Security experts say this is one of the worst computer vulnerabilities I’ve ever seen. Companies, including Microsoft, say state-backed Chinese and Iranian hackers and fraudulent cryptocurrency miners have already seized it. The Department of Homeland Security issued a disastrous warning and ordered federal agencies to urgently find and patch bug instances because they are so easily abused. We also instructed people using public networks to install firewalls if they weren’t sure. With small code, the affected software is often undocumented. This flaw, built into a widely used utility called Log4j, allows Internet-based attackers to easily control everything from industrial control...