Security researchers at Anthropic have built a working exploit for a critical Linux kernel vulnerability that allows unauthenticated attackers to remotely execute commands as root on servers running Multipath TCP (MPTCP) listeners.

The flaw, tracked as CVE-2026-98432, is a use-after-free in the kernel’s IPv4 TCP networking code and received a severity score of 9.8 out of 10 from the Linux kernel CVE team. It affects kernels dating back to version 4.4, putting widely used distributions including Ubuntu, Red Hat Enterprise Linux, and Rocky Linux at risk.

According to the Linux kernel advisory, the bug stems from a race condition in how the kernel handles TCP connection requests. The function tcp_v4_syn_recv_sock() transfers ownership of request socket options to a child socket without copying them, and a concurrent CPU processing a retransmitted SYN can trigger a use-after-free when building and sending the SYN-ACK packet. A similar issue can occur during TCP request socket migration when net.ipv4.tcp_migrate_req is enabled or a BPF_SK_REUSEPORT_SELECT_OR_MIGRATE program is in use.

The vulnerability also affects ordinary TCP connections, not just MPTCP, although remote code execution has only been publicly demonstrated against servers with an MPTCP listener. Red Hat classifies the issue primarily as a denial-of-service risk, scoring it 5.9 due to the complexity of exploitation. Anthropic’s research shows that assessment undersells the risk: an unreleased Claude AI model developed an exploit that bypassed kernel ASLR and achieved remote root code execution against an MPTCP-enabled server in about 53 minutes.

Why MPTCP exposure may be wider than expected

MPTCP isn’t typically enabled by default, but some software turns it on automatically. Starting with Go 1.24, the Go programming language enables MPTCP by default for TCP listeners on Linux whenever the kernel supports it, meaning some Go-based network services may be running MPTCP listeners without administrators realizing it.

Patch status

  • Ubuntu 22.04, 24.04, 26.04, and 26.10 remain vulnerable, with fixes in progress as of October 30
  • RHEL 8, 9, and 10 are affected, with fixes planned; RHEL 7 is not affected
  • Rocky Linux 8, 9, and 10 are affected, per CIQ
  • Debian 12 and 13 have received fixed kernel packages
  • Oracle Linux, Amazon Linux, and SUSE have released fixes for some affected versions

macOS is not affected since it uses Apple’s XNU kernel rather than Linux.

Administrators should apply updated kernel packages as they become available. Where patches aren’t yet ready, CIQ recommends restricting access to trusted networks and using nftables rules to block IPv4 TCP packets containing IP options before they reach the vulnerable code path. Disabling MPTCP alone is not sufficient, since the underlying flaw can still be triggered via ordinary TCP traffic. There are no confirmed reports of in-the-wild exploitation at this time.