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Linux kernel TCP flaw with 9.8 CVSS score affects Ubuntu, Red Hat and other distributions

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Linux Kernel TCP Flaw With 9.8 CVSS Score Affects Ubuntu, Red Hat and Other Distributions

Swati KhandelwalOct 29, 2026Vulnerability / Linux

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

Anthropic said on October 27 that its AI model wrote an exploit that uses the flaw to let a remote attacker with no credentials run commands as root on Linux servers with a multipath TCP listener.

As of October 29, Ubuntu and RHEL have not released a fixed kernel. Red Hat, Amazon, Oracle, and SUSE score the flaw between 5.9 and 8.1, lower than the kernel team's 9.8.

The flaw is a race condition in the code that sets up new IPv4 TCP connections. It can make the kernel read memory it has already freed, a bug type known as a use-after-free.

The kernel fix credits Anthropic researcher Xinyang Ge with reporting the bug. Neither the CVE record nor the fix mentions multipath TCP (MPTCP). The link between that feature and this bug comes from Anthropic's post.

MPTCP lets one connection use several network paths at once. On Linux, an application must request MPTCP when it opens a socket. As of Go 1.24, Go programs do so by default for the sockets they listen on, the Go Security Team said on October 29.

Anthropic's post says the flaw "was patched upstream and backported to all maintained stable kernel branches." For distribution users, that fix arrives only in the vendor's kernel package. Debian, Oracle Linux, SUSE, and Amazon Linux have shipped fixed packages, at least for some of their kernels, but Ubuntu and RHEL have not.

Which Distributions Have a Fix

Distribution Vendor rating Status as of October 29 Fixed package or advisory

Ubuntu 22.04 LTS, 24.04 LTS, 26.04 LTS and 26.10

Priority High Work in progress None released

Ubuntu 16.04, 18.04 and 20.04 Priority High Vulnerable None released

RHEL 8, 9 and 10, OpenShift 4

Moderate, 5.9 (CVE page)

Important, 8.1 ( advisory file)

Affected None released

RHEL 7 Not applicable Not affected Not needed

Amazon Linux 2023, kernel6.18 package

Important, 7.0 Fixed October 24

kernel6.18-6.18.56-125.164.amzn2023 (ALAS2023-2026-3251)

Amazon Linux 2023, kernel6.12 package Important, 7.0 Fixed October 29 in Amazon's advisory; Amazon's CVE page still says pending

kernel6.12-6.12.113-138.209.amzn2023 (ALAS2023-2026-3335)

Amazon Linux 2

Amazon Linux 2023, kernel package

Important, 7.0 Pending fix None released

Oracle Linux 7, 8, 9 and 10, UEK kernel

Important, 8.1 Fixed October 16 ELSA-2026-500397, ELSA-2026-500398, ELSA-2026-500399

Debian trixie (stable)

No score listed Fixed October 16

linux 6.12.113-1 (DSA-6568-1)

Debian 12 (bookworm) No score listed Fixed October 17 and October 22

linux 6.1.190-1 (DLA-4841-1)

linux-6.12 6.12.113-1~deb12u1 (DLA-4850-1)

SUSE Linux Micro 6.2, openSUSE Leap 16.0

Important, 8.1 Fixed

kernel-default 6.12.0-160000.40.1 or later

SLES 15 SP4 to SP6 LTSS, SLES 16.0, SLE Micro 5.3 to 5.5 Important, 8.1 In testing None released

SLES 15 SP7 Important, 8.1 Affected, fix in progress None released

Ubuntu also lists its cloud, FIPS, and real-time kernels as vulnerable or as work in progress. On October 29, Red Hat said its fixes were "currently deferred to a future update." By October 30, it listed RHEL 8, 9 and 10 and OpenShift 4 as Affected rather than Fix deferred. Neither Red Hat nor Ubuntu gives a release date.

How the Bug Works

The race happens during the TCP handshake. When a connection is completed, 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 the options to answer a resent connection request, the kernel reads freed memory.

IPv6 is not affected by this race because the kernel copies the options for IPv6 connections.

The bug was introduced in Linux 4.4. Kernel developers applied a fix on October 5, ten days before the CVE was published.

The fix makes the kernel wait until other CPUs finish reading the options before it frees them.

