Linux kernel bugs which can be triggered remotely, without any login, are rare. Most kernel vulnerabilities need an attacker to already be running code on the machine. CVE-2026-98432 is one of those: a use-after-free in the IPv4 TCP handshake code, rated CVSS 9.8, which Atlas has written up in detail. It goes all the way back to Linux 4.4.

A use-after-free is exactly what it sounds like: the program releases a chunk of memory, but some other code keeps using it. If an attacker can get their own data placed into that freed memory before it’s used again, they can influence what the kernel does next. In this case, when a server accepts a connection whose SYN packet carries IPv4 header options (a rarely used feature of IPv4), the kernel hands those options from the half-open connection to the full connection without copying them, then frees them while another CPU core may still be reading them. The fix is a four-line change, from “kfree()” to “kfree_rcu()”, which waits until every reader is done before freeing the memory. A lost race simply fails the handshake, so an attacker can keep trying.

The bug was reported by Xinyang Ge at Anthropic, and Anthropic says its Claude model wrote a working proof-of-concept that gets root on “Linux servers with a multipath TCP listener.” Multipath TCP (MPTCP) lets a single connection use several network paths at once, like WiFi and cellular, and you might assume almost nobody runs it on servers. Unfortunately, as Marcus Hutchins points out, Debian kernels enable MPTCP by default, and Go 1.24 and newer open MPTCP listening sockets by default when the kernel allows it, so anything built in Go (including the Caddy web server) is potentially exposed. Hutchins’ scans found about 2.1 million servers reachable over MPTCP on port 443, and he’s already seeing scanning from residential proxies and Russian VPNs which looks like it’s specifically hunting for this bug.

Upstream stable kernels were fixed in mid October, but as of October 31, Ubuntu and Red Hat were both still listed as vulnerable, with fixes in progress. If you can’t patch yet, you can check for MPTCP listeners with “ss -Mlnp”, and disable MPTCP with “sysctl net.mptcp.enabled=0”, but note that this doesn’t affect sockets which are already open, so you’ll need to restart any services using them. Red Hat and AlmaLinux have also published firewall rules which drop TCP packets carrying IPv4 options, which basically no legitimate traffic uses.

We haven’t seen bulk exploitation of this vulnerability yet, which might hint that it’s less trivial to trigger than the severity might indicate, but it’s a great time to get servers up to date. “Not exploited yet” has a nasty habit of turning into “every vulnerable server exploited in a day”.

Meanwhile, on the local side, OpenAI has published the results of pointing an automated pipeline, built on an unreleased “cyber-focused” model, at the kernel. In a run in mid October, it found 2,300 candidate bugs, reproduced 1,700 of them, and produced 948 working local privilege escalation exploits against a stock desktop Linux distribution, for about $1.2 million in API credits, or roughly $1,279 per root exploit. That’s not cheap, but for comparison, Pwn2Own has historically paid $20,000 for a single Linux kernel privilege escalation. Local privilege escalation is the step after an attacker gets a foothold: turning an ordinary user account, a compromised web app, or a sandboxed process into full control of the machine. Coupled with even a basic vulnerability in other code, it can allow an attacker to take full control of the system and hide the compromise much more effectively.

OpenAI says it’s working with the kernel community to report the bugs and submit patches, starting with the most critical, and that during the ten days the scan was running, more than 100 other kernel privilege escalation bugs were patched by other contributors, with about 300 more being discussed publicly. The most sobering line in the report is that, for now, a low-privileged account on an unpatched system “should be considered functionally equivalent to root access for a well-resourced threat actor.” It’s been quite a week for kernel developers, and it’s not going to slow down any time soon.

The flood of AI discovered vulnerabilities seems to continue unabated, and it’s easy to feel under attack from them. Consider, however, how often the open source community has called for companies using open source tools and libraries as core elements of their business to help support those tools and community. There’s still a significant burden placed on the community maintainers when almost 2000 bug reports land in a week, but over $1 million investment in securing Linux isn’t nothing, and the patches at least appear to be reasonably scoped and accurate. I’m pretty sure we’d see an article covering Linus’s response if they weren’t.

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