Security researchers at Anthropic have developed an 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.
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 in environments where devices have multiple network connections, such as Wi-Fi and cellular networks.
CVE-2026-98432 is a use-after-free vulnerability in the kernel's IPv4 TCP networking code that received a severity score of 9.8 out of 10 from the Linux kernel CVE team.
The vulnerability affects Linux kernels dating back to version 4.4, including widely used distributions such as Ubuntu, Red Hat Enterprise Linux (RHEL), and Rocky Linux.
According to the Linux kernel advisory, the vulnerability stems from a race condition in how the kernel handles TCP connection requests, potentially causing a use-after-free condition.
"tcp_v4_syn_recv_sock() transfers ownership of ireq->ireq_opt to the child socket (newinet->inet_opt) without copying it," reads the advisory.
"Another cpu can concurrently 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, and ip_build_and_send_pkt() and ip_options_build() then read opt->optlen twice."
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.
Although triggering the vulnerability requires no authentication, Red Hat says exploiting the race condition is more complex than just sending a malicious network packet.
Red Hat classifies the vulnerability as a potential denial-of-service issue that could cause a kernel crash, assigning it a lower severity score of 5.9 because exploitation is complex.
However, researchers at Anthropic demonstrated that the vulnerability could be exploited for remote code execution under specific conditions.
An Anthropic report explained that 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.
According to Anthropic, 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.
While MPTCP is not typically enabled by default in applications, some software enables it automatically.
Beginning in Go 1.24, the programming language enables MPTCP by default for TCP listeners on Linux as long as the kernel supports the feature. Due to this, some Go-based network services may enable MPTCP listeners without administrators knowing.
Linux distros begin releasing patches
The vulnerability was fixed in the upstream Linux kernel and backported to maintained kernel branches. However, security updates have not yet been pushed out to all affected Linux distributions.
According to Canonical's security tracker, Ubuntu 22.04 LTS, 24.04 LTS, 26.04 LTS, and 26.10 remain vulnerable, with kernel fixes in progress as of October 30.
Red Hat also 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.
In mitigation guidance, Rocky Linux support provider CIQ confirms Rocky Linux 8, 9, and 10 are also affected and shared temporary firewall protections for affected systems.
According to Debian's security tracker, the vulnerability was fixed in kernel package version 6.1.190-1 for Debian 12 (Bookworm) and 6.12.113-1 for Debian 13 (Trixie). Fixed packages are also available for Debian testing and unstable.
Other Linux vendors, including Oracle Linux, Amazon Linux, and SUSE, have also released fixes for some affected kernel versions.
The vulnerability does not affect macOS, which uses Apple's XNU kernel rather than the Linux kernel.
Administrators are advised to check their distribution's security advisories and install updated kernel packages as they become available.
For systems where patches are not yet available, restricting access to trusted networks or IP addresses can reduce the risk.
CIQ also recommends temporarily blocking IPv4 TCP packets containing IP options before they reach the vulnerable TCP processing code, and shared examples of nftables rules that can block this type of traffic.
The company cautions that disabling MPTCP alone is insufficient because the underlying vulnerability can still be triggered through ordinary TCP traffic.
There are currently no confirmed reports of CVE-2026-98432 being exploited in the wild.
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