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Response snapshot Oct 27, 2026, 07:42 UTC

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VulnerabilityCVE-2026-113659

ApostropheCMS: Client-side Prototype Pollution in apos.http.parseQuery leading to DOM-based XSS against authenticated users

The client-side helper `apos.http.parseQuery()` in ApostropheCMS builds a nested object from a URL query string without sanitizing the keys `__proto__`, `constructor`, or `prototype`. Parsing an attacker-controlled query string therefore writes arbitrary properties onto `Object.

Published Updated

At a glance

Exploited
Not known
Severity
6.1 Medium (CVSS 3.1)
Chance of exploitation
0.3% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: apostrophe: 4.33.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-m2m5-rw3w-cwmj (opens a new tab)

What happened

## Summary

The client-side helper `apos.http.parseQuery()` in ApostropheCMS builds a nested object from a URL query string without sanitizing the keys `__proto__`, `constructor`, or `prototype`. Parsing an attacker-controlled query string therefore writes arbitrary properties onto `Object.prototype` (client-side prototype pollution).

ApostropheCMS's admin UI calls this parser with `window.location.search` on **every authenticated page load** (the admin context bar's `data()` initializer). Because the admin UI is built on Vue, a polluted, enumerable property on `Object.

prototype` is then picked up by Vue's standard element-mount routine and written to the DOM as an element property, turning the prototype pollution into DOM-based XSS in the victim's authenticated session. The escalation gadget is a well-known, general Vue behavior; the novel, reportable defect is the unsanitized `parseQuery` source together with the authenticated trigger, both of which are ApostropheCMS code.

Continue reading

Any user who can get **any logged-in user (from the lowest-privileged `guest` role up to the highest-privileged `admin` role)** to open a crafted URL can execute arbitrary JavaScript in that user's session.

This is **distinct from CVE-2026-53609** (server-side prototype pollution via `apos.util.set()` / `$pullAll`, fixed in 4.31.0): different side (client vs server), different code path, and the vulnerable code here is present in the current release.

## Package

- **Ecosystem:** npm - **Package:** `apostrophe` - **Affected versions:** all **4.x** releases, **4.0.0 through 4.32.2** (current `latest`). Verified by inspecting the published npm tarballs: `apostrophe@4.0.0` already contains the vulnerable descent (`parentKey = 'root'`) with no `__proto__`/`constructor`/`prototype` filtering, and `apostrophe@4.32.2` is unchanged in this respect.

The vulnerable `parseQuery()` logic was introduced in commit `0255086bf` (2020-11-12), well before the 4.0 version, so **ApostropheCMS 3.x is also affected** (the same code ships in 3.x at the older build path `modules/@apostrophecms/util/ui/public/2-http.js`). - **Patched versions:** none at time of writing.

## Details

The issue has three parts. Parts 1 and 2 are ApostropheCMS code and constitute the reportable vulnerability. Part 3 is the escalation gadget, which is standard Vue behavior and is included for completeness.

### 1. The pollution primitive: source (ApostropheCMS)

`modules/@apostrophecms/util/ui/src/http.js`, function `apos.http.parseQuery()` (line ~301) and its inner `patch()` (lines ~317–354):

```js function patch(key, value) { let match; let parentKey = 'root'; let context = data; // data = {} (ordinary object) key = decodeURIComponent(key); const path = key.split('['); path.forEach(function(subKey) { if (subKey === ']') { /* array handling */ } else if (subKey.match(/^\d+]/)) { /* numeric index handling */ } else { match = subKey.replace(']', ''); if (!

context[parentKey]) { // (A) truthy check, not an own-property check context[parentKey] = {}; } context = context[parentKey]; // (B) descends into whatever this resolves to parentKey = match; } }); value = (value === null) ? value : decodeURIComponent(value); if (Array.isArray(context[parentKey])) { context[parentKey].push(value); } else if (context[parentKey] !

== undefined) { context[parentKey] = [ context[parentKey], value ]; } else { context[parentKey] = value; // (C) assignment onto the resolved object } } ```

No `__proto__`, `constructor`, or `prototype` key is rejected. Walking the input `__proto__[x]=v`:

1. Segment `__proto__`: at **(A)**, `context['__proto__']` resolves to the object's inherited prototype (truthy), so the `{}` initializer is skipped; at **(B)**, `context` becomes `Object.prototype`. 2. Segment `x`: at **(C)**, `Object.prototype['x'] = v`.

The truthy guard at **(A)** (rather than an own-property check) is what allows the descent into the inherited prototype. Because the assignment at **(C)** is an ordinary property write, the resulting `Object.prototype` property is **enumerable**,which matters for the gadget (3).

### 2. The trigger: reached on every authenticated page load (ApostropheCMS)

`modules/@apostrophecms/admin-bar/ui/apos/components/TheAposContextBar.vue`. The component's `data()` initializer (line ~64) runs when the admin context bar mounts (i.e. on every authenticated page load), and passes the raw query string straight into the vulnerable parser:

```js data() { const query = apos.http.parseQuery(location.search); // (1) pollution, on mount const draftMode = query.aposMode || 'published'; // ... } ```

The same component's `refresh()` method (line ~606) passes `window.location.search` into `parseQuery()` again on admin-bar refresh events, and folds it into `doc._url` (re-parsed inside `apos.http.remote()`), providing additional trigger points.

