A critical use-after-free vulnerability in the Linux kernel has been weaponized by security researchers at Anthropic, who developed a working exploit that grants unauthenticated attackers remote root access on systems running Multipath TCP (MPTCP) listeners. The flaw, designated CVE-2026-98432, also impacts conventional TCP connections, though public demonstrations of code execution have been limited to servers with active MPTCP listeners.
Multipath TCP extends standard TCP functionality by allowing a single connection to traverse multiple network paths simultaneously, enhancing both performance and fault tolerance. This capability is particularly valuable in scenarios where devices maintain multiple simultaneous network interfaces, such as systems with both Wi-Fi and cellular connectivity.
The Linux kernel CVE team assigned CVE-2026-98432 a severity rating of 9.8 out of 10. The vulnerability traces back through Linux kernel versions beginning with 4.4 and affects major distributions including Ubuntu, Red Hat Enterprise Linux (RHEL), and Rocky Linux.
The underlying issue stems from a race condition in TCP connection handling. According to the kernel advisory, "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(). 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."
The race condition can also manifest during TCP request socket migration when net.ipv4.tcp_migrate_req is enabled or when a BPF_SK_REUSEPORT_SELECT_OR_MIGRATE program is active.
While the vulnerability requires no authentication to trigger, Red Hat notes that successful exploitation demands considerable technical sophistication beyond simply transmitting a crafted network packet. Red Hat consequently categorizes the issue as primarily a denial-of-service risk capable of triggering kernel crashes, assigning it a lower severity score of 5.9.
Anthropic's research contradicts this assessment. The organization demonstrated that an unreleased Claude AI model successfully generated an exploit that circumvented kernel address space layout randomization (KASLR) and executed arbitrary commands with root privileges on a Linux server hosting an MPTCP listener. The exploit required approximately 53 minutes to achieve remote root code execution, establishing that the vulnerability extends well beyond denial-of-service scenarios.
MPTCP typically remains disabled by default in most applications, yet certain software automatically activates it. Starting with version 1.24, Go enables MPTCP by default for TCP listeners on Linux systems where kernel support exists. This means Go-based network services may inadvertently expose MPTCP listeners without administrator awareness.
Patch deployment underway across distributions
The upstream Linux kernel has incorporated a fix, which has been backported to maintained kernel branches. However, security updates have not yet reached all affected distributions.
Canonical's security tracker indicates that Ubuntu 22.04 LTS, 24.04 LTS, 26.04 LTS, and 26.10 remain vulnerable as of October 30, with kernel fixes currently in development. Red Hat lists RHEL 8, 9, and 10 as affected, with patches scheduled for upcoming releases; RHEL 7 remains unaffected because its kernel lacks the vulnerable code.
CIQ, the support provider for Rocky Linux, confirms that Rocky Linux 8, 9, and 10 are vulnerable and has published temporary firewall mitigations for impacted systems. Debian's security tracker documents that the fix was released in kernel package version 6.1.190-1 for Debian 12 (Bookworm) and 6.12.113-1 for Debian 13 (Trixie), with corrected packages also available for Debian testing and unstable branches.
Oracle Linux, Amazon Linux, and SUSE have released patches for certain affected kernel versions. The vulnerability does not impact macOS systems, which rely on Apple's XNU kernel rather than the Linux kernel.
Mitigation strategies for unpatched systems
System administrators should consult their distribution's security advisories and deploy updated kernel packages as they become available. For environments where patches remain unavailable, restricting network access to trusted sources and authorized IP ranges can reduce exposure.
CIQ recommends temporarily filtering IPv4 TCP packets that contain IP options before they reach vulnerable TCP processing routines, and has provided nftables rule examples for implementing this traffic blocking. The organization cautions that disabling MPTCP alone provides insufficient protection, as the underlying vulnerability can still be triggered through standard TCP traffic.
No confirmed instances of CVE-2026-98432 exploitation in production environments have been reported to date.