A critical Linux kernel flaw in IPv4 TCP handling is now a patching priority for server operators after Anthropic said an unreleased Claude research model produced a working remote-root exploit against a Linux server with a Multipath TCP listener.
Anthropic reported on Oct. 27 that its Frontier Red Team used an internal research model to build a two-stage exploit for CVE-2026-98432. The first stage leaks enough kernel memory to defeat kernel address space layout randomization. The second hijacks control flow and runs an attacker-chosen command as root.
Anthropic said its proof of concept took about 48 minutes to leak a kernel pointer and another five minutes to reach command execution. A failed race only breaks that handshake, so an attacker can try again without necessarily crashing the host.
AWS, in a bulletin updated Nov. 4, said it had no evidence the issue had been leveraged. Public exploit code for the Anthropic chain was not published in the sources reviewed for this article.
The vulnerable path is in ordinary IPv4 TCP
The issue is a use-after-free bug in the kernel code that handles incoming IPv4 TCP connection requests. The Linux kernel CVE Numbering Authority and Ubuntu rate it CVSS 9.8 Critical. Other vendors, including AWS and SUSE, use lower platform-specific scores, reflecting different views of exploit complexity and default configurations rather than a different underlying bug.
During an incoming TCP handshake, a SYN packet can carry IPv4 options. The kernel stores those options with a request socket so it can build the SYN-ACK response. The bug appears when the kernel hands those options to a new child socket without copying them while another CPU is still using the same data to process a retransmitted SYN.
If the memory is freed before the second CPU finishes reading it, the kernel can read freed memory. A related path exists with TCP request socket migration, including configurations using net.ipv4.tcp_migrate_req=1 or certain BPF migration programs. The upstream fix changes the affected code so the memory waits for a read-copy-update grace period before release.
IPv6 is not affected by the main path described in vendor advisories, although Ubuntu and Debian both note a separate IPv6 migration issue tied to CALIPSO that is expected to be addressed separately.
MPTCP is the demonstrated entry point, not the whole bug
Multipath TCP lets one TCP connection use multiple network paths. It matters here because Anthropic’s remote-root exploit targeted a host with an MPTCP listener.
The Go advisory adds an operational wrinkle. Go 1.24 and later enable MPTCP by default for Linux listen sockets. The Go Security Team says setting GODEBUG=multipathtcp=0 or configuring the application can turn it off, but calls this a fragile mitigation because a single remaining MPTCP listener can leave the demonstrated path exposed and because future exploits might not depend on MPTCP.
Red Hat makes the same point from another angle. Red Hat Enterprise Linux disables MPTCP by default, but Red Hat still says affected RHEL systems remain vulnerable because the flaw is in IPv4 TCP handling itself.
Patch status is changing by distribution
The Canadian Centre for Cyber Security says Linux kernels prior to 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.56, 7.2.10 and 7.3 are affected. That upstream version list is useful, but Linux distributions commonly backport fixes without changing to a visibly newer upstream kernel version, so distribution advisories are the safer source for patch decisions.
Debian’s tracker lists fixes for bookworm security, trixie security, forky and sid. Amazon Linux advisories list fixed packages for Amazon Linux 2 and Amazon Linux 2023. AlmaLinux updated its advisory on Nov. 2 to say regular kernel updates now include the fix for AlmaLinux 8, 9 and 10.
Ubuntu’s status was more complicated at the time of review. Canonical said on Nov. 2 that fixes for Ubuntu 22.04 LTS and newer were available in the -proposed pocket while testing continued, and its CVE page last updated Nov. 4 still listed several supported linux packages as vulnerable or work in progress. Red Hat’s pages listed fixes for RHEL 8, 9 and 10 kernels while some layered products, including OpenShift entries, remained affected.
What operators should do now
The consistent advice across Red Hat, Ubuntu, AWS, the Go Security Team and the Canadian Centre for Cyber Security is to install the vendor’s fixed kernel as soon as it is available and make sure the fixed kernel is running. For ordinary package-based systems, that usually means a reboot unless a vendor-supported live patch is active.
Temporary controls can reduce exposure where a kernel fix cannot be installed immediately. Red Hat and AWS describe filtering inbound IPv4 TCP packets that carry IP options. AWS and the Go Security Team also describe disabling MPTCP or setting Go’s MPTCP GODEBUG control. Those measures have tradeoffs. Filtering IP options can break traffic that depends on them, and disabling MPTCP does not remove the underlying kernel bug.
Internet-facing servers, cloud instances, Kubernetes nodes, container hosts and appliances with reachable IPv4 TCP ports deserve priority. Containers do not carry their own kernel for this risk; the host kernel is the boundary that matters.
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