Prototype pollution via unchecked __proto__ path traversal

Prototype pollution in Intlify Vue I18n allows remote attackers to alter the global prototype chain through crafted flat JSON. handleFlatJson splits dotted keys and traverses or creates nested objects without rejecting proto, so assignment follows the inherited setter into Object.prototype. Exploitation requires an integrating application to pass attacker-controlled messages into this function; the demonstrated result is prototype modification, while denial of service has no published failure mode and command execution depends on later use by sensitive Node.js APIs.