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

{
  "id": "0a4131bc64f16675f06fb628e47ba4fc80dfa564263755110117fc0652c35e39",
  "url": "https://osv.dev/vulnerability/GHSA-m2m5-rw3w-cwmj",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "ApostropheCMS: Client-side Prototype Pollution in apos.http.parseQuery leading to DOM-based XSS against authenticated users",
    "summary": "## 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. 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:** `apostrop",
    "evidence": "osv:GHSA-m2m5-rw3w-cwmj",
    "products": [
      {
        "name": {
          "value": "apostrophe",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "4.33.0",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "medium",
    "structured": {
      "id": "GHSA-m2m5-rw3w-cwmj",
      "aliases": [
        "CVE-2026-113659",
        "GHSA-m2m5-rw3w-cwmj"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "4.33.0"
                }
              ]
            }
          ],
          "package": {
            "name": "apostrophe",
            "purl": "pkg:npm/apostrophe",
            "ecosystem": "npm"
          },
          "database_specific": {
            "source": "https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-m2m5-rw3w-cwmj/GHSA-m2m5-rw3w-cwmj.json"
          }
        }
      ],
      "modified": "2026-10-20T18:30:04.605117382Z",
      "provider": "osv",
      "references": [
        "https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-m2m5-rw3w-cwmj",
        "https://github.com/apostrophecms/apostrophe/commit/9500b8b0e1d630d5e07fbb5fd2c25ed0943845ce",
        "https://github.com/apostrophecms/apostrophe/commit/f84fd41cbf6c22eb8d0008fc9c25df1f7130b350",
        "https://github.com/apostrophecms/apostrophe",
        "https://github.com/apostrophecms/apostrophe/releases/tag/apostrophe@4.33.0"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-1321",
          "CWE-79"
        ],
        "severity": "MODERATE",
        "github_reviewed": true,
        "nvd_published_at": null,
        "github_reviewed_at": "2026-10-20T18:18:49Z"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-113659",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 390151,
  "published_at": "2026-10-20T18:18:49+00:00"
}