(The Washington Post/The Washington Post illustration; iStock) Tens of thousands of South Koreans had their financial data exposed online in recent weeks after at least nine major banks there suffered cyberattacks that suggested a dark future for online security.One man, a 26-year-old living in southern China, was allegedly behind the attacks, according to an investigation by cybersecurity company CrowdStrike. Traces left online and failure to secure his own systems allowed investigators to piece together the man’s name, age, location and what he allegedly did.The clues revealed that the devastating assault on some of Asia’s most sophisticated financial institutions was the…