A flaw in Node.js HTTP request handling causes an uncaught `TypeError` when a request is received with a header named `__proto__` and the application accesses `req.headersDistinct`. When this occurs, `dest["__proto__"]` resolves to `Object.prototype` rather than `undefined`, causing `.push()` to be called on a non-array. This exception is thrown synchronously inside a property getter and cannot be intercepted by `error` event listeners, meaning it cannot be handled without wrapping every `req.headersDistinct` access in a `try/catch`. * This vulnerability affects all Node.js HTTP servers on **20.x, 22.x, 24.x, and v25.x**

• cbl2 nodejs18 18.20.3-12
• cbl2 nodejs18 18.20.3-12 on CBL Mariner 2.0
• azl3 nodejs 20.14.0-14 on Azure Linux 3.0
• azl3 nodejs24 24.13.0-3 on Azure Linux 3.0
• Microsoft Visual Studio 2019 version 16.11 (includes 16.0 - 16.10)
• Microsoft Visual Studio 2022 version 17.14
• cbl2 nodejs18 18.20.3-12 on CBL-Mariner 2.0

CBL-Mariner Releases

1. Apr 1, 2026 at 1:16 AM UTC
   Published by Microsoft Security Response Center

   The publication date reported by the vendor.

2. Sep 7, 2026 at 1:53 AM UTC
   Updated by Microsoft Security Response Center

   The vendor changed the advisory after it was first published.

3. Oct 2, 2026 at 12:40 PM UTC
   Added to SecurityAlert

   We collected the advisory from the official source.

4. Oct 3, 2026 at 1:00 PM UTC
   Confirmed at the source

   Our collector saw this advisory during a later source check.