An exploit developed by Anthropic researchers managed to remotely execute commands as root on a Linux server with an MPTCP listener. The vulnerability, identified as CVE-2026-98432, affects the kernel’s IPv4 networking code and can be triggered without authentication, although the attack depends on specific conditions.
BleepingComputer reported on October 30 that an unreleased Claude model developed the exploit and managed to bypass KASLR, the protection that randomizes kernel memory addresses. According to the outlet, the process took about 53 minutes. The demonstration was carried out against a server that accepted Multipath TCP (MPTCP) connections; remote code execution on systems without an MPTCP listener has not been publicly demonstrated.
A race condition in TCP connection handling
The flaw is a use-after-free: the kernel can free options associated with a socket while another operation is still reading them. The condition arises while concurrently processing retransmitted TCP requests or during socket migration. A remote attacker can send network requests with carefully timed delays to trigger this race condition.
The vulnerability dates back to Linux kernels since 4.4 and affects the IPv4 TCP subsystem. Although Anthropic’s demonstration used MPTCP, the issue also affects ordinary TCP connections. Red Hat says the flaw can cause memory corruption or a system crash; its portal notes that exploiting the race requires precision, but no privileges or user interaction.
MPTCP allows a connection to use multiple network paths at the same time, such as Wi-Fi and mobile connections. It is not usually enabled by default in applications, but BleepingComputer warns that Go 1.24 enables MPTCP for TCP listeners on Linux when the kernel supports the feature. As a result, some services written in Go could expose an MPTCP listener without their administrators explicitly configuring it.
Published severity assessments are not identical. BleepingComputer described a Red Hat rating of 5.9, focused on a possible denial-of-service attack and the complexity of exploiting the race. By contrast, Red Hat’s CVE page rates it as critical and shows a preliminary CVSS score of 9.8; the company’s security bulletin also classifies it as critical.
Patches roll out by distribution
The flaw has already been fixed in the mainline Linux kernel, and the fix has been backported to maintained branches, but updates are not yet available for all distributions. Canonical said Ubuntu 22.04 LTS, 24.04 LTS, 26.04 LTS, and 26.10 remained affected, with fixes in preparation. Red Hat lists RHEL 8, 9, and 10 among the affected products; RHEL 7 is not affected because its kernel does not contain the vulnerable code.
Debian released fixed versions of the kernel package for Debian 12 and 13: 6.1.190-1 and 6.12.113-1, respectively. Amazon Web Services also announced that fixed kernel packages are now available for Amazon Linux 2 and Amazon Linux 2023. On self-managed Linux instances on EC2, updating the kernel and rebooting are the customer’s responsibility.
For systems still awaiting the patch, Red Hat and CIQ recommend filtering IPv4 TCP traffic containing IP options as a temporary measure. Red Hat warns that this filtering may affect workloads and is not a substitute for updating. AWS suggests disabling MPTCP or, for applications built with Go 1.24 or later, setting GODEBUG=multipathtcp=0; the company clarifies that these measures reduce the conditions for code execution but do not fix the vulnerability.
Disabling MPTCP alone is also not enough to eliminate the risk: the flaw can be triggered through ordinary TCP traffic. As of BleepingComputer’s report, there were no confirmed reports of CVE-2026-98432 being exploited in real-world attacks. Administrators are advised to review their distribution’s security advisory and install the fixed kernel as soon as it becomes available.
Sources: BleepingComputer, Red Hat Customer Portal, Red Hat, Amazon Web Services, Inc.
