Linux kernel flaw leaves Ubuntu and RHEL awaiting fixes
The Linux kernel flaw CVE-2026-98432 affects the kernel, the core software controlling a Linux system, and its handling of Transmission Control Protocol (TCP) connections. The Linux kernel CVE team assigned it a 9.8 score out of 10 under the Common Vulnerability Scoring System (CVSS), which ranks vulnerability severity. Anthropic reported on October 27 that its AI model produced an exploit capable of running commands as root, the system's highest-privilege account, without credentials against a Linux server with a multipath TCP listener. As of October 29, Ubuntu and Red Hat Enterprise Linux (RHEL) did not have fixed kernel packages available.
The underlying defect is a race condition, meaning two operations can interact in an unsafe order, during the setup of new IPv4 TCP connections. Once the connection handshake finishes, the kernel gives the new connection access to stored IP options without first making a separate copy. If the connection is destroyed while another processor is still consulting those options to respond to a repeated request, the kernel can access memory that has already been released. This is known as a use-after-free flaw. IPv6 does not have the same problem because its connection code copies the relevant options.
The bug entered Linux in version 4.4. Kernel developers applied an upstream correction on October 5, ten days before the CVE was published. The change delays freeing the options until other processors have finished reading them. Anthropic said the correction was also added to maintained stable kernel branches, but people running distribution-supplied kernels must wait for their Linux vendor to package and release it. For organisations compiling their own kernels, fixed versions listed by the kernel team are 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 team recommends moving to the newest stable kernel rather than selecting and applying this change by itself.
Anthropic connected the vulnerability to Multipath TCP (MPTCP), a feature that allows one connection to use several network paths. On Linux, software must specifically request MPTCP when opening a connection. Neither the CVE record nor the upstream correction identifies MPTCP as a requirement, and Anthropic did not establish whether the same takeover technique works against servers without an MPTCP listener.
According to Anthropic, its AI model wrote a two-stage exploit after initial instructions. The first stage repeatedly triggered the race until a response disclosed nearby kernel memory, including an address that allowed the exploit to bypass Kernel Address Space Layout Randomization (KASLR), which normally makes important kernel locations harder to predict. The second stage used the flaw to overflow a memory area and replace a callback pointer with the location of attacker-controlled instructions already placed in kernel memory. When the kernel followed that pointer, it executed an attacker-selected command as root.
Anthropic said an unsuccessful race attempt merely caused that connection attempt to fail, allowing repeated tries without crashing the system. In its test, obtaining a kernel address took 48 minutes, followed by another five minutes to reach command execution. The company did not publish the exploit, identify the tested kernel version or disclose the configuration used. No vendor advisory, CVE entry or Anthropic publication had reported attacks exploiting the flaw as of October 29. It was also absent from the copy of CISA's Known Exploited Vulnerabilities catalogue last updated by Luxembourg's CIRCL on October 28.
Severity assessments differ because vendors disagree about how difficult the race is to win. The kernel CVE team added its 9.8 rating on October 24 and treated attack complexity as low because an attacker can create both competing events by sending an IPv4 connection request with Record Route or Timestamp options and then repeating it. The team says possible results include disclosure of nearby kernel memory, memory corruption or a system crash. The National Vulnerability Database had not supplied a separate score as of October 28 and displayed the kernel team's assessment.
Red Hat's CVE page rated the flaw Moderate at 5.9, reasoning that winning the race is difficult and that a crash is the most likely outcome. It said confidentiality or integrity damage had not been demonstrated, although memory corruption was theoretically possible. In contrast, Red Hat's machine-readable advisory, updated October 29, classified it as Important at 8.1 with high confidentiality and integrity impact, while its bug tracker moved the severity from medium to high. The enterprise Linux vendor and SUSE also assigned 8.1, treating attack complexity as high. Amazon rated it 7.0.
Distribution patch availability remains uneven. Ubuntu marked versions 22.04 LTS, 24.04 LTS, 26.04 LTS and 26.10 as work in progress, while 16.04, 18.04 and 20.04 were listed as vulnerable. Its cloud, FIPS and real-time kernels were also vulnerable or awaiting work. RHEL 8, 9 and 10 and OpenShift 4 had their corrections deferred to a future update, with no release date, while RHEL 7 was not affected. Red Hat said it had no mitigation meeting its requirements for broad suitability, stability and ease of deployment.
Fixed packages were available for several other systems. Debian trixie received linux 6.12.113-1 through DSA-6568-1 on October 16. Debian 12 received linux 6.1.190-1 through DLA-4841-1 on October 17 and linux-6.12 6.12.113-1~deb12u1 through DLA-4850-1 on October 22. The enterprise Linux distribution's versions 7, 8, 9 and 10 received UEK fixes on October 16 through ELSA-2026-500397, ELSA-2026-500398 and ELSA-2026-500399. SUSE Linux Micro 6.2 and openSUSE Leap 16.0 were fixed in kernel-default 6.12.0-160000.40.1 or later, while several SLES and SLE Micro releases remained in testing or had work under way.
Amazon Linux 2023 fixed its kernel6.18 package on October 24 with kernel6.18-6.18.56-125.164.amzn2023 under ALAS2023-2026-3251. Its kernel6.12 advisory listed kernel6.12-6.12.113-138.209.amzn2023 as fixed on October 29 under ALAS2023-2026-3335, although Amazon's CVE page still showed that work as pending. Amazon Linux 2 and the standard Amazon Linux 2023 kernel package did not yet have fixes.
Administrators should install their distribution's corrected kernel as soon as it becomes available and continue checking vendor advisories where a release is pending. The advisories do not establish that disabling MPTCP or blocking connection requests carrying IP options would reduce exposure, so those steps should not be treated as confirmed substitutes for patching. For organisations reviewing exposed Linux systems and patch plans, AEU-I provides security-first IT, infrastructure and consulting.