{"id":"CVE-2026-113659","aliases":["GHSA-m2m5-rw3w-cwmj"],"title":"ApostropheCMS: Client-side Prototype Pollution in apos.http.parseQuery leading to DOM-based XSS against authenticated users","summary":"ApostropheCMS: Client-side Prototype Pollution in apos.http.parseQuery leading to DOM-based XSS against authenticated users","severity":"medium","cvss":6.1,"cwe":["CWE-79","CWE-1321"],"vendor":"apostrophe","product":"apostrophe","ecosystem":"npm","affected":["apostrophe < 4.33.0"],"patched":["apostrophe 4.33.0"],"published":"2026-10-20","updated":"2026-10-20","sourceUpdated":"2026-10-20T18:18:52Z","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-m2m5-rw3w-cwmj","references":[{"url":"https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-m2m5-rw3w-cwmj"},{"url":"https://github.com/apostrophecms/apostrophe/commit/9500b8b0e1d630d5e07fbb5fd2c25ed0943845ce"},{"url":"https://github.com/apostrophecms/apostrophe/commit/f84fd41cbf6c22eb8d0008fc9c25df1f7130b350"},{"url":"https://github.com/apostrophecms/apostrophe/releases/tag/apostrophe@4.33.0"},{"url":"https://github.com/advisories/GHSA-m2m5-rw3w-cwmj"}],"tags":["ghsa","npm","cve.org"],"ingestedAt":"2026-10-20T18:14:37.225Z","slug":"CVE-2026-113659","body":"## Overview\n\n## Summary\n\nThe client-side helper `apos.http.parseQuery()` in ApostropheCMS builds a nested object\nfrom a URL query string without sanitizing the keys `__proto__`, `constructor`, or\n`prototype`. Parsing an attacker-controlled query string therefore writes arbitrary\nproperties onto `Object.prototype` (client-side prototype pollution).\n\nApostropheCMS's admin UI calls this parser with `window.location.search` on **every\nauthenticated page load** (the admin context bar's `data()` initializer). Because the admin\nUI is built on Vue, a polluted, enumerable property on `Object.prototype` is then picked up\nby Vue's standard element-mount routine and written to the DOM as an element property,\nturning the prototype pollution into DOM-based XSS in the victim's authenticated session.\nThe escalation gadget is a well-known, general Vue behavior; the novel,\nreportable defect is the unsanitized `parseQuery` source together with the authenticated\ntrigger, both of which are ApostropheCMS code.\n\nAny user who can get **any logged-in user (from the lowest-privileged `guest` role up to the highest-privileged `admin` role)** to\nopen a crafted URL can execute arbitrary JavaScript in that user's session.\n\nThis is **distinct from CVE-2026-53609** (server-side prototype pollution via\n`apos.util.set()` / `$pullAll`, fixed in 4.31.0): different side (client vs server),\ndifferent code path, and the vulnerable code here is present in the current release.\n\n## Package\n\n- **Ecosystem:** npm\n- **Package:** `apostrophe`\n- **Affected versions:** all **4.x** releases, **4.0.0 through 4.32.2** (current `latest`).\n  Verified by inspecting the published npm tarballs: `apostrophe@4.0.0` already contains the\n  vulnerable descent (`parentKey = 'root'`) with no `__proto__`/`constructor`/`prototype`\n  filtering, and `apostrophe@4.32.2` is unchanged in this respect. The vulnerable\n  `parseQuery()` logic was introduced in commit `0255086bf` (2020-11-12), well before the 4.0\n  version, so **ApostropheCMS 3.x is also affected** (the same code ships in 3.x at the older\n  build path `modules/@apostrophecms/util/ui/public/2-http.js`).\n- **Patched versions:** none at time of writing.\n\n## Details\n\nThe issue has three parts. Parts 1 and 2 are ApostropheCMS code and constitute the\nreportable vulnerability. Part 3 is the escalation gadget, which is standard Vue behavior\nand is included for completeness.\n\n### 1. The pollution primitive: source (ApostropheCMS)\n\n`modules/@apostrophecms/util/ui/src/http.js`, function `apos.http.parseQuery()`\n(line ~301) and its inner `patch()` (lines ~317–354):\n\n```js\nfunction patch(key, value) {\n  let match;\n  let parentKey = 'root';\n  let context = data;                    // data = {}  (ordinary object)\n  key = decodeURIComponent(key);\n  const path = key.split('[');\n  path.forEach(function(subKey) {\n    if (subKey === ']') {\n      /* array handling */\n    } else if (subKey.match(/^\\d+]/)) {\n      /* numeric index handling */\n    } else {\n      match = subKey.replace(']', '');\n      if (!context[parentKey]) {         // (A) truthy check, not an own-property check\n        context[parentKey] = {};\n      }\n      context = context[parentKey];      // (B) descends into whatever this resolves to\n      parentKey = match;\n    }\n  });\n  value = (value === null) ? value : decodeURIComponent(value);\n  if (Array.isArray(context[parentKey])) {\n    context[parentKey].push(value);\n  } else if (context[parentKey] !