If there is a lesson to be drawn from Internet search giant Yahoo’s hellish past year, it is a grimly illustrative one: Never assume a cybersecurity disaster can’t get worse. Last September, the Internet portal disclosed that it had suffered the most damaging and far-reaching data breach in history — only to then announce in December the discovery of a second, earlier, and even larger hack. Yahoo has began a probe into the hack that is expected to take several weeks, but we may not know the full extent of these hacks’ effects for years.