Microsoft developer Andres Freund was doing routine maintenance on his computer when he stumbled upon one of the largest Internet hacks in history. Noticing that his computer was a little slow, he discovered a “backdoor” that could give hackers access to millions of computers and servers used by banks, hospitals and governments around the world. The hack, which could have taken years to plan and execute, has been called a “nightmare scenario” by cybersecurity experts, who say the evidence points to state actors. We break down what happened and who was behind it.