CVE-2026-98432, Linux kernel IPv4 TCP: a use-after-free in connection setup, present since 4.4, has an Anthropic-built remote root exploit against MPTCP listeners; Debian is fixed, Ubuntu and Red Hat kernels are not yet (CVSS 9.8)
Linux TCP use-after-free: Anthropic built a remote root exploit; Ubuntu and RHEL kernels still unfixed
Defender actions
- On every internet-facing Linux server, install the distribution kernel that fixes CVE-2026-98432 and reboot as soon as it ships (Debian bookworm 6.1.190-1 and trixie 6.12.113-1 are out; Ubuntu and RHEL kernels were not as of 2026-10-30), and until then drop IPv4 TCP packets that carry IP options ahead of the TCP stack as CIQ describes, after testing on one non-production host because it also drops legitimate option-bearing traffic; disabling MPTCP alone does not close the path.
Analysis
The Linux kernel CVE team published CVE-2026-98432 on 2026-10-15: a use-after-free in the IPv4 TCP connection setup path, introduced in Linux 4.4 and fixed upstream in 6.12.113, 6.18.56, 7.2.10 and 7.3-rc7 (Linux kernel CVE team, 2026-10-15). An unauthenticated peer starts it by sending TCP SYN packets that carry IPv4 options, and a retransmitted SYN or the ACK of the syncookie path can then touch the freed option memory (CIQ, 2026-10-30). Anthropic wrote on 2026-10-27 that Claude produced an exploit that lets a remote attacker send packets to a Linux server with a multipath TCP (MPTCP) listener and run arbitrary commands as root without credentials (Anthropic, 2026-10-27). The Go team says the bug is not specific to MPTCP, that systems with no MPTCP listener may be open to similar attacks, and that since Go 1.24 Go programs enable MPTCP on Linux listen sockets by default (Go team, 2026-10-29). NCSC-NL judges large-scale abuse unlikely given the exploit's complexity and the mitigations distributions carry (translated from Dutch) (NCSC-NL, 2026-10-30), and BleepingComputer found no confirmed report of exploitation in the wild (BleepingComputer, 2026-10-30).
Severity is disputed: the CNA scores it 9.8, upstream 8.1 with high attack complexity (CIQ, 2026-10-30), and Red Hat rates it Critical for RHEL because it lets an unauthenticated remote attacker cause kernel memory corruption or an immediate crash over ordinary IPv4 TCP traffic, while saying reliable code execution needs precise heap control (Red Hat, 2026-10-31). Patch state as read on 2026-10-30 and 2026-10-31: Debian's security kernels 6.1.190-1 (bookworm) and 6.12.113-1 (trixie) are fixed (Debian Security Tracker, 2026-10-31); Ubuntu 22.04, 24.04, 26.04 and 26.10 are marked vulnerable with work in progress and 20.04, 18.04 and 16.04 vulnerable (Canonical, 2026-10-30); Red Hat lists RHEL 8, 9 and 10 as affected and RHEL 7 as not affected (BleepingComputer, 2026-10-30); BleepingComputer reports fixes for some kernel versions from Oracle Linux, Amazon Linux and SUSE (BleepingComputer, 2026-10-30); and CIQ had published no patched kernel for its RLC Pro LTS 9.6, RLC Pro LTS 8.10 or RLC Pro 10 variants and refers Rocky Linux community releases to Rocky errata (CIQ, 2026-10-30).
Cited evidence
Issue introduced in 4.4
allows a remote attacker to send packets to Linux servers with a multipath TCP listener to run arbitrary commands as root, without any credentials or user interaction
While the demonstrated exploit requires the target system to expose an MPTCP listener, the underlying bug is not specific to MPTCP.
An unauthenticated network peer can start the flaw by sending TCP SYN packets that carry IPv4 options.
There are currently no confirmed reports of CVE-2026-98432 being exploited in the wild.
Sources9
AI-generated · no human review · this permalink is the shareable record for the finding · verify operationally critical claims against the linked primary source.