1) and ?return_url=//evil.com — look for single-click XSS via improper redirect sanitization.

Probe UTM and analytics parameters — utmsource, utmmedium, utm_campaign are often reflected without sanitization on marketing pages.

Test CSP bypass opportunities — If CSP is present, look for: JSONP endpoints on allowed domains, unsafe-inline in style-src, SVG that bypasses script-src, script gadgets on whitelisted CDNs.

Attempt stored XSS in profile/metadata fields — Username, bio, tag names, label colors, organization names — these render in many contexts and often have weaker validation.

Check cache poisoning — Test if reflected XSS payloads can be cached and served to other users (especially on CDN-fronted pages), transforming reflected XSS into stored-equivalent.

Validate in target browser — Always confirm in a real browser before reporting. Many payloads echo back in Burp but fail to execute in a real browser due to CSP, output encoding, framework auto-escaping, context mismatch, WAF normalization, or browser HTML-parsing differences. (Note: Chrome's XSS Auditor was removed in Chrome 78 / Oct 2019 and no shipping browser has one — never attribute a failed PoC to an "XSS auditor".)

Payload & Detection Patterns

Basic context probing:

aaa"bbb'ccc<ddd>eee`fff

Reflected XSS — URL parameter baseline:

?q=<script>alert(document.domain)</script>
?q="><script>alert(1)</script>
?utm_source=<svg onload=alert(1)>
?redirect=javascript:alert(document.domain)

Attribute context escapes:

" onmouseover="alert(1)
' onmouseover='alert(1)
`onmouseover=alert(1)

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