CVE-2026-98432: Linux Kernel TCP Use-After-Free

• Your servers run Ubuntu or RHEL

  Including Rocky Linux, AlmaLinux, CentOS Stream, OpenShift and RHEL CoreOS nodes

  Act now

  No fixed kernel from Ubuntu or Red Hat yet. Apply the packet filter now and put internet-facing hosts first in line for the fix. Until you patch

• You run Go 1.24+ services on Linux

  Go opens multipath TCP listeners by default where the kernel allows it

  Act now

  These hosts match the setup Anthropic’s exploit used. Patch them first, and turn off multipath TCP if you do not need it. Multipath TCP

• Your servers run Debian, Amazon Linux, Oracle UEK or current SUSE

  Fixed kernels are published

  Update needed

  Install the fixed kernel and reboot. The fix is not active until the host boots into it. Fixed versions

• You run containers or Kubernetes

  Docker, Podman, EKS, GKE, AKS or self-managed nodes

  Update needed

  Containers use the host kernel, so new images do not help. Patch and reboot the nodes, or roll to a fixed node image from your provider.

• You use managed or serverless platforms only

  Lambda, Cloud Run, Fargate, managed databases and similar services

  Low risk

  The provider runs the kernel. Watch for its notice, and record the dependency in your vendor review.

• Your hosts run Linux older than 4.4, or RHEL 6 or 7

  Kernels that predate the vulnerable code

  Protected

  Not affected by this flaw, according to the CVE record and Red Hat. These kernels are out of support for other reasons.

1. 01

   List your Linux hosts and their kernels

   Include container hosts, Kubernetes nodes, appliances and cloud images, not just application servers. Compare each against your distribution’s status, not the upstream version list. How to check
2. 02

   Install the fixed kernel and reboot

   Debian, Amazon Linux, Oracle UEK and several SUSE releases have one now. A kernel update does nothing until the host boots into it. Fixed versions
3. 03

   Filter IPv4 options where you cannot patch yet

   On Ubuntu and the RHEL family, apply the nftables rule that drops incoming TCP packets carrying IPv4 options. Ordinary traffic does not use them. The rule
4. 04

   Find and remove multipath TCP listeners

   Anthropic’s exploit targeted them, and Go 1.24 and later open them by default. Turn multipath TCP off where you do not use it, and patch hosts that have these listeners first.
5. 05

   Record the decisions

   Note which hosts were exposed, which controls you applied, and when each moved to a fixed kernel. For compliance

Upstream Linux

Every kernel from 4.4 onward without the fix is affected, according to the CVE record. The kernel team recommends moving to the latest stable release for your branch rather than cherry-picking the commit.

Distributions, as of 1 November 2026

Trackers: Ubuntu · Red Hat · AlmaLinux · Debian · SUSE · Oracle · Amazon Linux

Red Hat says any product built on the RHEL kernel, including RHEL CoreOS, OpenStack Platform and Virtualization, is potentially affected. Oracle’s page lists only its UEK kernels; check the Red Hat compatible kernel separately.

Kernel version and pending reboots

uname -r                            # running kernel
dpkg -l 'linux-image*' | grep ^ii   # Debian and Ubuntu: installed kernels
rpm -q kernel                       # RHEL family: installed kernels
needs-restarting -r                 # RHEL family: is a reboot pending?

If the newest installed kernel is fixed but uname -r shows an older one, the host still needs a reboot.

Multipath TCP listeners

sysctl net.mptcp.enabled    # 1 means applications may open MPTCP sockets
sudo ss -Mlnp               # MPTCP listeners and the processes that own them

ss -M lists only multipath TCP sockets, so empty output means no such listener. A host has one only if an application asks for it. Go programs built with Go 1.24 or later do so by default for listening sockets, per the Go Security Team; the default follows the go line in the program’s go.mod (GODEBUG docs). AlmaLinux and CIQ say multipath TCP is off by default on their kernels.

On some RHEL 8 era kernels, CIQ found ss did not show the multipath row, so use the sysctl check there.

