Joi.string().email({ multiple: true }) takes quadratic time to split a value that contains a long run of whitespace. One request with 100 KB of spaces blocks the Node.js event loop for several seconds, which causes a denial of service.
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 …