--- schema: 1 kind: vulnerability title: "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)" headline: "Linux TCP use-after-free: Anthropic built a remote root exploit; Ubuntu and RHEL kernels still unfixed" summary: > The Linux kernel CVE team published CVE-2026-98432 on 2026-10-15, a use-after-free in the IPv4 TCP connection setup path that dates to Linux 4.4 and is reached by a SYN packet carrying IP options; the CNA scores it CVSS 9.8. Anthropic reports that Claude wrote an exploit giving a remote, unauthenticated attacker root command execution on servers with a multipath TCP (MPTCP) listener, and the Go team says the bug is not specific to MPTCP and that Go programs enable MPTCP on Linux listeners by default. Debian's security kernels are fixed while Ubuntu 22.04 to 26.10 and RHEL 8 to 10 had no fixed kernel on 2026-10-30; no source reports public exploit code or attacks. discovered_at: "2026-10-31T03:48:00Z" updated_at: null event_date: "2026-10-27" run_id: 2026-10-31T0256Z-intel priority: notable immediate_action: null tags: [vulnerabilities, pre-auth, zero-click, rce, dos, patch-available] regions: [global] sectors: [technology] entities: ["product:linux-kernel", "product:ubuntu", "product:red-hat-enterprise-linux"] techniques: [T1190, T1499.004] affected_products: ["Linux kernel", "Ubuntu", "Red Hat Enterprise Linux"] cves: - id: CVE-2026-98432 cvss: "9.8 (CVSS 3.1, kernel.org CNA; upstream rates it 8.1 per CIQ)" epss: null type: memory-corruption vector: zero-click auth: pre-auth status: [patch-available] affected: "Linux kernels from 4.4 on the IPv4 TCP path; Ubuntu 22.04, 24.04, 26.04 and 26.10 (fix in progress) and 20.04, 18.04 and 16.04; RHEL 8, 9 and 10; Rocky Linux 8, 9 and 10 (CIQ's RLC Pro LTS 9.6, RLC Pro LTS 8.10 and RLC Pro 10 have no patched kernel, community releases follow Rocky errata); RHEL 7 and Ubuntu 14.04 are not affected" fixed: "upstream 6.12.113, 6.18.56, 7.2.10 and 7.3-rc7; Debian bookworm 6.1.190-1 and trixie 6.12.113-1; fixes for some kernels from Oracle Linux, Amazon Linux and SUSE; no fixed kernel from Ubuntu, Red Hat or CIQ as of 2026-10-30 and 2026-10-31" sources: - url: "https://lists.openwall.net/linux-cve-announce/2026/10/15/1" publisher: "Linux kernel CVE team (linux-cve-announce)" date: "2026-10-15" role: primary - url: "https://www.anthropic.com/research/discovering-vulnerabilities-with-claude" publisher: "Anthropic Frontier Red Team" date: "2026-10-27" role: primary - url: "https://groups.google.com/g/golang-announce/c/PCoQcdxoScw/m/aD-MgXEvAAAJ" publisher: "Go team (golang-announce)" date: "2026-10-29" role: corroborating - url: "https://kb.ciq.com/article/security-advisories/cve-2026-98432-mitigation" publisher: "CIQ (Rocky Linux mitigation article)" date: "2026-10-30" role: corroborating - url: "https://ubuntu.com/security/CVE-2026-98432" publisher: "Canonical (Ubuntu security tracker)" date: "2026-10-30" role: corroborating - url: "https://security-tracker.debian.org/tracker/CVE-2026-98432" publisher: "Debian Security Tracker" date: "2026-10-31" role: corroborating - url: "https://access.redhat.com/security/cve/cve-2026-98432" publisher: "Red Hat (CVE page)" date: "2026-10-31" role: corroborating - url: "https://advisories.ncsc.nl/advisory?id=NCSC-2026-0461" publisher: "NCSC-NL (NCSC-2026-0461)" date: "2026-10-30" role: corroborating - url: "https://www.bleepingcomputer.com/news/security/critical-linux-kernel-flaw-allows-remote-root-attacks-on-mptcp-servers/" publisher: "BleepingComputer" date: "2026-10-30" role: corroborating closed_sources: [] evidence: - quote: "Issue introduced in 4.4" publisher: "Linux kernel CVE team (linux-cve-announce)" source_url: "https://lists.openwall.net/linux-cve-announce/2026/10/15/1" - quote: "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" publisher: "Anthropic Frontier Red Team" source_url: "https://www.anthropic.com/research/discovering-vulnerabilities-with-claude" - quote: "While the demonstrated exploit requires the target system to expose an MPTCP listener, the underlying bug is not specific to MPTCP." publisher: "Go team (golang-announce)" source_url: "https://groups.google.com/g/golang-announce/c/PCoQcdxoScw/m/aD-MgXEvAAAJ" - quote: "An unauthenticated network peer can start the flaw by sending TCP SYN packets that carry IPv4 options." publisher: "CIQ (Rocky Linux mitigation article)" source_url: "https://kb.ciq.com/article/security-advisories/cve-2026-98432-mitigation" - quote: "There are currently no confirmed reports of CVE-2026-98432 being exploited in the wild." publisher: "BleepingComputer" source_url: "https://www.bleepingcomputer.com/news/security/critical-linux-kernel-flaw-allows-remote-root-attacks-on-mptcp-servers/" verification: multi-source sourcing_note: > The kernel CVE team's announcement is the primary for the bug and the fixed upstream versions; the exploit claim rests on Anthropic's own post, which publishes no exploit code, and no second party is reported to have reproduced it. Severity statements disagree (the CNA's 9.8, upstream's 8.1 as CIQ reports it, Red Hat's Critical for RHEL on its page of 2026-10-31, and a 5.9 that BleepingComputer reported from Red Hat on 2026-10-30), and distribution status changes daily, so each tracker should be re-read before relying on a row. confidence: medium references: ["2026-10-25/zdi-linux-kernel-reports-public-skb-zerocopy-root-exploit"] deep_dive: false deep_dive_category: null org_triage: null classification: reliability: B credibility: 2 watchlist_hit: false 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." updates: [] migrated_from: null --- 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](https://lists.openwall.net/linux-cve-announce/2026/10/15/1)). 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](https://kb.ciq.com/article/security-advisories/cve-2026-98432-mitigation)). 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](https://www.anthropic.com/research/discovering-vulnerabilities-with-claude)). 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](https://groups.google.com/g/golang-announce/c/PCoQcdxoScw/m/aD-MgXEvAAAJ)). 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](https://advisories.ncsc.nl/advisory?id=NCSC-2026-0461)), and BleepingComputer found no confirmed report of exploitation in the wild ([BleepingComputer, 2026-10-30](https://www.bleepingcomputer.com/news/security/critical-linux-kernel-flaw-allows-remote-root-attacks-on-mptcp-servers/)). Severity is disputed: the CNA scores it 9.8, upstream 8.1 with high attack complexity ([CIQ, 2026-10-30](https://kb.ciq.com/article/security-advisories/cve-2026-98432-mitigation)), 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](https://access.redhat.com/security/cve/cve-2026-98432)). 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](https://security-tracker.debian.org/tracker/CVE-2026-98432)); 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](https://ubuntu.com/security/CVE-2026-98432)); Red Hat lists RHEL 8, 9 and 10 as affected and RHEL 7 as not affected ([BleepingComputer, 2026-10-30](https://www.bleepingcomputer.com/news/security/critical-linux-kernel-flaw-allows-remote-root-attacks-on-mptcp-servers/)); BleepingComputer reports fixes for some kernel versions from Oracle Linux, Amazon Linux and SUSE ([BleepingComputer, 2026-10-30](https://www.bleepingcomputer.com/news/security/critical-linux-kernel-flaw-allows-remote-root-attacks-on-mptcp-servers/)); 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](https://kb.ciq.com/article/security-advisories/cve-2026-98432-mitigation)). **Exposure:** a Linux host on a kernel below the fixed builds that accepts IPv4 TCP connections from an attacker; the demonstrated exploit needs an MPTCP listener, which `sysctl net.mptcp.enabled` shows as possible when it returns 1 and which matters for software built with Go 1.24 or newer, but CIQ's testing found that setting it to 0 leaves the plain-TCP path reachable ([CIQ, 2026-10-30](https://kb.ciq.com/article/security-advisories/cve-2026-98432-mitigation)). **Detection:** no source publishes an indicator. Because the trigger is a SYN packet with IP options ([CIQ, 2026-10-30](https://kb.ciq.com/article/security-advisories/cve-2026-98432-mitigation)) and a lost race only fails the handshake, so the attacker can retry ([Anthropic, 2026-10-27](https://www.anthropic.com/research/discovering-vulnerabilities-with-claude)), repeated option-bearing SYNs from one source toward a listener in firewall or flow telemetry are the pattern the mechanism implies (an inference, since no source describes the traffic). **Defender takeaway:** install the distribution kernel and reboot as soon as it ships; Debian's is out, Ubuntu's and Red Hat's were not on 2026-10-30. Until then, CIQ's interim measure is a packet filter that drops IPv4 TCP packets whose header carries IP options before the TCP stack sees them, to be tested on one non-production host first because it also drops legitimate TCP traffic that carries IP options; disabling MPTCP alone does not remove the path ([CIQ, 2026-10-30](https://kb.ciq.com/article/security-advisories/cve-2026-98432-mitigation)).