joi: Prototype pollution via a `__proto__` language key in custom messages

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.