Windows 10 was just released, but ancient versions of the operating system are still used across the globe. We’re not just talking about Windows XP, either. In a bizarre turn of events, it appears Windows 3.1 forced the temporary closure of a French airport in Paris last Saturday.

Why does a modern airport use Windows 3.1? It has to do with legacy software — in this case, a program called DECOR, which is used by air traffic control to communicate weather information with pilots in poor conditions.

Without it, pilots flying in or out of the airport couldn’t receive information about visibility conditions, making flights more hazardous. That wouldn’t have forced a closure under clear skies, but the glitch occurred as fog was rolling in, forcing the airport to halt all flights as a precaution.

The situation highlighted the use of aging software across infrastructure both public and private. According to the secretary general of the French air traffic controller union, the use of Windows 3.1 to run DECOR is no oddity. Airports in the country use a hodge-podge of operating systems, including various versions of Windows from 3.1 to XP, and UNIX.

Despite the glitch, it seems unlikely that a replacement system will be available any time soon. Software like DECOR, once implemented, becomes difficult to replace because it’s not easy to update. Every addition to the system must be rigorously vetted to ensure safety.

The security of software used by airports has become a greater concern in 2015. Earlier this year, United Airlines’ entire fleet was grounded because of software problems. It responded by starting a bug bounty program, which was paid out in the form of a one million sky miles to two white-hat hackers. The specific details of what vulnerability the hackers found remain unknown.

In May, a man named Chris Roberts was questioned by the FBI for allegedly hacking an in-flight entertainment system and then issuing a “climb” command to one of the engines. However, Boeing states that is not possible because the entertainment and flight systems are separate. An investigation by Wired confirmed that.

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