External risk intelligence

Linux Kernel TCP ISN Prediction Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.4)

CVE-2026-64024

This vulnerability exists in the Linux kernel's TCP stack, which handles network traffic. Because the kernel processes TCP packets directly from the network interface, including those originating from the public internet, this component is inherently and universally exposed on any internet-facing host.

Halo Surface Signal: 5 out of 5 — more likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability in the Linux kernel's TCP networking could allow predictable connection information. This issue arises from how certain network packets are handled, potentially leading to unintended consequences if not properly managed. The primary concern is to determine if our systems are affected and to what extent.

Predictable network data in Linux TCP.
Matters for network security and connection integrity.
Confirm relevance and exposure to Linux TCP.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending specially crafted network packets to a vulnerable system. The Linux kernel's TCP processing, when handling these packets, may incorrectly leave a predictable value in a per-CPU variable. If subsequent legitimate network connections are processed on the same CPU, they could inadvertently use this predictable value, potentially leading to predictable initial sequence numbers. This could then enable further attacks, such as spoofing or hijacking connections.

Network access required.
Packets dropped by specific kernel paths trigger.
Predictable TCP initial sequence numbers.

Live Threat

Current exploitation, exposure, and threat context

When specific network conditions are met, an attacker could potentially manipulate the sequence number used in TCP connections. This manipulation could lead to disruptions in network service behavior.

Network connection sequence numbers.
Packet drops on specific kernel paths.
Potential for denial of service.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in the Linux kernel's TCP stack impacts infrastructure and platform teams responsible for kernel maintenance and network services. The first step is to identify all systems running the affected kernel version, determine their exposure to the network, and confirm their business criticality to prioritize remediation efforts.

Infrastructure/Platform teams own the fix.
Verify system network exposure and criticality.
Plan kernel updates or apply mitigations.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is the Linux kernel's TCP stack?

The TCP stack is the core part of the Linux operating system that manages how data is transmitted and received over a network. It handles the low-level rules for establishing, maintaining, and closing connections. Because it sits deep within the kernel, it processes every incoming network packet before passing data to applications, making it a foundational component for the security and reliability of any Linux-based server or workstation.

How does CVE-2026-64024 make TCP connection numbers predictable?

This vulnerability is a state management error. When a network packet is dropped due to security checks or other filters, the kernel accidentally leaves behind a piece of data called an Initial Sequence Number (ISN) in a shared memory area. Because this leftover value is not properly cleared, the next legitimate network connection processed by the same CPU might pick it up. This allows an attacker to predict the sequence number, which is a key element in maintaining secure communication.

Does any dropped packet trigger this vulnerability?

No. The issue occurs specifically when a packet is dropped by certain kernel code paths, such as those involving security policy checks, MD5 or AO signature mismatches, or eBPF filters. These drops must happen in a way that interrupts the normal processing flow, leaving the stale data in a per-CPU variable. If a packet is processed normally or dropped through a path that does not invoke this specific faulty logic, the vulnerability is not triggered.

Is my system exposed to CVE-2026-64024?

According to Halo Surface Signal, this vulnerability resides in the kernel's TCP stack, which is inherently exposed on any internet-facing host. Because the kernel processes packets directly from the network interface, any system with a network connection is potentially reachable. Systems that are fully exposed to the public internet are at the highest level of concern, as they can receive the specifically crafted packets required to trigger the underlying flaw.

What should I do to address this Linux kernel issue?

Your first step is to perform an inventory of all systems in your environment to identify those running the affected Linux kernel versions. Once you have identified these assets, evaluate their exposure to network traffic to prioritize those that are internet-facing. Infrastructure and platform teams should then plan for kernel updates to receive the necessary fix, which removes the problematic per-CPU variable and restores the correct sequence number handling.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.