Summary A prototype pollution vulnerability exists in the latest version of the convict npm package (6.2.4). Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input started with a forbidden key, it is still possible to pollute "Object.prototype" via a crafted input using "String.prototype". Details The vulnerability resides in line 564 of https://github.com/mozilla/node-convict/blob/master/packages/convict/src/main.js where "startsWith()" function is used to check whether user provided input contain forbidden strings. PoC Steps to reproduce 1. Install latest version of convict using "npm install" or cloning from git 2. Run the following code snippet: String.prototype.startsWith = () => false; const convict = require('convict'); let obj = {}; const config = convict(obj); console.log({}.polluted); config.set('constructor.prototype.polluted', 'yes'); console.log({}.polluted); // prints yes -> the patch is 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 convict is used by downstream applications. Any application that processes attacker-controlled input using "convict.set" may be affected. It could potentially lead to the following problems: 3. Authentication bypass 4. Denial of service 5. Remote code execution (if polluted property is passed to sinks like eval or child_process)