How the bug works in the Linux IPv4 TCP stack
The flaw, tracked as CVE-2026-98432, is a race condition in how the kernel handles TCP connection requests that can produce a use-after-free. The Linux kernel CVE team gave the bug a severity score of 9.8 out of 10. According to the kernel advisory, the problem arises because request-socket options are transferred to a child socket without copying them and can be read concurrently during retransmission handling. The advisory explains: "tcp_v4_syn_recv_sock() transfers ownership of ireq->ireq_opt to the child socket (newinet->inet_opt) without copying it."
It continues: "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 advisory also notes a similar exposure can occur when TCP request socket migration (net.ipv4.tcp_migrate_req) is enabled or when a BPF_SK_REUSEPORT_SELECT_OR_MIGRATE program is used.
Although the vulnerability can be triggered via ordinary TCP traffic, remote code execution has only been publicly demonstrated against servers running MPTCP listeners.
Anthropic’s exploit: an AI-developed path to remote root on MPTCP servers
Security researchers at Anthropic reported a working exploit that elevates the issue from a potential denial-of-service to remote code execution under specific conditions. Anthropic said 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 company reported the exploit required about 53 minutes to achieve remote root code execution in their test.
Red Hat, by contrast, classifies the bug as a potential denial-of-service that could crash the kernel and assigned a lower severity score of 5.9, noting that exploiting the race condition requires more than simply sending a malformed network packet.
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Fix the backlogWhich kernels and distributions are affected — and who has fixes
The vulnerability affects Linux kernels going back to version 4.4. Upstream fixes and backports have been produced, but security updates have not been pushed to all affected distributions as of October 30, 2026.
- Canonical (Ubuntu): 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: RHEL 8, 9, and 10 are listed as affected with fixes planned; RHEL 7 is not affected because its kernel lacks the vulnerable code.
- Debian: Fixed in kernel package 6.1.190-1 for Debian 12 (Bookworm) and 6.12.113-1 for Debian 13 (Trixie); fixed packages are available for testing and unstable.
- Rocky Linux: CIQ confirms Rocky Linux 8, 9, and 10 are affected and has published temporary firewall mitigations.
- Other vendors: Oracle Linux, Amazon Linux, and SUSE have released fixes for some affected kernel versions.
Mitigations vendors and support providers recommend
Administrators are advised to monitor their distribution security advisories and install updated kernel packages when available. For systems without immediate patches, recommended temporary mitigations include restricting access to trusted networks or IP addresses.
CIQ, the Rocky Linux support provider, recommends temporarily blocking IPv4 TCP packets that contain IP options before they reach the vulnerable TCP processing code and shared nftables rule examples to do so. CIQ cautions that disabling MPTCP alone is insufficient because the underlying kernel vulnerability can still be triggered over ordinary TCP traffic.
There are currently no confirmed reports of CVE-2026-98432 being exploited in the wild.
What this means for technologists, administrators, and Go developers
Technologists and security teams: The Anthropic demonstration shows the vulnerability can be weaponized for remote code execution under specific conditions (MPTCP listeners present and additional exploitation steps). The high severity score from the kernel team and the lower operational-severity view from Red Hat underline a technical tension: the bug is powerful in theory but non-trivial to exploit in practice.
Administrators and IT operations: Check whether your kernels are among the versions vendors have patched, prioritize installation of kernel updates when they arrive, and consider temporary network-level mitigations such as limiting incoming TCP connections to trusted IPs or using the nftables IP-options block CIQ published.
Go developers and service owners: Beginning with Go 1.24, the language enables MPTCP by default for TCP listeners on Linux if the kernel supports it. That change means some Go-based network services may be accepting MPTCP connections without operators realizing it — a detail worth auditing when evaluating exposure and patch timelines.
For now, the record is clear and narrow: an upstream kernel defect dating back to 4.4 has been fixed, but not all distributors have pushed updates; an AI-assisted exploit has demonstrated remote root against MPTCP listeners in lab conditions; and temporary network controls exist that can reduce exposure until patched kernels are deployed. Administrators should treat kernel updates and packet-filtering mitigations as complementary steps while watching distribution advisories closely.
