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Ubuntu security tracker status table for CVE-2026-98432 showing the linux package as vulnerable, with work in progress, on Ubuntu 26.10, 26.04 LTS, 24.04 LTS and 22.04 LTS.
Image: Canonical Ubuntu security tracker (fair use), captured Oct. 29, 2026. Ubuntu lists its supported kernels as vulnerable, with fixes in progress.

Response

Anthropic Says Claude Wrote a Remote Root Exploit for Linux Kernel Flaw CVE-2026-98432, Rated 9.8, as Ubuntu and Red Hat Still Have No Fixed Kernel to Install

The IPv4 TCP handshake race, present since Linux 4.4, is fixed upstream and in Debian, Oracle Linux and some SUSE and Amazon Linux kernels. Vendors score it from 7.0 to 9.8, no advisory reports exploitation in the wild, and Red Hat says no mitigation meets its standards.

Noah Park, ResponseNew York5 min read

NEW YORK - A race condition in the Linux kernel's IPv4 TCP code that Anthropic says its AI model turned into a remote root exploit is fixed upstream and in several distributions, but as of Thursday Ubuntu and Red Hat Enterprise Linux still listed their kernels as vulnerable with no fixed package to install.

The flaw, CVE-2026-98432, was published on Oct. 15 and carries a 9.8 CVSS score from the Linux kernel's own CVE team, which rates it as exploitable over the network with low attack complexity, no privileges and no user interaction. It was introduced in Linux 4.4. No vendor advisory reviewed by SOCtember reports exploitation in the wild, and the flaw was not in the copy of CISA's Known Exploited Vulnerabilities catalog released on Oct. 28.

What the bug is

According to the kernel CVE record, when a new IPv4 TCP connection completes, the kernel hands the IP options stored with the pending request to the child socket without copying them. Another CPU can at the same moment process a retransmitted SYN for the same request and build a SYN-ACK from those options. Because child sockets free the options with a plain kfree() rather than after an RCU grace period, the result is a use-after-free in ip_options_build(). The record describes a similar problem when request sockets are migrated, with net.ipv4.tcp_migrate_req set to 1 or a BPF reuseport migration program. IPv6 is not affected by the first issue because the kernel duplicates the options there.

Anthropic's exploit

Section of Anthropic's Oct. 27 post describing a zero-click remote code execution exploit in the Linux kernel networking stack.
Image: Anthropic (fair use).The section of Anthropic's post that describes the Linux exploit.

Anthropic described the exploit in an Oct. 27 post on vulnerabilities its models found and exploited. "After initial prompting, Claude wrote an exploit that allows a remote attacker to send packets to Linux servers with a multipath TCP listener to run arbitrary commands as root, without any credentials or user interaction," the company wrote, naming CVE-2026-98432.

The exploit has two stages. The first abuses the race to make the kernel copy extra bytes back to the attacker until a reply leaks a kernel pointer, defeating KASLR. The second uses the same bug to overflow a buffer and overwrite a callback pointer with the address of a payload already staged in kernel memory, ending in a shell command run as root. A lost race "only fails the handshake without crashing the whole system, so a determined attacker can retry indefinitely until they succeed," Anthropic said. In its proof of concept, leaking a kernel pointer took 48 minutes and reaching command execution took another five. The post does not publish exploit code.

Who has a fix

The upstream 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. Debian has fixed packages for bookworm and trixie through DLA-4841-1, DLA-4850-1 and DSA-6568-1. Oracle shipped Unbreakable Enterprise Kernel errata for Oracle Linux 7, 8, 9 and 10 on Oct. 16. SUSE lists released fixes for many products and others still in progress.

Ubuntu rates the issue High priority and shows the 9.8 score. Its tracker lists the linux package as vulnerable, with work in progress, on 26.10, 26.04 LTS, 24.04 LTS and 22.04 LTS, and as vulnerable on 20.04, 18.04 and 16.04 LTS.

Red Hat CVE page for CVE-2026-98432 showing Important severity, a CVSS v3 score of 8.1 and a last-modified time of Oct. 29, 2026.
Image: Red Hat (fair use), captured Oct. 29, 2026.Red Hat rates the flaw Important and scores it 8.1.

Red Hat rates the flaw Important with a score of 8.1, judging attack complexity high rather than low, and its machine-readable advisory lists affected RHEL kernels with no fix yet available. Its description says the issue "primarily results in a system crash leading to a Denial of Service (DoS) or potential memory corruption." On mitigation, Red Hat says the options available "do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability." Oracle and SUSE also score it 8.1.

Amazon Linux Security Center table for CVE-2026-98432 showing pending fixes for Amazon Linux 2 and the base Amazon Linux 2023 kernel, and fixes for kernel6.12 and kernel6.18.
Image: Amazon Web Services (fair use), captured Oct. 29, 2026.Amazon has fixed two Amazon Linux 2023 kernel streams; the rest are pending.

Amazon scores it 7.0. It released fixes for the Amazon Linux 2023 kernel6.18 stream on Oct. 24 and the kernel6.12 stream on Oct. 29, while Amazon Linux 2 and the base Amazon Linux 2023 kernel remain marked "Pending Fix."

What responders should do

Where a fixed kernel exists, install it and reboot; the fix does not take effect until the new kernel is running. The kernel CVE team recommends updating to the latest stable kernel and says cherry-picking individual commits "is not recommended or supported by the Linux kernel community at all."

For Ubuntu and RHEL fleets, the job for now is inventory and exposure. List internet-facing Linux hosts with TCP listeners by kernel version, flag any that run a multipath TCP listener, which is the configuration Anthropic's exploit targeted, and check whether net.ipv4.tcp_migrate_req is enabled. None of the vendor advisories or Anthropic's post says whether disabling multipath TCP or filtering SYNs that carry IP options reduces the risk, so treat any such change as a judgment call to test, not a vendor-backed workaround. Because a lost race fails a single handshake and Red Hat says the most likely outcome is a crash, unexplained kernel crashes on exposed servers deserve a closer look while patches are pending.

Sources:


Noah Park covers incident response and blue-team operations for SOCtember from New York.

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