No, it's built into the format that implementations can extend it with their own types. Then library authors decided it would be neat if any object in their lan

No, it's built into the format that implementations can extend it with their own types. Then library authors decided it would be neat if any object in their language could be serialized as yaml, and so extended it with types that happen to be able to execute arbitrary code, because that's what objects do in dynamic languages. A more conservative implementation wouldn't have that particular vulnerability.