Critical Linux kernel flaw allows remote root attacks on MPTCP servers

What it is

CVE-2026-98432 is a use-after-free in the Linux kernel IPv4 TCP code. The Linux kernel CVE team scores it 9.8. Red Hat scores it 5.9 and treats it as a denial-of-service (kernel crash) issue. Anthropic researchers say an unreleased Claude model built a working remote root exploit against an MPTCP listener. Remote code execution has not been publicly demonstrated on servers without an MPTCP listener.

How it works

tcp_v4_syn_recv_sock() passes ownership of ireq->ireq_opt to the child socket (newinet->inet_opt) without copying it. Another CPU can process a retransmitted SYN for the same request socket and send a SYNACK from tcp_check_req(). tcp_v4_send_synack() and inet_csk_route_req() read ireq->ireq_opt under rcu_read_lock() only. ip_build_and_send_pkt() and ip_options_build() then read opt->optlen twice. The same flaw can occur during request socket migration when net.ipv4.tcp_migrate_req is enabled or a BPF_SK_REUSEPORT_SELECT_OR_MIGRATE program is used.

Preconditions / Affected

• Kernels back to 4.4 are affected, including Ubuntu, RHEL and Rocky Linux. RHEL 7 is not affected because its kernel lacks the vulnerable code.
• No authentication is needed, but Red Hat says winning the race is harder than sending one malicious packet.
• MPTCP is usually off by default, but Go 1.24 and later enable it by default on TCP listeners when the kernel supports it. Go services may therefore expose MPTCP listeners without the administrator knowing.
• macOS is not affected.

What to do

Patch status as of 30 October 2026:

• Ubuntu 22.04, 24.04, 26.04 and 26.10 are still vulnerable, with fixes in progress (Canonical tracker).
• RHEL 8, 9 and 10 are affected, with fixes planned.
• Rocky Linux 8, 9 and 10 are affected.
• Debian 12 is fixed in kernel package 6.1.190-1, and Debian 13 in 6.12.113-1. Testing and unstable also have fixes.
• Oracle Linux, Amazon Linux and SUSE have fixes for some affected kernel versions.

Where no patch exists, restrict access to trusted networks or IP addresses. CIQ also recommends temporarily dropping IPv4 TCP packets that carry IP options before they reach the TCP code. The page mentions CIQ's nftables examples but does not reproduce the rules, so take them from CIQ's guidance.

Caveats

• Disabling MPTCP alone is not enough, because ordinary TCP traffic can trigger the bug.
• The 53-minute, KASLR-bypassing exploit comes from Anthropic's own report. The page does not say whether it is public.
• No in-the-wild exploitation has been reported.