== undefined) {\n    context[parentKey] = [ context[parentKey], value ];\n  } else {\n    context[parentKey] = value;          // (C) assignment onto the resolved object\n  }\n}\n```\n\nNo `__proto__`, `constructor`, or `prototype` key is rejected. Walking the input\n`__proto__[x]=v`:\n\n1. Segment `__proto__`: at **(A)**, `context['__proto__']` resolves to the object's\n   inherited prototype (truthy), so the `{}` initializer is skipped; at **(B)**, `context`\n   becomes `Object.prototype`.\n2. Segment `x`: at **(C)**, `Object.prototype['x'] = v`.\n\nThe truthy guard at **(A)** (rather than an own-property check) is what allows the descent\ninto the inherited prototype. Because the assignment at **(C)** is an ordinary property\nwrite, the resulting `Object.prototype` property is **enumerable**,which matters for the\ngadget (3).\n\n### 2. The trigger: reached on every authenticated page load (ApostropheCMS)\n\n`modules/@apostrophecms/admin-bar/ui/apos/components/TheAposContextBar.vue`. The component's\n`data()` initializer (line ~64) runs when the admin context bar mounts (i.e. on every\nauthenticated page load), and passes the raw query string straight into the vulnerable\nparser:\n\n```js\ndata() {\n  const query = apos.http.parseQuery(location.search);   // <- attacker-controlled -> (1) pollution, on mount\n  const draftMode = query.aposMode || 'published';\n  // ...\n}\n```\n\nThe same component's `refresh()` method (line ~606) passes `window.location.search` into\n`parseQuery()` again on admin-bar refresh events, and folds it into `doc._url`\n(re-parsed inside `apos.http.remote()`), providing additional trigger points.\n\nThe admin context bar mounts only when a session is present and the admin bundle loads, so\nnone of this executes for anonymous visitors, which is why the pollution is observed **only\nwhen logged in**. All of this is core ApostropheCMS code (`node_modules/apostrophe/...`);\nit is not introduced by any project or starter kit content.\n\n### 3. The escalation gadget: Vue element mount (framework behavior; known, not novel)\n\nThe demonstrated payload pollutes the property `innerHTML`\n(`__proto__[innerHTML]=<img/src/onerror=alert(1)>`). The sink that consumes it is **Vue's\nstandard element-mount path**, not ApostropheCMS code. Confirmed in the bundled framework\n(`vue` / `@vue/runtime-core` / `@vue/runtime-dom` all **3.5.42**):\n\n`@vue/runtime-core` `mountElement`, prop enumeration (line ~5741):\n\n```js\nif (props) {\n  for (const key in props) {                              // for...in walks the prototype chain\n    if (key !== \"value\" && !shared.isReservedProp(key)) { // isReservedProp does NOT list innerHTML\n      hostPatchProp(el, key, null, props[key], ...);      // props[\"innerHTML\"] → inherited polluted value\n    }\n  }\n```\n\n`@vue/runtime-dom` `patchDOMProp` (line ~583), reached via `hostPatchProp`:\n\n```js\nfunction patchDOMProp(el, key, value, ...) {\n  if (key === \"innerHTML\" || key === \"textContent\") {\n    if (value != null) {\n      el[key] = key === \"innerHTML\" ? unsafeToTrustedHTML(value) : value;  // el.innerHTML = payload\n    }\n    return;\n  }\n```\n\nMechanism: whenever Vue mounts an element that has a non-null `props` object without its own\n`innerHTML` (routine during the admin render), `for (const key in props)` enumerates the\ninherited, enumerable `innerHTML` from `Object.prototype`; it passes the `\"value\"` and\n`isReservedProp` filters; `props[\"innerHTML\"]` resolves to the attacker's payload; and\n`patchDOMProp` writes it to `el.innerHTML`. `unsafeToTrustedHTML` is Vue's Trusted-Types\nwrapper, not a sanitizer, a passthrough unless Trusted Types is enforced (not the default).\n\nThis gadget is a well-documented, essentially universal client-side-PP > XSS escalation in Vue applications.\nIt is **not claimed as a novel finding** and is **not ApostropheCMS's to fix**, it cannot\nbe removed without changing Vue. Its relevance here is that it makes the (1) pollution\nreliably exploitable in any ApostropheCMS admin session.\n\n## Proof of Concept\n\nPrerequisite: a running ApostropheCMS instance and any authenticated session (a `guest`\naccount suffices). The tester/victim must be logged in.\n\n**A. Prototype pollution (console-verifiable):**\n\n1. Log in as any user.