Security researchers at Anthropic have developed an exploit for a critical Linux kernel vulnerability that can allow an unauthenticated attacker to execute commands remotely as root on a server running a Multipath TCP (MPTCP) listener. Tracked as CVE-2026-98432, the flaw affects Linux kernels dating back to version 4.4 and can also be triggered through regular TCP connections, although systems without an MPTCP listener are not exposed to the demonstrated remote-code-execution scenario.
Critical Linux kernel use-after-free vulnerability
Multipath TCP is a TCP feature that allows a single connection to use multiple network paths at the same time. It can improve performance and reliability on devices with 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. The Linux kernel CVE team assigned it a severity score of 9.8 out of 10.
The flaw affects Linux kernels dating back to version 4.4, including kernels used by distributions such as Ubuntu, Red Hat Enterprise Linux (RHEL), and Rocky Linux.
According to the Linux kernel advisory, the vulnerability is caused by a race condition in the way the kernel handles TCP connection requests. Under certain circumstances, the race can result in 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,” the advisory reads.
“Another CPU can concurrently process re-sent SYNs for the same request socket and send SYNACKs from tcp_check_req(). tcp_v4_send_synack() and inet_csk_route_req() only work with ireq->ireq_opt on rcu_read_lock() and ip_build_and_send_pkt() and ip_options_build() read opt->optlen twice.”
A similar issue can occur during TCP request-socket migration when net.ipv4.tcp_merge_req is enabled or when the BPF_SK_REUSEPORT_SELECT_OR_MIGRATE program is used.
Anthropic demonstrates potential remote code execution
Authentication is not required to trigger the vulnerability, but Red Hat says that race conditions are more complex to exploit than simply sending malicious network packets.
Red Hat classifies CVE-2026-98432 as a potential denial-of-service vulnerability that can cause a kernel crash and assigns it a lower severity score of 5.9 because of the complexity of exploitation.
However, Anthropic researchers demonstrated that the vulnerability can lead to remote code execution under certain conditions. According to Anthropic’s report, an unreleased Claude AI model developed an exploit that bypasses kernel address space layout randomization (KASLR) and executes arbitrary commands as root on a Linux server running an MPTCP listener.
Anthropic said the exploit took approximately 53 minutes to execute root-level code remotely, indicating that the vulnerability may be usable for more than denial-of-service attacks.
MPTCP is not typically enabled by default in applications, but some software enables it automatically. Since Go 1.24, the programming language has enabled MPTCP by default for TCP listeners on Linux when the kernel supports the feature. As a result, some Go-based network services may create MPTCP listeners without an administrator’s knowledge.
Linux distributions begin releasing patches
The vulnerability has been fixed in the upstream Linux kernel and backported to a maintained kernel branch. However, security updates have not yet reached all affected Linux distributions.
According to the Canonical security tracker, Ubuntu 22.04 LTS, 24.04 LTS, 26.04 LTS, and 26.10 remained 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.
Rocky Linux 8, 9, and 10 are also affected, according to Rocky Linux support provider CIQ. The company has published mitigation guidance that includes temporary firewall protection for affected systems.
The Debian security tracker states that 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 and unstable packages for Debian testing are also available.
Other Linux vendors, including Oracle Linux, Amazon Linux, and SUSE, have released fixes for some affected kernel versions.
Mitigation guidance for Linux administrators
The vulnerability does not affect macOS, which uses Apple’s XNU kernel rather than the Linux kernel.
Administrators should review their distribution’s security advisories and install updated kernel packages as they become available. Where patches are not yet available, access can be restricted to trusted networks or IP addresses to reduce risk.
CIQ also recommends temporarily blocking IPv4 TCP packets containing IP options before they reach vulnerable TCP processing code and provides an example nftables rule to block this traffic.
Simply disabling MPTCP is not sufficient because the underlying vulnerability can still be triggered through normal TCP traffic.
There are currently no confirmed reports of CVE-2026-98432 being exploited in the wild.
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Source: www.bleepingcomputer.com