One of the largest IT outages in history has disrupted companies worldwide, affecting sectors from airlines to financial services. The issue, caused by a security update from CrowdStrike that impacted Microsoft Windows, left thousands of workers unable to log in, grounding flights and halting business operations. CrowdStrike and Microsoft have identified and isolated the problem, but residual impacts continue. The outage has highlighted the fragility of global internet infrastructure, with significant disruptions in Europe, the US, and Asia. Shares in CrowdStrike and Microsoft fell as a result.