The admin context bar mounts only when a session is present and the admin bundle loads, so none of this executes for anonymous visitors, which is why the pollution is observed **only when logged in**. All of this is core ApostropheCMS code (`node_modules/apostrophe/...`); it is not introduced by any project or starter kit content.

### 3. The escalation gadget: Vue element mount (framework behavior; known, not novel)

The demonstrated payload pollutes the property `innerHTML` (`__proto__[innerHTML]= `). The sink that consumes it is **Vue's standard element-mount path**, not ApostropheCMS code. Confirmed in the bundled framework (`vue` / `@vue/runtime-core` / `@vue/runtime-dom` all **3.5.42**):

`@vue/runtime-core` `mountElement`, prop enumeration (line ~5741):

```js if (props) { for (const key in props) { // for...in walks the prototype chain if (key !== "value" && !shared.isReservedProp(key)) { // isReservedProp does NOT list innerHTML hostPatchProp(el, key, null, props[key], ...); // props["innerHTML"] → inherited polluted value } } ```

`@vue/runtime-dom` `patchDOMProp` (line ~583)

Affected package ranges reported by GitHub: apostrophe: < 4.33.0

Scores and how they were measured

CVSS base severity

6.1

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.3%

Scored Percentile 27.8%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-m2m5-rw3w-cwmj (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-m2m5-rw3w-cwmj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-m2m5-rw3w-cwmj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-m2m5-rw3w-cwmj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-m2m5-rw3w-cwmj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-m2m5-rw3w-cwmj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-m2m5-rw3w-cwmj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-m2m5-rw3w-cwmj, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  9. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-113659, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 20). ApostropheCMS: Client-side Prototype Pollution in apos.http.parseQuery leading to DOM-based XSS against authenticated users (CVE-2026-113659). https://kevwire.com/record/CVE-2026-113659

MLA

"ApostropheCMS: Client-side Prototype Pollution in apos.http.parseQuery leading to DOM-based XSS against authenticated users (CVE-2026-113659)." KEVWire, 20 Oct. 2026, kevwire.com/record/CVE-2026-113659.

Chicago

KEVWire. "ApostropheCMS: Client-side Prototype Pollution in apos.http.parseQuery leading to DOM-based XSS against authenticated users (CVE-2026-113659)." Last modified October 20, 2026. https://kevwire.com/record/CVE-2026-113659.

Evidence summary (plain text)

ApostropheCMS: Client-side Prototype Pollution in apos.http.parseQuery leading to DOM-based XSS against authenticated users (Oct 20, 2026).
The client-side helper `apos.http.parseQuery()` in ApostropheCMS builds a nested object
from a URL query string without sanitizing the keys `__proto__`, `constructor`, or
`prototype`. Parsing an attacker-controlled query string therefore writes arbitrary
properties onto `Object.prototype` (client-side prototype pollution).

ApostropheCMS's admin UI calls this parser with `window.location.search` on **every
authenticated page load** (the admin context bar's `data()` initializer). Because the admin
UI is built on Vue, a polluted, enumerable property on `Object.prototype` is then picked up
by Vue's standard element-mount routine and written to the DOM as an element property,
turning the prototype pollution into DOM-based XSS in the victim's authenticated session.
The escalation gadget is a well-known, general Vue behavior; the novel,
reportable defect is the unsanitized `parseQuery` source together with the authenticated
trigger, both of which are ApostropheCMS code.
CVSS 6.1 (version 3.1; evidence as of Oct 20, 2026; https://github.com/advisories/GHSA-m2m5-rw3w-cwmj).
EPSS 0.3% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-113659&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-m2m5-rw3w-cwmj: https://github.com/advisories/GHSA-m2m5-rw3w-cwmj
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMTozNTo1OFrOAAfwCQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMTozMDoxM1rOAAfv2A%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyODowOVrOAAfv1g%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyNDoyMFrOAAfv1Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDo0NTowN1rOAAfvzQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDozMDoyNlrOAAfvxw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDowMzo1N1rOAAfvxQ%3D%3D
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-113659&date=2026-10-26
Record ID: CVE-2026-113659.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 07:41 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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