External risk intelligence

Linux Kernel Netfilter Synproxy Checksum Corruption Vulnerability.

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-64007

This vulnerability exists in the Linux kernel's netfilter synproxy module. While it resides in the network stack, it is a specific feature that must be explicitly configured by an administrator. It is not exposed by default in standard deployments, meaning public internet reachability is only possible if the feature is actively enabled and in use.

Halo Surface Signal: 3 out of 5 — possibly public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability was identified in the Linux kernel's networking component that could lead to a corrupted network packet. This issue arises when the system attempts to modify packet data, potentially causing the packet's checksum to become invalid.

Corrupted network packets due to a checksum error.
Requires confirmation of synproxy feature usage.
Confirm relevance and exposure for critical systems.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending specially crafted network packets to a system where the Linux kernel's synproxy feature is enabled. The synproxy feature, part of netfilter, processes TCP connection requests. By manipulating these packets, an attacker could cause the kernel to incorrectly update a TCP checksum, potentially leading to system instability or allowing for further malicious activities.

Network access required.
Malformed packets trigger vulnerability.
Potential for denial-of-service or memory corruption.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could affect the integrity of network packets processed by the Linux kernel's synproxy feature when TCP timestamp options are adjusted. If the system is configured to use synproxy with these options, a specially crafted network packet could lead to a situation where the TCP checksum is recalculated incorrectly, potentially causing packet corruption or leading to a write into freed memory.

Network packet integrity.
Incorrect checksum calculation.
Network instability or crashes.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability impacts the Linux kernel's netfilter synproxy component, requiring explicit administrator configuration to be exploitable. Owners of systems where synproxy is enabled and exposed to network traffic should prioritize identifying these systems. The immediate first step is to confirm the presence of synproxy, assess its network exposure and business criticality, and then identify the accountable system owner to plan remediation based on observed risk.

Linux infrastructure or platform teams own remediation.
Verify synproxy configuration and network exposure.
Plan maintenance for targeted system updates.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is the Linux kernel netfilter synproxy component?

The netfilter synproxy is a specialized feature within the Linux kernel networking stack designed to help handle TCP connection requests. It acts as a protective buffer, allowing servers to defer creating full connection state until they are certain the incoming request is legitimate. This is often used to mitigate high volumes of connection attempts, such as those seen during network floods.

How does CVE-2026-64007 impact TCP packet processing?

This vulnerability is a pointer-management error that occurs during TCP timestamp adjustments. When the kernel modifies these options, it may unintentionally free the memory address holding the TCP header information while still trying to use it to update the checksum. This leads to the kernel writing checksum data into the wrong location or potentially accessing invalid memory, resulting in malformed packets being sent across the network.

When does this vulnerability trigger?

The vulnerability triggers only when an attacker sends specifically crafted network traffic to a system where the synproxy feature is actively running and enabled. If your Linux kernel is not configured to use the synproxy module within netfilter, this specific code path is not executed, meaning the vulnerability cannot be triggered.

Is my system at risk according to Halo Surface Signal?

Halo Surface Signal identifies this as a potential risk only if you have manually enabled the synproxy feature. Because it is not a default configuration for most Linux systems, it is generally not exposed to the internet unless you have explicitly turned it on. You should focus your investigation on network infrastructure or servers where you have specifically configured these advanced synproxy capabilities.

What is the first step to address this CVE?

Your first step is to verify whether the synproxy feature is active on your Linux systems. You do not need to take immediate action on systems where this feature is disabled. For systems where it is enabled, coordinate with your infrastructure or platform teams to assess the network exposure of those specific machines and prepare for kernel updates provided by your distribution maintainers.

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.