Linux kernel TCP flaw rated 9.8 still unpatched on Ubuntu and RHEL

A flaw in the Linux kernel's TCP code, tracked as CVE-2026-98432, affects Ubuntu, Red Hat Enterprise Linux and other distributions. The kernel's own CVE team rates it 9.8 out of 10 on the CVSS scale.

On October 27, Anthropic said its AI model Claude had written an exploit for the bug. The exploit lets a remote, unauthenticated attacker run commands as root on servers with a multipath TCP (MPTCP) listener.

How the bug works

The flaw is a race condition in the code that sets up new IPv4 TCP connections. When the handshake completes, the kernel hands the IP options stored with the pending request to the new socket without copying them. If that socket is destroyed while another CPU is still using those options to answer a resent request, the kernel reads memory that has already been freed. This class of bug is known as a use-after-free.

IPv6 is not affected, because the kernel copies the options for those connections. The bug was introduced in Linux 4.4. A fix landed on October 5, ten days before the CVE was published, and makes the kernel wait until other CPUs finish reading the options before freeing them. Anthropic researcher Xinyang Ge is credited with the report. Neither the CVE record nor the patch mentions MPTCP; the link to that feature comes only from Anthropic's post.

Why the scores differ

The kernel team added its 9.8 score on October 24. It assumes an attacker who sends a request carrying IPv4 Record Route or Timestamp options and resends it to try to win the race, and it rates attack complexity as low.

Vendors disagree. Red Hat's CVE page lists the flaw as Moderate at 5.9, arguing that the race is hard to win and a kernel crash is the likeliest outcome. Red Hat's machine-readable advisory file from October 29, however, lists it as Important at 8.1. Oracle and SUSE also score it 8.1, differing from the kernel team only on attack complexity. Amazon puts it at 7.0.

What Claude built

Anthropic says Claude wrote the exploit after "initial prompting." The attack has two stages. First, the race makes the kernel send extra bytes back to the attacker; once one reply contains a kernel address, KASLR can be bypassed. Then the same bug overflows a buffer and overwrites a callback pointer with the address of a payload planted in advance. That payload runs a command of the attacker's choice as root.

A lost race only fails one connection attempt and does not crash the system. In Anthropic's test, leaking an address took 48 minutes, and command execution took five more. The company did not publish the exploit code, the kernel version or the configuration it tested. It is also unclear whether servers without MPTCP can be taken over the same way.

Patch status

Debian, Oracle Linux, SUSE and Amazon Linux have shipped fixed packages, at least for some of their kernels. As of October 29, Ubuntu and RHEL have not. Red Hat says its fixes are "currently deferred to a future update," and neither Red Hat nor Canonical gives a release date. RHEL 7 is not affected. For those who build their own kernels, the fix is in 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.56, 7.2.10 and 7.3.

Red Hat says no mitigation meets its standards. No attacks exploiting the flaw have been reported, and it does not appear in CISA's KEV catalog.

Source: The Hacker News