Security professionals say it’s one of the worst computer vulnerabilities they’ve ever seen. Companies, including Microsoft, claim that state-backed Chinese and Iranian hackers and rogue cryptocurrency miners have already taken hold of it. The Department of Homeland Security has sounded the alarm, ordering federal agencies to urgently find and fix instances of bugs because they are so easily exploitable – and telling those with public networks to set up firewall if they cannot be sure. A small piece of code, the often undocumented affected software. Housed in a widely used utility called Log4j, the flaw allows internet-based attackers to easily...