@hey-api/openapi-ts's `buildClientParams` template: prototype chain substitution via unknown `$<slot>___proto__` key
dist/clients/core/params.ts in @hey-api/openapi-ts ships a runtime template that is copied verbatim into every generated SDK as params.gen.ts. When a caller passes an object argument containing an unknown key starting with a slot prefix ($body_, $headers_, $path_, $query_), the function strips the prefix and writes the remainder directly to that slot without validation. The key "$query___proto__" causes the returned params.query object to have its prototype chain substituted with attacker-controlled data. The …