Here’s a story that should make anyone who runs a website sit up.
This week Anthropic — the company behind the Claude AI — announced that one of its research models didn’t just find a nasty bug in the Linux kernel, it wrote a working attack that uses it. No password, no clicks, nothing. Point it at the wrong server and it ends up running commands as root, which in Linux terms means it owns the machine outright.
What actually happened, in plain English
The bug is in the code Linux uses to set up new internet connections. Two parts of the kernel can end up racing each other over the same bit of memory — one side frees it while the other side is still reading it. Programmers call that a use-after-free, and it’s the sort of flaw that’s been hiding in plain sight: this one has been sitting in the kernel since version 4.4, roughly a decade, in the most read, most argued-over code on the planet.
Anthropic’s model chained it together step by step. First it used the flaw to leak a kernel memory address, which defeats the protection that’s supposed to randomise where things live in memory. Then it used the same flaw to hijack what the kernel does next and run a shell command of its choosing. The whole thing, first leak to root, took about 53 minutes.
The technical name is CVE-2026-98432, and the kernel team scored it 9.8 out of 10. For scale, 10 is as bad as the scale goes.
The MPTCP bit — and the honest caveat
Anthropic’s attack went in through servers listening on Multipath TCP — MPTCP, the clever extension that lets one connection hop across several network paths at once. Worth knowing: since Go 1.24, programs written in Go turn MPTCP listening on by default, so more servers have this door fitted than their owners probably realise.
And before you assume this is some exotic corner of the internet: MPTCP keeps turning up in ordinary tools whether their makers asked for it or not. Take Tailscale, the VPN half the tech world now runs on. Tailscale itself is WireGuard, not MPTCP — but it’s written in Go, and Go’s habit of switching MPTCP on by default meant Tailscale’s own developers have had to write code specifically disabling it in places where it got in the way. That’s the point, really: a feature can be quietly sitting in your stack because your programming language put it there, not because you ever chose it.
But fairness matters, so here’s the caveat the headlines are skipping. The bug itself isn’t really an “MPTCP bug” — it lives in the ordinary TCP handshake code, and neither the official CVE record nor the kernel fix even mentions MPTCP. That was just the doorway Anthropic chose to walk through. Some vendors also score it lower than 9.8 — Red Hat now rates it Important at 8.1 (up from 5.9) — and nobody has reported it being used out in the wild. The kernel fix is already written and backported to every maintained branch. What’s lagging, as of the end of October, is some of the big distributions: Ubuntu and Red Hat hadn’t shipped patched kernels yet.
Why I think this matters more than one bug
Bugs like this have always existed. What’s new is who found it and what happened next. The same model didn’t stop at “there’s a problem here” — it built the exploit, chained the steps, and timed itself doing it. And this Linux one was just one of five working exploit chains Anthropic showed off, alongside Chrome, Safari, Android and Windows.
That’s the future arriving, and it cuts both ways. Pointed at the code by the good guys, this is the best bug-hunter the open-source world has ever had — a decade-old flaw in the Linux kernel, found and fixed because an AI went looking. In the wrong hands, the same ability is an attack that writes itself in under an hour.
What I’d do if you run anything on Linux
- Patch. When your distribution ships the fixed kernel, install it. The fixed versions are already known — 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.56, 7.2.10 and the 7.3 release candidates.
- Check your listeners. If you don’t need Multipath TCP, don’t leave it on — and if you run Go services, know that MPTCP may be on by default.
- Don’t panic, but don’t dawdle either. No wild attacks reported, patches exist, this is a “this week, not this quarter” job.
Anyway. A machine just out-hacked ten years of human code review on the world’s most important piece of software — and then the humans fixed it within weeks. Both halves of that sentence are worth remembering.
This is a higher severity than the PHP bug that was just found.