\n2. Navigate to: `https://TARGET/?__proto__[prop]=polluted`\n3. In the browser console: `Object.prototype.prop` > returns `\"polluted\"`.\n\n**B. DOM-based XSS:**\n\n1. Log in as any user.\n2. Navigate to:\n   `https://TARGET/?__proto__[innerHTML]=%3Cimg/src/onerror%3dalert(1)%3E`\n3. `alert(1)` executes in the victim's authenticated origin.\n\nTo capture the sink stack for evidence: DevTools > Sources > open the admin bundle >\npretty-print > search `unsafeToTrustedHTML` (or `=== \"innerHTML\"`) > line breakpoint in the\n`patchDOMProp` innerHTML branch > reload the logged-in payload URL > inspect `value` and\nwalk the call stack up through `hostPatchProp` to `mountElement`'s `for…in`.\n\nBoth A and B were reproduced against ApostropheCMS 4.32.2 (Vue 3.5.42) in Chrome, Firefox, and Edge browsers.\n\n## Impact\n\nAny party who can coerce an authenticated ApostropheCMS user to open a crafted URL can\nexecute arbitrary JavaScript in that user's session. Because the escalation gadget is a\nguaranteed-present Vue behavior, this is a reliable exploit, not a theoretical primitive.\nConsequences scale with the victim's role:\n\n- **Any authenticated victim:** actions and data reads available to that account; theft of\n  the session cookie / CSRF token from the authenticated context.\n- **Editor/admin victim:** the injected script can invoke the authenticated management REST\n  APIs as the victim, creating or modifying documents and user accounts, changing\n  configuration and possibly leading to account takeover and potential full compromise of the CMS\n  instance.\n\nPrototype pollution can additionally affect unrelated code paths that read inherited\nproperties, so other gadgets beyond the demonstrated Vue `innerHTML` sink may exist.\n\n## Remediation\n\nThe fix belongs in ApostropheCMS, at the source (1). The gadget (3) is framework behavior\nand cannot be relied upon to change, so sanitizing `parseQuery` is the correct fix.\n\n**Primary fix: reject prototype-affecting keys in `parseQuery`/`patch()`**:\n\n```js\napos.http.parseQuery = function(query) {\n  query = query.replace(/^\\?/, '');\n  const data = {};\n  const DANGEROUS = new Set([ '__proto__', 'constructor', 'prototype' ]);\n  query.split('&').forEach(function(pair) {\n    // ... unchanged ...\n  });\n  return data.root || {};\n\n  function patch(key, value) {\n    let match;\n    let parentKey = 'root';\n    let context = data;\n    key = decodeURIComponent(key);\n    const path = key.split('[');\n    let dangerous = false;\n    path.forEach(function(subKey) {\n      const cleanKey = subKey.replace(']', '');\n      if (DANGEROUS.has(cleanKey)) {   // refuse to traverse into prototype-affecting keys\n        dangerous = true;\n        return;\n      }\n      // ... existing subKey handling ...\n    });\n    if (dangerous) {\n      return;                          // discard this parameter entirely\n    }\n    // ... existing assignment ...\n  }\n};\n```\n\n**Defense in depth:**\n- Replace the truthy guard at **(A)** with an own-property check:\n  `if (!Object.prototype.hasOwnProperty.call(context, parentKey))`, so a key can never\n  descend into an inherited prototype.\n- Build `data` and any created containers with `Object.create(null)` so intermediate objects\n  have no prototype chain to reach.\n\n**Interim mitigation for operators:** restrict who holds authenticated accounts, and be\naware that opening untrusted links while logged in can trigger this. There is no server-side\nconfiguration toggle that disables the vulnerable client behavior.\n\n## References\n\n- Source (ApostropheCMS): `apostrophe/modules/@apostrophecms/util/ui/src/http.js` —\n  `parseQuery()` / `patch()`\n- Trigger (ApostropheCMS):\n  `apostrophe/modules/@apostrophecms/admin-bar/ui/apos/components/TheAposContextBar.vue` —\n  `data()` (~line 64), `refresh()` (~line 606)\n- Escalation gadget (Vue 3.5.42, framework behavior): `@vue/runtime-core` `mountElement`\n  prop enumeration (~line 5741); `@vue/runtime-dom` `patchDOMProp` innerHTML branch\n  (~line 583)\n- Related but distinct: CVE-2026-53609 / GHSA-6h5j-32cf-4253 (server-side prototype\n  pollution via `apos.util.set()` / `$pullAll`, fixed in 4.31.0)\n- CWE-1321; CWE-79\n\n## Credit\n\n### Github Account Username: `Xx-LoGiCaL-xX`\n\n---\n\n## Affected packages\n\n- `apostrophe < 4.33.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `apostrophe 4.33.0`","depth":"sunlit","depthScore":34,"depthScoreParts":{"impact":33.6,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}