Linux kernel IPv4 TCP use-after-free, remote root on MPTCP servers: CVE-2026-98432

• Your Linux hosts have an MPTCP listener

  Including any Go 1.24+ service on a kernel with MPTCP enabled

  Act now

  Patch and reboot these first. Until you can, apply the packet filter and turn MPTCP off. How

• You run Ubuntu on internet-facing servers

  Fixed kernels only in the -proposed pocket as of 5 November 2026

  Act now

  Apply the interim mitigations now and install the fixed kernel the day it reaches the updates pocket. Status

• Any other Linux host that accepts IPv4 TCP connections

  Web, SSH, database, load balancer, Kubernetes node

  Update needed

  Install your distribution's fixed kernel and reboot. Fixed kernels

• You run containers or Kubernetes

  Pods and containers use the node's kernel

  Update needed

  Patch and reboot the nodes. New container images do not fix a kernel bug.

• You ship an appliance or image with a bundled kernel

  Firmware, virtual appliance, custom cloud image

  Update needed

  Rebuild on a fixed kernel and tell customers which release carries it.

• Your kernel is older than 4.4

  For example the RHEL 7 3.10 kernel

  Low risk

  Not affected by this bug: the vulnerable code arrived in 4.4. Red Hat confirms RHEL 6 and 7 are not affected.

1. 01

   Find the running kernel on every host and node

   Run these on hosts and Kubernetes nodes, not inside containers. Compare the result with your distribution's status, not with the upstream version number.

   ShellCopy

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

2. 02

   Find hosts with multipath TCP listeners

   These match the setup of the only demonstrated exploit. An empty ss -M result means no MPTCP listener. Go services built with Go 1.24 or later show up here whenever the kernel allows MPTCP. Red Hat cautions that an empty result is not a test for exposure: every host still needs the fixed kernel.

   ShellCopy

   sysctl net.mptcp.enabled    # 1 means applications may open MPTCP sockets
   sudo ss -Mlnp               # MPTCP listening sockets and their processes

3. 03

   Install the fixed kernel and reboot, exposed hosts first

   Order: internet-facing hosts with MPTCP listeners, then other internet-facing hosts, then internal ones. A kernel update does nothing until the machine boots into it.
4. 04

   No fixed kernel yet? Mitigate today

   On Ubuntu, drop IPv4 TCP packets that carry IP options and turn off MPTCP if you do not use it. Commands
5. 05

   Confirm after the reboot

   Rerun uname -r and check it against the fixed version, then remove any temporary packet filter.

Kernels before 4.4 are not affected. The kernel CVE team recommends moving to the latest stable release for your line rather than cherry-picking the fix. Distribution kernels carry backports under their own version numbers.

Red Hat says any product built on the RHEL kernel, including RHEL CoreOS, OpenStack Platform and Virtualization, is potentially affected. Rocky Linux, CloudLinux and other RHEL rebuilds follow the same kernel, so check their own trackers.

When a SYN arrives with IPv4 options, such as Record Route or Timestamp, the kernel stores them with the half-open connection (the request socket) so it can echo them in the SYN-ACK. When the handshake completes, tcp_v4_syn_recv_sock() hands that options block to the new socket without copying it.

Since Linux 4.4, request sockets are processed without the listener lock. If the peer retransmits its SYN at the right moment, another CPU can be building a SYN-ACK from the shared options while the new socket is destroyed and inet_sock_destruct() frees them with a plain kfree(). ip_options_build() then reads freed memory. A similar race exists during request socket migration (net.ipv4.tcp_migrate_req=1, off by default).

• Before authentication. The CVE record says the path runs during the TCP handshake, before any application sees the connection. Any listening IPv4 TCP port is a way in.
• Read, write, crash. Per the kernel CNA, a reclaimed object can leak kernel heap memory to the peer in the SYN-ACK, steer out-of-bounds kernel writes, or panic the host.
• Free retries. Anthropic says a lost race only fails that handshake, so an attacker can retry indefinitely. In its proof of concept, leaking a kernel pointer to defeat KASLR took 48 minutes and reaching root command execution another five.
• The fix. Eric Dumazet's commit changes both frees to kfree_rcu(), which waits until every reader has finished. IPv6 is not affected by the main issue because it copies the options; a related IPv6 migration case that needs CALIPSO will get a separate patch.

