Anthropic security researchers have developed an exploit for a critical Linux kernel vulnerability that allows unauthenticated attackers to execute remote commands as root on servers running Multipath TCP (MPTCP) listeners.
The fault, followed as CVE-2026-98432also 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 allows a single connection to 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 TCP IPv4 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 ReviewsThe vulnerability arises from a race condition in the way 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,” the advisory reads.
“Another processor can simultaneously 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 both times.”
A similar issue may 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 exploit the race condition is more complex than simply sending a malicious network packet.
Red Hat classifies the vulnerability as a potential denial of service issue that could cause a kernel crash, giving it a lower severity score of 5.9 because exploitation is complex.
However, Anthropic researchers demonstrated that the vulnerability could be exploited for remote code execution under specific conditions.
A Anthropogenic report explained that a previously 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 approximately 53 minutes to achieve remote root code execution, showing that the vulnerability could be exploited for much more than denial of service attacks.
Although MPTCP is not usually enabled by default in applications, some software enables it automatically.
Since Go 1.24, the programming language enable MPTCP by default for TCP listeners on Linux as long as the kernel supports this feature. For this reason, some Go-based network services may enable MPTCP listeners without administrators’ knowledge.
Linux distributions start releasing patches
The vulnerability was fixed in the upstream Linux kernel and backported to maintained kernel branches. However, security updates have not yet been released to all affected Linux distributions.
According to Canonical’s security trackerUbuntu 22.04 LTS, 24.04 LTS, 26.04 LTS and 26.10 remain vulnerable, with kernel patches 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 core does not contain vulnerable code.
In mitigation guidanceRocky Linux support provider CIQ confirms that Rocky Linux 8, 9, and 10 are also affected and share temporary firewall protections for affected systems.
According to Debian security trackerThe 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 and unstable testing.
Other Linux vendors, including Oracle Linux, Amazon Linux, and SUSE, have also released patches 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 soon as they become available.
For systems for which patches are not yet available, restricting access to trusted networks or IP addresses can reduce risk.
CIQ also recommends temporarily blocking IPv4 TCP packets containing IP options before they reach vulnerable TCP processing code, and shares examples of nftables rules that can block this type of traffic.
The company warns that disabling MPTCP alone is insufficient because the underlying vulnerability can still be triggered via ordinary TCP traffic.
There are currently no confirmed reports of CVE-2026-98432 being exploited in the wild.
Overall prevention scores can mask what happens after initial access. Once attackers use valid credentials, prevention drops significantly.
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