# CVE-2026-98432 - Linux Kernel IPv4 TCP SYN Race Condition / Use-After-Free (inet_opt / ireq_opt) Enabling Remote Kernel Memory Corruption and Reported Root Command Execution

> A race-condition use-after-free in the Linux kernel IPv4 TCP stack (tcp_v4_syn_recv_sock() / ip_options_build()) can be triggered by an unauthenticated remote attacker sending retransmitted SYN packets, causing kernel memory corruption. A secondary news report says Anthropic's Frontier Red Team built a lab exploit achieving root command execution; no public PoC or in-the-wild exploitation is documented by primary trackers.

- **Published:** 2026-10-29T15:41:39Z
- **Last reviewed:** 2026-10-29T17:13:04.293Z
- **Canonical:** https://intel.threadlinqs.com/threat/TL-2026-4751
- **ID:** TL-2026-4751
- **Severity:** CRITICAL (CVSS 9.8)
- **Category:** VULNERABILITY
- **Status:** MONITORING
- **Detections:** 9 · **IOCs:** 19 (full data via the Threadlinqs MCP server — Purple tier)
- **CVEs:** CVE-2026-98432

## Description

CVE-2026-98432 (published 2026-10-15) affects the IPv4 TCP handling in the Linux kernel. tcp_v4_syn_recv_sock() transfers ownership of ireq->ireq_opt to the child socket (newinet->inet_opt) without copying it. Readers such as tcp_v4_send_synack() and inet_csk_route_req() access the options under rcu_read_lock() only, while inet_sock_destruct() and tcp_v4_reqsk_destructor() free them with a plain kfree() without an RCU grace period. A second CPU processing a retransmitted SYN can therefore build a SYN-ACK from freed memory, producing a use-after-free in ip_options_build(). A similar issue exists in request-socket migration scenarios. IPv6 is not affected by the primary issue because tcp_v6_syn_recv_sock() duplicates options, although a related IPv6 migration issue (ipv6_opt) and a CALIPSO migration issue need separate fixes.

The upstream fix switches the destructors to kfree_rcu() so that inet_opt and ireq_opt are freed only after an RCU grace period. Affected versions run from Linux 4.4 (bug introduced by commit 079096f103fa 'tcp/dccp: install syn_recv requests into ehash table') up to the fixed stable releases 5.10.272, 5.15.223, 6.1.190, 6.6.159, 6.12.113, 6.18.56, 7.2.10 and 7.3-rc7. Wiz lists upstream fix commit e3f33b0a1d89d738e2b913dab519226eaf76e15b. Ubuntu 26.04, 24.04 and 22.04 were still work-in-progress and 20.04 and 18.04 vulnerable at time of review (including AWS, Azure, GCP and Oracle kernel flavors); SUSE shipped fixes for SLES 16.0 / Micro 6.2 while older LTSS products were pending.

According to Cyber Security News (2026-10-29), which cites an Anthropic Frontier Red Team disclosure dated 2026-10-27, an unreleased Claude model produced a working lab exploit in two phases: a race that leaks kernel pointers to defeat KASLR (about 48 minutes in lab testing), then a callback-pointer overwrite that runs an attacker payload as root (about 5 minutes). The report frames this as controlled lab research, not an in-the-wild incident. This claim is not corroborated by a primary source reviewed here: Anthropic's Frontier Red Team page lists a 2026-10-27 post titled 'Discovering and exploiting vulnerabilities in critical, real-world software with Claude' but no CVE-specific entry, and NVD, Wiz and other trackers state that no public PoC or exploitation was reported as of 2026-10-15.

Severity is inconsistent across sources. NVD / Linux CNA and Ubuntu rate it CVSS 3.1 9.8 (AV:N/AC:L/PR:N), while SUSE and Wiz rate it 8.1 (SUSE vector AV:N/AC:H, reflecting the race) and one aggregator reported 5.9 with a crash-only impact. Red Hat rates it High. The 9.8 value is used here with that caveat. The NVD record status was 'Received' (not yet analyzed). Not listed in CISA KEV per Wiz. Practical impact is at minimum remote kernel crash (DoS) for any host with an internet-reachable IPv4 TCP listener; reliable code execution depends on winning a race and is unconfirmed by primary sources.

## MITRE ATT&CK

- T1190 Exploit Public-Facing Application
- T1059 Command and Scripting Interpreter
- T1082 System Information Discovery
- T1068 Exploitation for Privilege Escalation
- T1059.004 Command and Scripting Interpreter
- T1499.004 Endpoint Denial of Service

## Sources

- [Claude AI Exploits Linux Kernel Flaw to Execute Root Commands Without User Interaction](https://cybersecuritynews.com/linux-kernel-flaw-root-commands/)
- [NVD - CVE-2026-98432](https://nvd.nist.gov/vuln/detail/CVE-2026-98432)
- [Red Hat Bugzilla 2553390 - CVE-2026-98432 kernel: ipv4: free inet_opt and ireq_opt after an RCU grace period](https://bugzilla.redhat.com/show_bug.cgi?id=2553390)
- [SUSE CVE-2026-98432](https://suse.com/security/cve/CVE-2026-98432)
- [Ubuntu Security - CVE-2026-98432](https://ubuntu.com/security/CVE-2026-98432)
- [Wiz Vulnerability Database - CVE-2026-98432](https://www.wiz.io/vulnerability-database/cve/cve-2026-98432)
- [stack.watch - Linux kernel TCP UAF in inet_opt handling (CVE-2026-98432)](https://stack.watch/vuln/CVE-2026-98432/)
- [OSV AZL-108246 (Azure Linux)](https://osv.dev/vulnerability/AZL-108246)
- [Anthropic Frontier Red Team research index (2026-10-27: Discovering and exploiting vulnerabilities in critical, real-world software with Claude)](https://www.anthropic.com/research/team/frontier-red-team)

## Full data

Detection queries (Splunk SPL / Microsoft KQL / Sigma) and IOC values require the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp

Canonical: https://intel.threadlinqs.com/threat/TL-2026-4751