BOSTON—A critical vulnerability in a widely used software tool—one quickly exploited in the online game Minecraft—is rapidly emerging as a major threat to organizations around the world. “The internet’s on fire right now,” said Adam Meyers, senior vice president of intelligence at the cybersecurity firm Crowdstrike. “People are scrambling to patch,” he said, “and all kinds of people scrambling to exploit it.” He said Friday morning that in the 12 hours since the bug’s existence was disclosed that it had been “fully weaponized,” meaning malefactors had developed and distributed tools to exploit it. The flaw may be the worst computer vulnerability discovered in years. It was uncovered in a utility that’s ubiquitous in cloud servers and enterprise software used across industry and government. Unless it is fixed, it grants criminals, spies, and programming novices alike easy access to internal networks where they can loot valuable data, plant malware, erase crucial …