Claude Built a Remote Root Exploit for a Linux Bug, and Ubuntu LTS Users Are Still Waiting on the Fix

It took an unreleased Claude model about 53 minutes to turn a Linux kernel race condition into full remote control of a server. Almost three weeks later, servers running Ubuntu's long term support releases still cannot get the fix through their normal updates.

The bug is CVE-2026-98432, a use after free in the kernel's IPv4 TCP connection setup code. It is triggered when a retransmitted SYN is handled on one CPU while another is still building the SYNACK, and it reaches back to kernel 4.4. The Linux kernel CVE team scores it 9.8 out of 10, and no authentication is needed to trigger it. Red Hat rates it 5.9, treating it mainly as a way to crash the kernel and arguing that winning the race takes more than one malicious packet.

53 minutes to root

Anthropic's researchers say the model wrote an exploit that remotely defeated kernel address space layout randomization (KASLR) and then ran arbitrary commands as root on a Linux server with a Multipath TCP (MPTCP) listener. No credentials, no user interaction. The exploit has not been released, and Anthropic says it reports bugs under coordinated disclosure with a 90 day patch window.

The speed fits a pattern. Days earlier an autonomous agent found four critical flaws in the software that runs particle accelerators and fusion reactors in under four hours. The gap between finding a serious bug and weaponizing it is shrinking fast.

Plain TCP is affected too, although remote code execution has not been publicly shown on servers without an MPTCP listener. MPTCP is usually off by default, with one catch: since Go 1.24, Go turns it on for TCP listeners on Linux when the kernel supports it. Some Go services may be exposing an MPTCP listener without their admins knowing.

Everyone else has shipped

The fix is upstream and backported to maintained kernel branches. Debian shipped it in 6.1.190-1 for Bookworm and 6.12.113-1 for Trixie. Oracle Linux, Amazon Linux and SUSE have fixes out for some affected kernels. Red Hat lists RHEL 8, 9 and 10 as affected, and CIQ says the same of Rocky Linux 8, 9 and 10.

Ubuntu is the outlier. Patched kernels for 22.04, 24.04 and 26.04 LTS are sitting in the -proposed pocket, which machines do not pull from unless admins opt in. Ubuntu 26.10 already has the fix in its regular stream, and older supported releases will follow.

Canonical says the delay is testing. "To ensure stability across a wide ecosystem we perform extensive tests on more than 2,000 certified servers and devices," the company wrote, adding that it will move the fixes to the updates pocket once those tests finish. That did not stop a Hacker News thread titled "Ubuntu still has not released patch for Linux 0day RCE in almost 3 weeks" from reaching the front page on November 2.

The workaround fight

The popular stopgap is to disable MPTCP. Canonical says that is not enough because the flaw is not specific to MPTCP, and its security team says "security updates are the only appropriate mitigation." CIQ also calls disabling MPTCP insufficient and recommends temporarily dropping IPv4 TCP packets that carry IP options with nftables. Limiting access to trusted networks cuts the risk further.

There is no confirmed exploitation in the wild, and no other public exploit is known. For now the only known working exploit sits with Anthropic, which has not released it.

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