Critical Linux kernel TCP vulnerability can allow remote code execution (CVE-2026-98432)

What it is

CVE-2026-98432 is a use-after-free in the Linux kernel's IPv4 TCP handshake code, introduced in Linux 4.4. The kernel CVE team rates it 9.8. The fix was applied upstream on 5 October 2026 and the CVE was published on 15 October. Anthropic's Frontier Red Team reports that Claude wrote an exploit that runs commands as root on a server with an MPTCP listener. In its test, leaking a kernel address took 48 minutes and reaching command execution took 5 more.

How it works

During the handshake the kernel keeps the SYN's IP options with the request socket. In tcp_v4_syn_recv_sock() the new child socket takes ownership of those options without copying them. If another CPU is still building a SYNACK for a retransmitted SYN, the options can be freed while ip_options_build() is reading them. Request socket migration (net.ipv4.tcp_migrate_req=1, or a BPF reuseport migrate program) opens a similar path. The upstream fix switches the frees to kfree_rcu(). IPv6 avoids the first issue because the kernel copies its options. A related IPv6 migration issue exists only where CALIPSO sets options, and a separate patch covers it.

A lost race fails only that handshake and does not crash the host, so an attacker can retry.

Preconditions / Affected

• Any Linux host on kernel 4.4 or later with an IPv4 TCP port an attacker can reach.
• Anthropic's exploit needed an MPTCP listener. Go 1.24 and later request MPTCP by default on listening sockets where the kernel allows it. AlmaLinux and CloudLinux ship with MPTCP off.
• Containers share the host kernel, so patch or replace the node, not the image.
• A proxy such as nginx or HAProxy on the same host gives no protection. A separate load balancer that terminates TCP does. AWS says WAF, ALB, CLB, NLB TLS listeners, CloudFront and API Gateway do not pass untrusted TCP straight through, and still advises updating instances behind them within 30 days. NLB TCP listeners and Global Accelerator need a platform update once patched versions exist.
• AWS Lambda is not affected.

What to do

Install a patched kernel and reboot. AWS says a livepatch also resolves it.

# Amazon Linux 2
sudo yum update kernel && sudo reboot
# Amazon Linux 2023
sudo dnf update kernel --releasever=latest && sudo reboot

• EKS: AMI release v20261031 or later. ECS on EC2: ECS-optimized AMI 20261031 or later.
• Fargate: tasks launched after 1 November 2026, 6:00 AM PDT use the patched platform.
• Bottlerocket: updated AMIs were expected from 2 November.
• Upstream fixed kernels: 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 kernel CVE team advises moving to the latest stable kernel rather than cherry-picking.
• Ubuntu (tracker as of 30 October): fixes in progress for 22.04, 24.04, 26.04 and 26.10. 20.04, 18.04 and 16.04 are vulnerable with no fix.
• Red Hat: RHEL 8, 9 and 10 affected, with no fixed kernel or date.
• Debian, Oracle Linux and SUSE: fixed packages have shipped for at least some kernels, per The Hacker News.

Interim measures, none of which fixes the bug:

sysctl -w net.mptcp.enabled=0
GODEBUG=multipathtcp=0        # Go 1.24+ programs
iptables -I INPUT 1 -m u32 --u32 '0>>24&0x0f=6:15' -j DROP

The iptables rule drops IPv4 packets carrying IP options. AWS also publishes an equivalent AWS Network Firewall stateful rule, which needs strict rule order.

To verify, run uname -r after the reboot and compare it with your vendor's fixed version, not the upstream one, since vendors backport. A host that installed the update but has not rebooted may show as fixed in scanners.

Caveats

• Anthropic has not published the exploit, kernel version or configuration. CloudLinux's kernel team says it could not reproduce the claim.
• Neither the CVE record nor the kernel fix mentions MPTCP. The Go Security Team says systems without MPTCP listeners may be open to similar attacks, so disabling MPTCP is a weak control. It also calls per-program disabling fragile, because one exposed MPTCP listener leaves the system vulnerable.
• Red Hat says no mitigation it reviewed is easy to deploy, suitable for most installations and stable.
• Vendors score it below 9.8 because they judge the race harder to win.
• No vendor advisory reported attacks in the wild as of 29 October 2026.
• The post's fix heading says CVE-2025-98432, which looks like a typo for CVE-2026-98432.