CVE-2026-84368

### Impact An application that passes attacker-controlled data into joi's custom message configuration (`messages()`, `message()`, `prefs({ messages })`, `Joi.extend({ messages })` or `rule({ message })`) lets the attacker write properties onto `Object.prototype`, where every object in the process then inherits them. A key named `__proto__` was treated as a language name, and the code reused the object it found at that key, which resolves to the prototype rather than to a new own property; a key named `constructor` did the same to the `Object` function's statics. A consuming application that gates on the mere presence of a property (`if (user.isAdmin)`) can be made to take the wrong branch for every object it inspects. This is not reachable from data that joi validates. Custom messages are schema-construction configuration, normally written by the application developer. Exploitation therefore requires an application that feeds untrusted input straight into schema construction. ### Patches Upgrade to version 18.2.5 or 17.13.6. ### Workarounds Do not pass untrusted input into `messages()`, `message()`, `prefs({ messages })`, `Joi.extend({ messages })` or `rule({ message })`. Or validate that they don't contain any `__proto__` or `constructor` property.