Anthropic's exploit works against "Linux servers with a multipath TCP listener". Neither the CVE record nor the fix mentions MPTCP, and Anthropic has not explained why the listener matters. A host has one only when an application opens an MPTCP socket and the kernel permits it (net.mptcp.enabled = 1, the upstream default). RHEL, AlmaLinux and CloudLinux ship with it off.

1. Drop IPv4 TCP packets that carry IP options

The CNA's scoring notes say the attacker supplies IPv4 Record Route or Timestamp options in the SYN. Legitimate TCP traffic almost never carries IPv4 options (TCP options such as SACK and window scaling live in the TCP header and are unaffected). ip hdrlength > 5 matches any IPv4 header longer than the 20-byte minimum, which only options produce. This is AlmaLinux's rule; it hooks prerouting before connection tracking, so it also covers traffic the host forwards to containers. It is safe to run more than once and does not survive a reboot.

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 dropped packets
sudo nft delete table ip cve_2026_98432   # remove after the fixed kernel

• Red Hat's variant hooks input at priority -310 in an inet table and says it "reduces exposure to this vulnerability but cannot fully prevent it."
• AlmaLinux warns that traffic between containers on the same host may bypass the rule, and that it must not be used on networks that rely on CIPSO security labels, which travel in IPv4 options.
• If your edge firewall or cloud network can drop IPv4 packets with options, doing it there protects every host behind it.

2. Turn off MPTCP if you do not use it

This removes the setup Anthropic's exploit needs, not the bug. New MPTCP sockets fail and Go programs fall back to plain TCP when they start, but listeners already open stay MPTCP until their service restarts. The setting is per network namespace, so set it inside containers too (Docker: --sysctl net.mptcp.enabled=0).

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

For a single Go service, the Go Security Team documents two switches. Code that calls SetMultipathTCP overrides both.

# Runtime, per service
GODEBUG=multipathtcp=0

# Build time, in the main module's go.mod
godebug multipathtcp=0

3. Prioritise and restrict

Keep MPTCP listeners off the internet, restrict management ports to known networks, and put hosts that must stay exposed at the top of the reboot schedule. Because the bug sits below every application, a stricter application config does not help.

A post on oss-security suggests booting with init_on_free=1, which zeroes freed memory and narrows what a use-after-free read can leak. It is a community suggestion, not a vendor recommendation, it needs a reboot and costs some performance. If you can reboot, reboot into a fixed kernel where one exists.

1. Kernel oopses in the vulnerable path

Per the CNA, a failed attempt can trigger skb_under_panic() or an oops in ip_options_build(). Unexplained reboots on exposed hosts deserve a look. Persistent journald is needed to see previous boots.

# Kernel crashes in the vulnerable path, including previous boots
sudo journalctl -k -b all | grep -E "skb_under_panic|ip_options_build|ip_build_and_send_pkt"

2. SYNs carrying IPv4 options

The packet filter's counter is a cheap sensor: a rising count on a host that sees no legitimate IP options means someone is sending them. Repeated option-bearing SYNs and retransmissions from one source in flow logs or an IDS are the same signal at the network edge.

3. Unexpected root processes

A successful exploit runs a command as root from kernel context, so its parent will not be a normal service. Look for shells, downloaders or new persistence with no matching login or deployment.

If you find signs of compromise, treat the host as lost: rebuild it on a fixed kernel and rotate every credential and key it held.

1. 2015-10-02

   2015-10-02

   Commit 079096f103fa ("tcp/dccp: install syn_recv requests into ehash table") lands for Linux 4.4 and introduces the race.

   Commit 079096f103fa

2. 2026-10-05

   2026-10-05

   Eric Dumazet's fix, crediting Xinyang Ge of Anthropic as reporter, is merged upstream. It ships in 7.3-rc7.

   Fix commit e3f33b0a1d89

3. 2026-10-15

   2026-10-1512:08 UTC

   The kernel CNA publishes CVE-2026-98432, CVSS 3.1 9.8, fixed in 6.12.113, 6.18.56, 7.2.10 and 7.3-rc7. Backports for 6.6, 6.1, 5.15 and 5.10 are added to the record later.

   linux-cve-announce

4. 2026-10-16

   2026-10-16

   Oracle publishes fixed UEK kernels for Oracle Linux 7 to 10.