1. Drop TCP packets that carry IPv4 options

The bug is only reachable through a connection request that carries IPv4 options, such as Record Route or Timestamp, according to the CNA’s scoring notes. Normal TCP traffic does not use them; TCP’s own options live in the TCP header and are unaffected. This is AlmaLinux’s rule, which hooks prerouting so it also covers traffic the host forwards to containers. It lives in its own table, works alongside firewalld, and is safe to run more than once.

sudo nft -f - <<'EOF'
add table ip cve_2026_98432
delete table ip cve_2026_98432
table ip cve_2026_98432 {
    chain prerouting {
        type filter hook prerouting priority -300; policy accept;
        meta l4proto tcp ip hdrlength > 5 counter drop
    }
}
EOF
sudo nft list table ip cve_2026_98432        # counter shows drops

sudo nft delete table ip cve_2026_98432

• The rule does not survive a reboot. Reapply it if you reboot before a fixed kernel is available.
• Traffic between containers on the same host may bypass it, AlmaLinux warns.
• Do not use it on networks that rely on CIPSO security labels, which travel in IPv4 options.
• Red Hat’s version hooks input instead, and Red Hat says it “reduces exposure to this vulnerability but cannot fully prevent it.”
• If your edge firewall or cloud load balancer can drop IPv4 packets with options, doing it there protects every host behind it.

2. Turn off multipath TCP if you do not use it

echo 'net.mptcp.enabled = 0' | sudo tee /etc/sysctl.d/90-disable-mptcp.conf
sudo sysctl -w net.mptcp.enabled=0
sudo ss -Mlnp      # existing listeners stay MPTCP: restart the services listed

This removes the setup Anthropic’s exploit used, not the bug. CIQ’s testing found a plain TCP listener still processed the trigger with multipath TCP off, so do not rely on this step alone. The setting is per network namespace: containers keep their own value unless you set it there too, for example with Docker’s --sysctl net.mptcp.enabled=0.

For a single Go program, GODEBUG=multipathtcp=0 in its environment, or godebug multipathtcp=0 in go.mod, turns the default off. The Go team calls program-by-program changes “a fragile mitigation” and is discussing its response in golang/go#82717.

3. Reduce what is exposed

Close TCP ports the internet does not need, keep multipath TCP listeners off public networks, and patch internet-facing hosts first. One extra hardening step has been proposed on oss-security: the init_on_free=1 boot parameter zeroes freed memory, which limits what the memory leak can reveal. It needs a reboot, costs some performance, and is not vendor guidance.

When a connection request arrives with IPv4 options, the kernel saves the options so it can echo them in its reply. Once the handshake completes, it hands that saved copy to the new connection without duplicating it. If the client resends its connection request at the right moment, another CPU can be building a reply from the same options while the new connection is closed and its memory freed. The reply code then reads memory that no longer belongs to it.

The CNA’s scoring notes say this can send leftover kernel memory back to the attacker, let stale lengths and offsets steer kernel writes out of bounds, or crash the host. It all happens before any application or login check runs. A similar race exists when request socket migration is on (net.ipv4.tcp_migrate_req=1, off by default). IPv6 is not affected by the main race because the kernel copies IPv6 options; a narrower IPv6 migration case will be fixed in a separate patch.

The fix, by Eric Dumazet, makes the kernel wait until no CPU can still be reading the options before freeing them, as other code paths already did. It credits Xinyang Ge of Anthropic with the report.

• Anthropic’s exploit. Anthropic says an unreleased Claude model wrote an exploit that lets a remote attacker “run arbitrary commands as root, without any credentials or user interaction” on servers with a multipath TCP listener. It first leaks a kernel address to defeat address randomisation, then overwrites a callback pointer. A lost race “only fails the handshake,” so the attacker can retry. In its test, the leak took 48 minutes and command execution another five. The code is not public.
• In the wild. No vendor reviewed reports exploitation, and the CVE is not in CISA’s KEV catalog as of 1 November 2026.
• Public code. About 40 GitHub repositories reference the CVE as of 1 November. Most are checkers and mitigations, but several describe race triggers or proofs of concept. We have not tested them and do not link them.
• Signs to watch. Failed attempts do not crash the host. Look for inbound SYNs with IPv4 options (the nftables counter above counts them) and for unexplained kernel oopses mentioning ip_options_build or skb_under_panic.

