Critical Linux Networking Flaw: Ubuntu Servers Still Have No Patch

A critical bug in the part of Linux that accepts new network connections is still unpatched on Ubuntu. The flaw, tracked as CVE-2026-98432, was fixed in the Linux kernel on October 5 and is rated 9.8 out of 10 by the kernel’s own security team.^{1, 2} Red Hat, Debian and Amazon have since released fixed kernels, but Ubuntu’s security page still lists its fixes as “work in progress”.^{3, 4, 5, 6}

Reporting checked November 2, 2026, 11:27 AM ET.

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An attacker on the network can send specially crafted connection requests to a Linux machine, with no password. In the worst case this leaks or corrupts the kernel’s memory or crashes the machine.^{2, 3}

Anthropic says one of its unreleased AI models wrote an exploit that takes full control of Linux servers that use a feature called Multipath TCP. Its post describes the exploit but doesn’t include the code.⁷

Red Hat released fixes on November 1, and Debian and Amazon Linux already have them. Ubuntu 22.04, 24.04, 26.04 and 26.10 are still listed as vulnerable, with no release date.^{3, 4, 5, 6}

No attacks using the flaw have been reported so far.^{4, 5}

If you run an Ubuntu server, limit who can reach it, consider a temporary firewall rule, and install the kernel update and reboot as soon as it arrives.

What the bug is

Whenever a computer accepts a new network connection, the two sides first go through a short “handshake”. The bug is in the Linux code that runs this handshake for IPv4 TCP connections, which nearly every networked service uses, from websites to remote logins.⁸ An attacker adds rarely used extras, called IP options, to a connection request and then sends the request again at just the right moment. That can make the kernel, the core of the operating system, keep using a piece of memory after it has already been freed.²

According to the kernel’s CVE record, this can send pieces of kernel memory back to the attacker, let them write to memory they shouldn’t reach, or crash the system. All of this happens before any login or password check.² The bug affects Linux kernels from version 4.4 onwards.^{2, 4} Connections over IPv6, the newer internet addressing system, are not affected by the main bug, according to the fix’s description.¹

The AI exploit claim

The fix credits a researcher at Anthropic, the company that makes the Claude AI models, with reporting the bug.¹ On October 27, Anthropic’s Frontier Red Team wrote that, “after initial prompting”, an unreleased internal research model wrote an exploit that lets a remote attacker run commands as root, the all-powerful administrator account. It works against Linux servers with a Multipath TCP listener, without any credentials.⁷ In Anthropic’s test, the first stage took 48 minutes and running a command as root took another five. A failed attempt only breaks that one connection and doesn’t crash the machine, so an attacker “can retry indefinitely”, Anthropic wrote.⁷

Multipath TCP (MPTCP) lets one connection use several network paths at once. Most software doesn’t switch it on. But since version 1.24, programs written in Go, a popular language for server software, turn it on by default for incoming connections on Linux, so some services may use it without their owners knowing.^{4, 9}

BleepingComputer notes that taking over a server without an MPTCP listener has not been publicly demonstrated.⁴ The Go Security Team still warns that “the underlying bug is not specific to MPTCP” and that systems without it may be vulnerable to similar attacks.⁹

How serious is it?

Experts have disagreed about how hard the bug is to use. Red Hat first treated it mainly as a way to crash servers and scored it 5.9, BleepingComputer reported.⁴ Red Hat now rates it Critical at 9.8. It says the flaw lets “an unauthenticated remote attacker” cause kernel memory corruption or an immediate crash “over standard IPv4 TCP network traffic”.³ Red Hat says systems that accept incoming TCP connections are the most directly exposed.⁸

There are no confirmed reports of attacks so far. BleepingComputer reported that on October 30, and Amazon Web Services, in a bulletin last updated November 1, says it is not aware of the flaw being exploited.^{4, 5}

Who has a fix

Red Hat Enterprise Linux 8, 9 and 10: fixed kernels released November 1. A few related packages, such as the RHEL 9 real-time kernel and OpenShift, are still listed as affected.³
Debian 12 and 13: fixed in kernel packages 6.1.190-1 and 6.12.113-1.⁴
Amazon Linux 2 and 2023: fixed kernels available. Reboot after updating.⁵
Oracle Linux and SUSE: fixes for some kernel versions.⁴
Ubuntu: 22.04 LTS, 24.04 LTS, 26.04 LTS and 26.10 are listed as “Vulnerable” with a fix “work in progress”. 16.04, 18.04 and 20.04 are listed as vulnerable. The page gives no release date.⁶
People who build their own kernels: the fix is in the stable versions 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.56 and 7.2.10, and in 7.3.²

What you can do

The machines most at risk are the ones that accept connections from the internet, such as web servers, rented cloud servers (VPS) and home servers you can reach from outside.⁸ Everyone running Ubuntu should still install the kernel update when it arrives.

Install the update and restart. Keep an eye on Ubuntu’s page for CVE-2026-98432. When your version shows as fixed, run sudo apt update && sudo apt full-upgrade and then reboot, because a new kernel is only used after a restart. AWS calls a kernel update and reboot “the only complete remediation”.^{5, 6}
Limit who can reach your server. Close ports you don’t need, and allow admin access such as SSH only from addresses you trust. BleepingComputer notes that restricting access to trusted networks or IP addresses can reduce the risk until a patch is out.⁴
Consider a temporary firewall rule. Red Hat publishes a rule for nftables, the standard Linux firewall tool, that drops incoming IPv4 TCP packets carrying IP options. Ordinary traffic doesn’t use these. Red Hat warns that the rule “reduces exposure to this vulnerability but cannot fully prevent it” and should be tested before use.⁸ Save this as cve-2026-98432.nft:
```
table inet cve_2026_98432 {
    chain input {
        type filter hook input priority -310; policy accept;
        ip protocol tcp ip hdrlength > 5 counter drop
    }
}
```
Check it with sudo nft --check --file cve-2026-98432.nft, then apply it with sudo nft --file cve-2026-98432.nft. If the nft command is missing, install it with sudo apt install nftables. A rule loaded this way is gone after a restart. Once you’re on a fixed kernel, remove it with sudo nft delete table inet cve_2026_98432.⁸
If you run Go programs: setting GODEBUG=multipathtcp=0 in a program’s environment turns off MPTCP for it. The Go Security Team calls this “a fragile mitigation” and recommends a patched kernel instead.⁹
If you rent a cloud server: updating its kernel is usually your job, not the provider’s. AWS says customers running their own Linux servers on EC2 must update the kernel themselves by following their distribution’s guidance.⁵

Critical Linux Networking Flaw: Ubuntu Servers Still Have No Patch

What the bug is

The AI exploit claim

How serious is it?

Who has a fix

What you can do

Sources

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What the bug is

The AI exploit claim

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