Critical Linux kernel flaw CVE-2026-98432 allows remote root attacks on MPTCP servers

Key points

• CVE-2026-98432 is a use-after-free vulnerability in the Linux kernel’s IPv4 TCP networking code. The Linux kernel CVE team scored it 9.8/10, and it affects kernels dating back to version 4.4.
• Anthropic demonstrated an exploit, developed by an unreleased Claude AI model, that bypassed KASLR and achieved remote root code execution on a server running an MPTCP listener in about 53 minutes.
• The flaw was fixed upstream and backported to maintained kernel branches, but Ubuntu 22.04/24.04/26.04 LTS and 26.10, RHEL 8/9/10, and Rocky Linux 8/9/10 were still vulnerable as of October 30. Debian has shipped patched kernels.

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, also affects ordinary TCP connections, although remote code execution has not been publicly demonstrated on servers without an MPTCP listener.

Details

Multipath TCP (MPTCP) is a TCP feature that lets a single connection use multiple network paths at the same time to improve performance and reliability. It is commonly used where devices have multiple network connections, such as Wi-Fi and cellular.

The vulnerability is a use-after-free — code that keeps using memory after it has been freed — caused by a race condition in how the kernel handles TCP connection requests. The Linux kernel advisory explains: “tcp_v4_syn_recv_sock() transfers ownership of ireq->ireq_opt to the child socket (newinet->inet_opt) without copying it. Another cpu can concurrently process a retransmitted SYN for the same request socket, and send a SYNACK from tcp_check_req().” 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 used.

Red Hat says exploiting the race condition is more complex than just sending a malicious network packet. It classifies the bug as a potential denial-of-service issue that could crash the kernel and assigns it a lower severity score of 5.9. Anthropic, however, demonstrated remote code execution under specific conditions: an unreleased Claude AI model developed an exploit that bypassed kernel address space layout randomization (KASLR) and executed arbitrary commands as root on a Linux server running an MPTCP listener. The exploit took about 53 minutes to achieve remote root code execution, showing the vulnerability could be exploited for more than denial-of-service attacks.

MPTCP is not typically enabled by default in applications, but some software turns it on automatically. Starting with Go 1.24, the language enables MPTCP by default for TCP listeners on Linux as long as the kernel supports the feature. Because of this, some Go-based network services may enable MPTCP listeners without administrators knowing.

The flaw was fixed in the upstream Linux kernel and backported to maintained kernel branches, but security updates have not yet reached all affected distributions. Canonical’s security tracker listed Ubuntu 22.04 LTS, 24.04 LTS, 26.04 LTS, and 26.10 as vulnerable, with kernel fixes in progress as of October 30. Red Hat lists RHEL 8, 9, and 10 as affected, with fixes planned for future updates. RHEL 7 is not affected because its kernel does not contain the vulnerable code. CIQ, the Rocky Linux support provider, confirmed Rocky Linux 8, 9, and 10 are also affected and shared temporary firewall protections. Debian fixed the issue in kernel package 6.1.190-1 for Debian 12 (Bookworm) and 6.12.113-1 for Debian 13 (Trixie), with fixed packages also available for Debian testing and unstable. Oracle Linux, Amazon Linux, and SUSE have released fixes for some affected kernel versions. macOS is unaffected because it uses Apple’s XNU kernel rather than the Linux kernel.

Background

There are currently no confirmed reports of CVE-2026-98432 being exploited in the wild. The Linux kernel CVE team scores the vulnerability 9.8 out of 10, while Red Hat scores it 5.9 and classifies it as a potential denial-of-service issue because it considers exploitation complex.

Why it matters

For engineers, the risk is concentrated on servers that listen with MPTCP — possibly without anyone realizing it, since Go 1.24 enables MPTCP by default for TCP listeners. Some Go-based network services on vulnerable kernels may enable MPTCP listeners without administrators knowing, and Anthropic’s demonstration shows the flaw can be exploited for more than denial-of-service attacks.

What to do

• Check your distribution’s security advisories and install updated kernel packages as they become available.
• If patches are not yet available, restrict access to trusted networks or IP addresses.
• CIQ recommends temporarily blocking IPv4 TCP packets containing IP options with nftables rules before they reach the vulnerable TCP processing code, and shared example rules.
• Do not rely on disabling MPTCP alone — the underlying vulnerability can still be triggered through ordinary TCP traffic.