Security researchers at Anthropic have developed an exploit for a critical Linux kernel vulnerability—tracked as CVE-2026-98432—that can allow unauthenticated attackers to execute commands as root on servers running Multipath TCP (MPTCP) listeners. The flaw is a use-after-free in the kernel’s IPv4 TCP networking code and has been scored 9.8/10 by the Linux kernel CVE team, although some vendors assess exploitation complexity differently.
What the flaw is
CVE-2026-98432 stems from a race condition in how the kernel handles TCP connection requests. The kernel function tcp_v4_syn_recv_sock() transfers ownership of ireq->ireq_opt to the child socket without copying it. Concurrent processing of a retransmitted SYN for the same request socket can lead to read-after-free and related inconsistencies when tcp_v4_send_synack(), inet_csk_route_req(), ip_build_and_send_pkt(), and ip_options_build() access the same structures. A similar issue can happen when TCP request socket migration is in use or when certain BPF programs are present.
Who and what is affected
- The vulnerability affects Linux kernels going back to version 4.4 and therefore impacts many mainstream distributions, including Ubuntu, Red Hat Enterprise Linux (RHEL), and Rocky Linux.
- The issue also affects ordinary TCP connections, though remote code execution has not been publicly demonstrated on servers that do not have an MPTCP listener.
- MPTCP is a feature that allows a single connection to use multiple network paths simultaneously; some software can enable it automatically (for example, Go 1.24 enables MPTCP by default for TCP listeners on Linux when the kernel supports it).
- Vendor responses vary: the upstream kernel has been fixed and fixes have been backported to maintained kernel branches, but not all distributions have pushed updates to users yet. As of October 30, Canonical’s tracker listed Ubuntu 22.04 LTS, 24.04 LTS, 26.04 LTS, and 26.10 as still vulnerable with fixes in progress. Red Hat lists RHEL 8, 9, and 10 as affected (RHEL 7 is not affected). Debian reports fixes in kernel package 6.1.190-1 (Debian 12) and 6.12.113-1 (Debian 13). Oracle Linux, Amazon Linux, and SUSE have also released fixes for some kernel versions; Rocky Linux confirmed it is affected and shared temporary firewall mitigations.
Anthropic’s exploit and its implications
Anthropic researchers demonstrated an exploit that achieved remote code execution against a Linux server running an MPTCP listener. According to their report, an unreleased Claude AI model helped develop an exploit that bypassed kernel address space layout randomization (KASLR) and executed arbitrary commands as root; the exploit reportedly took about 53 minutes to achieve remote root code execution. While the vulnerability requires no authentication, vendors such as Red Hat note that exploiting the race condition is more complex than simply sending a single malicious packet. There are no confirmed reports of CVE-2026-98432 being exploited in the wild.
Mitigations and short-term protections
- Install vendor-provided kernel updates as soon as they are available for your distribution; the upstream kernel and many maintained branches have already been patched.
- For systems without available patches, reduce exposure by restricting access to trusted networks or specific IP addresses.
- CIQ (Rocky Linux support provider) recommends temporarily blocking IPv4 TCP packets containing IP options before they reach the vulnerable TCP processing code and published example nftables rules to do so.
- Be aware that disabling MPTCP alone may be insufficient, because ordinary TCP traffic can still trigger the underlying vulnerability.
Practical steps for administrators
- Check your distribution’s security advisory and kernel package status immediately and apply updates when they arrive.
- Audit services for the presence of MPTCP listeners—note that some software, including Go 1.24 and later, can enable MPTCP by default on compatible kernels.
- Where patches are not yet available, consider network-level containment (firewalls, IP allowlists) and the nftables rule guidance shared by vendors or support providers.
- Monitor vendor channels for updated guidance and proofs-of-fix; keep an eye on distribution trackers for fixed package versions relevant to your environment.
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