Bizarre bug in macOS is a 'ticking time bomb' that takes out networking capabilities if a Mac is left on for too long

• A very strange bug in macOS has just been found
• If a Mac is left on for (just over) 49 days, its networking functionality will completely fail
• The only cure is a reboot, apparently, but presumably Apple will now be working on an official fix

Ever wondered what would happen if you left your Mac on for a couple of months solid? Probably not, but you might be interested to learn that if you did, the networking side of the OS would fall over.

Tom's Hardware reports that Photon wrote a blog post on how it "found a ticking time bomb in macOS TCP networking", an explosive element in the code that "detonates after exactly 49 days."

Well, 49 days, 17 hours, two minutes, and 47 seconds to be precise. When macOS has been running continuously for that exact length of time, the operating system will experience an 'integer overflow' that "freezes the internal TCP timestamp clock".

When that happens, existing TCP network connections won't expire as they should, remaining frozen in place, and eventually, as Photon explains: "Ephemeral ports slowly exhaust, and eventually no new TCP connections can be established at all. ICMP (ping) keeps working. Everything else dies."

In short, networking on the Mac goes completely kaput, and the only cure is to reboot the machine. Yes, the old 'turn it off and turn it on again' solution.

Photon — a company that facilitates building AI agents — found this bug on the Macs it uses to monitor Apple's Messages service, and the company successfully reproduced the glitch on two systems.

Analysis: Apple following in Microsoft's footsteps

Obviously, this isn't a problem that most of you — assuming that you own a Mac — will have to worry about. No everyday user leaves their machine on for 50 days solid; but in case you're ever inclined to do so, at least you're now forewarned. This is, of course, more of a glitch that'll hit servers (which do run continuously for long periods), and one which businesses like Photon need to be aware of.

The root cause of the issue is, as mentioned, integer overflow. This is where macOS assumes a counter is only going to go up in numerical value, when in fact, it wraps back round to zero after 50 days of ticking up – and this is something that's caught out Microsoft in the past, too. Photon reminds us that Windows 95 suffered a similar 49.7-day crash where the kernel's 32-bit millisecond tick counter overflowed, in this case causing the PC to completely freeze up.

Apparently Photon is working on a solution to avoid having to reboot in order to fix the Mac, but presumably now that this bug has been brought to Apple's attention we should see an official fix before long.

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