Linux kernel patches 11-year-old TCP bug after Claude writes a root exploit

Overview

Updated Yesterday

Linux kernels have carried a race condition in their IPv4 TCP code since version 4.4, which entered development in October 2015. On October 15, 2026, the kernel's CVE team published it as CVE-2026-98432 and gave it a severity score of 9.8 out of 10.

The bug is a use-after-free: a socket's saved IP options can be freed while another processor is still building a SYN-ACK from them. An attacker who wins the race can overwrite a callback pointer and run commands as root, and Anthropic says its Claude model produced a working exploit in 53 minutes.

Why it matters

Any Linux server taking IPv4 connections from untrusted networks needs the patch, whether or not it enables Multipath TCP.

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Key Indicators

9.8

Kernel team CVSS score

The Linux kernel CVE team rated the flaw 9.8 out of 10, citing low attack complexity.

5.9

Red Hat moderate score

Red Hat called the race hard to win, though its advisory file rated it Important at 8.1.

11 years

Time the flaw went undetected

The faulty commit entered the kernel on October 2, 2015.

53 min

Claude's time to remote root

Anthropic says an unreleased Claude model wrote a working exploit in under an hour.

Kernel branches with fixes

Fixes landed in 6.12.113, 6.18.56, 7.2.10 and 7.3-rc7.

Voices

Curated perspectives — historical figures and your fellow readers.

People Involved

Greg Kroah-Hartman

Linux kernel maintainer and CVE announcer

Active kernel maintainer

Organizations Involved

Linux kernel CVE team

Open-source security response group

Assigned CVE-2026-98432 and published the fix list

The group that assigns CVE identifiers to Linux kernel flaws and publishes the affected and fixed versions.

Anthropic

Artificial intelligence company

Reported the bug and disclosed a Claude-written exploit

The AI lab whose researcher reported the flaw and whose Claude model wrote a working exploit.

Red Hat, Inc.

Linux distributor

Rated the flaw Moderate 5.9 in analysis and Important 8.1 in its advisory

The enterprise Linux vendor whose severity split highlighted disagreement over how winnable the race is.

AlmaLinux OS

Linux distribution

Published patched kernels in testing and an interim filter recipe

The distribution that shipped test kernels and noted its default install cannot open Multipath TCP listeners.

National Vulnerability Database

United States government vulnerability database

Still displayed the kernel team's 9.8 score on October 28

The NIST-run database that had not yet applied its own severity analysis to the entry.

Cybersecurity and Infrastructure Security Agency (CISA)

United States federal agency

Had not listed the flaw in its exploited-vulnerabilities catalog as of October 28

The agency whose Known Exploited Vulnerabilities catalog signals confirmed exploitation.

Go security team

Programming language security group

Warned that Go 1.24 enables Multipath TCP by default for Linux listeners

The team that flagged how a language runtime default widens exposure to the flaw.

Timeline

October 2015 October 2026

7 events Latest: Yesterday

Tap a bar to jump to that date

October 2026
init_on_free=1 floated as interim mitigation
Latest Mitigation

An oss-security post suggests zeroing freed memory to narrow what a use-after-free read exposes.
Yesterday
AlmaLinux ships patched kernels in testing
Distribution

The advisory recommends an nftables rule dropping inbound TCP with IPv4 options.
2 days ago
NVD still lists 9.8; CISA catalog omits the flaw
Scoring

Red Hat's split between Moderate 5.9 and Important 8.1 is also reported that day.
4 days ago
Anthropic says Claude wrote a working root exploit
Research disclosure

The company reports 48 minutes to leak a kernel address and five more to run commands as root.
5 days ago
Kernel CVE team publishes CVE-2026-98432
Disclosure

The announcement assigns a 9.8 score and lists fixes in 6.12.113, 6.18.56, 7.2.10 and 7.3-rc7.
October 15th, 2026
Upstream patch switches to kfree_rcu
Fix

Kernel developers replace plain frees in inet_sock_destruct() and tcp_v4_reqsk_destructor() with kfree_rcu().
October 5th, 2026
October 2015
Faulty TCP code enters Linux 4.4
Origin

Commit 079096f103fa installs syn_recv request sockets in the ehash table without the listener lock.
October 2nd, 2015

Scenarios

No public exploit lands and patching completes quietly

Likely Resolves by Apr 30, 2027

Discussed by: Linux kernel CVE team and AlmaLinux advisories, which both urge patching rather than exploitation watches

Distributions backport the kfree_rcu fix to supported kernels, administrators apply them or the nftables filter, and no confirmed attack appears. The 9.8 score draws attention but the race remains hard to win without the Multipath TCP setup Anthropic used.

CISA adds the flaw to its exploited-vulnerabilities catalog

Possible Resolves by Apr 30, 2027

Discussed by: Security researchers tracking the Anthropic exploit and the gap between kernel and vendor severity scores

A working exploit circulates or attackers reproduce the race against Multipath TCP listeners, giving CISA evidence of use in the wild. Listing in the catalog would trigger patching deadlines for US federal agencies and raise pressure on slower distributors.

A weaponized exploit module ships in Metasploit

Possible Resolves by Apr 30, 2027

Discussed by: Penetration testers and exploit developers following Anthropic's write-up

Anthropic described the exploit but did not publish code. If independent researchers rebuild it and merge a module into a common exploitation framework, attacks become scripted and the pool of capable attackers widens beyond kernel specialists.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

October 2016

Dirty COW (2016)

A race condition in the Linux kernel's copy-on-write memory handling let any local user gain root. It had existed since 2007, and researchers disclosed it with a working exploit and patches in October 2016.

Then

Distributions shipped emergency kernel updates within days.

Now

It became a reference case for how long kernel race conditions can hide and how fast major distributions can patch when pressed.

Why this matters now

Dirty COW was also an old race condition in core kernel memory handling, disclosed with a proof of concept. It shows how a fix can arrive years after the code does.

April 2014

Heartbleed (2014)

A buffer over-read in OpenSSL's heartbeat extension let attackers read adjacent memory from servers. It was assigned CVE-2014-0160 and disclosed alongside a fix in April 2014.

Then

Large numbers of servers needed certificate and software replacement, and the bug drove a broad audit of OpenSSL.

Now

It pushed memory-safety bugs in core network libraries into mainstream security planning.

Why this matters now

Heartbleed was a memory-safety flaw in widely deployed network code, reachable before applications act. CVE-2026-98432 has the same shape: a stale or out-of-bounds read in software almost every server runs.

March-May 2017

EternalBlue and WannaCry (2017)

Microsoft patched a remote Windows SMB flaw in March 2017. An exploit leaked in April, and the WannaCry ransomware used it in May to spread across unpatched machines.

Then

Hundreds of thousands of computers were affected across more than 150 countries.

Now

It became the standard example of patching lag turning a known flaw into a global incident.

Why this matters now

CVE-2026-98432 has a fix and a public description but no published exploit code. EternalBlue shows what happens when the gap between patch and deployment stays open.