Why the Scores Differ

The kernel's CVE team added the 9.8 score to the CVE record on October 24, nine days after publishing the CVE.

Its score assumes an attacker who sends a TCP connection request carrying IPv4 Record Route or Timestamp options, then resends the request to try to win the race. The team rated attack complexity low, saying both sides of the race are "attacker-induced."

In that case, the team says, the bug could send nearby kernel memory back to the attacker, corrupt kernel memory, or crash the system.

As of October 28, the National Vulnerability Database (NVD) had not added its own score and showed the kernel team's 9.8. Ubuntu's page also shows that score.

Red Hat's CVE page rates the flaw Moderate, scoring it 5.9 because the race is hard to win. It says the most likely result is a kernel crash, with memory corruption possible in theory, and that "no confidentiality or integrity impact has been demonstrated."

Red Hat's machine-readable advisory file, updated on October 29, lists the impact as Important and scores the flaw 8.1, with high confidentiality and integrity impact. Red Hat's bug tracker changed the issue's severity from medium to high the same day. On that day, the CVE page showed Moderate and 5.9.

Oracle and SUSE also score the flaw 8.1, with the same CVSS vector as Red Hat's advisory file. Those scores match the kernel team's on every metric except attack complexity, which the vendors rate high and the kernel team rates low. Amazon scores it 7.0.

What Anthropic Says It Built

Anthropic described the Linux exploit in an October 27 post about flaws that its AI model, Claude, found and exploited. The Hacker News covered the post on October 28. Claude wrote the exploit after "initial prompting," Anthropic said.

The exploit works in two stages. First, the race makes the kernel send extra bytes back to the attacker. Once one of those replies contains a kernel address, the attacker can get around KASLR, the kernel's memory address randomization.

Second, the same bug overflows a buffer and overwrites a callback pointer with the address of a payload the attacker had already placed in kernel memory. When the kernel calls that pointer, the payload executes a shell command of the attacker's choice as root.

A lost race only fails that one connection attempt and does not crash the system, so an attacker can keep trying, the company said. In its test, leaking a kernel address took 48 minutes, and reaching command execution took 5 more minutes.

The post does not include the exploit code, the kernel version, or the configuration Anthropic tested.

Red Hat says a remote attacker who can reach an exposed TCP service could trigger the race. Anthropic's root exploit, however, targeted servers with a multipath TCP listener. The post does not say whether servers without one can be taken over the same way.

As of October 29, none of the vendor advisories, the CVE record, or Anthropic's post reports attacks using the flaw. It does not appear in a copy of CISA's Known Exploited Vulnerabilities catalog that Luxembourg's CIRCL last updated on October 28.

What to Do Now

Where your distribution has released a fixed kernel, install it. The table above lists the packages and advisories for Debian, Oracle Linux, SUSE and Amazon Linux.

If you build your own kernel, the fix is in Linux 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.56, 7.2.10 and 7.3. The kernel's CVE team recommends updating to the latest stable kernel. It says the kernel community does not support cherry-picking individual fixes.

Ubuntu, RHEL and some Amazon Linux and SUSE releases have no fixed kernel to install yet. Red Hat says no available mitigation meets its standards for being easy to deploy, suitable for most installations and stable. None of the other distributions' advisories lists one.

The distributions' advisories and Anthropic's post do not say whether turning off multipath TCP in the kernel, or blocking incoming connection requests that carry IP options, would lower the risk.

For Go programs, the Go Security Team says setting GODEBUG=multipathtcp=0 in a program's environment disables MPTCP by default at runtime. Adding godebug multipathtcp=0 to the main module's go.mod file does the same at build time, but code that calls SetMultipathTCP overrides both.

The team calls turning off MPTCP program by program a fragile mitigation. A single MPTCP listener left exposed still leaves a system vulnerable, and the approach assumes every working exploit depends on MPTCP.

The team also says the underlying bug is not specific to MPTCP, that systems with no MPTCP listeners may be vulnerable to similar attacks, and that the best fix is to upgrade to a patched kernel.

(The story was updated on October 30 to add the Go Security Team's guidance for Go programs and to reflect that Red Hat now lists RHEL 8, 9 and 10 and OpenShift 4 as Affected rather than Fix deferred.)

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artificial intelligence, linux, network security, Vulnerability

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