This is definitely a bug (a 'value of out range' for prepend > 255 should have at least been emitted), but IOS devices crashing because of it is noone but Cisco

This is definitely a bug (a 'value of out range' for prepend > 255 should have at least been emitted), but IOS devices crashing because of it is noone but Cisco's fault. And then, not having a 'filter out if AS path > 64' config line while being a transit provider is just careless.