NVD has not published its own analysis; scanners that read NVD alone may show no score yet.

1. 2016-01-10

   2016-01-10

   Linux 4.4 ships the change that lets request sockets be processed without the listener lock, which introduced the race.

   Linux kernel CVE announcement

2. 2026-10-15

   2026-10-15 · 12:08 UTC

   The Linux kernel CNA publishes CVE-2026-98432. 7.2.10, 6.18.56, 6.12.113 and 7.3-rc7 already carry the fix; backports to older longterm branches follow.

   linux-cve-announce

3. 2026-10-16

   2026-10-16

   Oracle publishes Unbreakable Enterprise Kernel errata for Oracle Linux 7 to 10.

   Oracle Linux CVE page

4. 2026-10-24

   2026-10-24

   Amazon Linux 2023 fixes its kernel6.18 package.

   Amazon Linux Security Center

5. 2026-10-27

   2026-10-27

   Anthropic describes a remote root exploit for the bug, written by an unreleased Claude model, against servers with a multipath TCP listener.

   Anthropic

6. 2026-10-29

   2026-10-29

   The Go Security Team warns that Go 1.24 and later open multipath TCP listeners by default on Linux. SUSE publishes fixed kernels for several products.

   oss-security

7. 2026-10-30

   2026-10-30

   Red Hat raises its rating to Critical and publishes bulletin RHSB-2026-016 with an nftables mitigation. AlmaLinux puts fixed kernels in testing. Amazon Linux fixes its remaining kernels.

   Red Hat RHSB-2026-016

8. 2026-10-31

   2026-10-31

   The CVE record lists fixes for all eight maintained branches. NVD still shows it as Received, without its own analysis.

   CVE record

Evidence to keep: the date you assessed it, the host inventory with kernel versions, which hosts had multipath TCP listeners, the filter and sysctl changes with their dates, and each host’s reboot into a fixed kernel. If your patch policy sets a deadline for critical vulnerabilities that Ubuntu or Red Hat cannot meet, record the exception and the compensating controls.

For how these controls work day to day, see our guides to ISO 27001 vulnerability management, SOC 2 vulnerability management and ISO 27001 change management.

• CVE-2026-98432, CVE record (Linux kernel CNA)Description, affected and fixed versions, CVSS 9.8 score and the CNA’s scoring notes.
• Fix commit e3f33b0, ipv4: free inet_opt and ireq_opt after an RCU grace periodThe upstream fix by Eric Dumazet, crediting Xinyang Ge of Anthropic.
• Red Hat RHSB-2026-016Critical rating, affected products and the nftables mitigation.
• Red Hat CVE pagePer-product status for RHEL and OpenShift.
• Ubuntu CVE trackerPer-release and per-kernel status for Ubuntu.
• Debian security trackerFixed versions and advisories for bookworm, trixie and unstable.
• SUSE CVE pageFixed packages by product and SUSE’s scoring.
• Oracle Linux CVE pageUEK errata for Oracle Linux 7 to 10.
• Amazon Linux Security CenterAdvisories for Amazon Linux 2 and 2023 kernels.
• AlmaLinux: patched kernels available in testingFixed kernel versions, the prerouting filter, its limits, and multipath TCP checks.
• Anthropic: Discovering and exploiting vulnerabilities with ClaudeThe remote root exploit against multipath TCP listeners.
• Go Security Team on oss-securityGo’s multipath TCP default and how to change it.
• CIQ: Mitigating CVE-2026-98432 on Rocky LinuxTesting that shows disabling multipath TCP alone leaves plain TCP reachable.
• CISA Known Exploited Vulnerabilities catalogNo entry for CVE-2026-98432 as of 1 November 2026.