   Oracle Linux

5. 2026-10-27

   2026-10-27

   Anthropic's Frontier Red Team describes a zero-click remote root exploit for the bug against servers with a multipath TCP listener. The exploit is not published.

   Anthropic

6. 2026-10-29

   2026-10-29

   SUSE releases fixed kernels for SLES 16.0 and several SLES 15 service packs.

   SUSE

7. 2026-10-2923:34 UTC

   The Go Security Team warns that Go 1.24 and later open MPTCP listeners by default on Linux, and that the bug is not specific to MPTCP.

   Go Security Team

8. 2026-10-30

   2026-10-30

   Red Hat raises its rating to Critical, 9.8, and publishes RHSB-2026-016 with an nftables mitigation. AlmaLinux puts fixed kernels in testing. Amazon Linux fixes its remaining kernels.

   Red Hat RHSB-2026-016

9. 2026-11-01

   2026-11-01

   Red Hat issues fixed kernels for RHEL 8, 9 and 10 (RHSA-2026:93414, RHSA-2026:93412, RHSA-2026:93413).

   RHSA-2026:93414

10. 2026-11-02

    2026-11-02

    AlmaLinux moves its fixed kernels to the production repositories. Oracle publishes fixed Red Hat Compatible Kernels and further UEK errata.

    AlmaLinux

11. 2026-11-02

    Canonical puts fixed kernels for Ubuntu 22.04, 24.04 and 26.04 in the -proposed pocket for qualification testing.

    Ubuntu blog

12. 2026-11-03

    2026-11-03

    SUSE releases further kernel updates on 3 and 4 November. Its overall state remains Pending.

    SUSE

13. 2026-11-05

    2026-11-05

    Ubuntu's fixed kernels are still not in the updates pocket. The CVE is not in CISA KEV and NVD has not completed its analysis.

    Ubuntu tracker

• CVE-2026-98432 recordLinux kernel CNA: description, affected and fixed versions, CVSS 3.1 vector and scoring rationale.
• linux-cve-announce: CVE-2026-98432Greg Kroah-Hartman's announcement with the introducing commit and the first fixed releases.
• Fix commit e3f33b0a1d89"ipv4: free inet_opt and ireq_opt after an RCU grace period" by Eric Dumazet.
• Anthropic: Discovering and exploiting vulnerabilities in critical, real-world software with ClaudeFrontier Red Team post describing the remote root exploit against MPTCP listeners.
• Go Security Team: Protecting against CVE-2026-98432Go's MPTCP default since 1.24, the multipathtcp GODEBUG setting, and why per-program opt-out is fragile.
• Red Hat RHSB-2026-016Red Hat bulletin: Critical rating, affected products, the nftables mitigation and why disabling MPTCP is not enough.
• Red Hat RHSA-2026:93414 (RHEL 8)Critical kernel update, 1 November 2026: 4.18.0-553.182.1.el8_10.
• Red Hat RHSA-2026:93412 (RHEL 9)Kernel update, 1 November 2026: 5.14.0-687.64.1.el9_8.
• Red Hat RHSA-2026:93413 (RHEL 10)Kernel update, 1 November 2026: 6.12.0-211.72.1.el10_2.
• Red Hat CVE pageRed Hat's rating, statement and CWE-825 classification.
• AlmaLinux: patched kernels releasedFixed AlmaLinux 8, 9 and 10 kernels in production since 2 November, the prerouting nftables rule and its limits, and MPTCP steps.
• Ubuntu: fixes available in -proposed pocket for CVE-2026-98432Canonical's per-release remediation status and how to install the -proposed kernels.
• Ubuntu CVE trackerPer-release kernel status.
• Debian security trackerFixed package versions and DSA/DLA references.
• SUSE CVE pageSUSE's rating, advisories and fixed packages per product.
• Oracle Linux CVE pageUEK and Red Hat Compatible Kernel errata for Oracle Linux 7 to 10.
• Amazon Linux Security CenterFixed kernels for Amazon Linux 2 and 2023.
• oss-security: init_on_free=1 as an interim mitigationCommunity hardening suggestion, not a vendor recommendation.
• NVD: CVE-2026-98432NIST entry. Analysis pending at the time of writing.
• CISA Known Exploited Vulnerabilities catalogCVE-2026-98432 not listed as of 5 November 2026.