External risk intelligence

Linux Kernel TLS Zero-Length Record Handling Flaw

CVE advisoryKnown Exploit

CVE-2025-39682

The vulnerability exists in the Linux kernel's TLS receive path. While the kernel handles network traffic, this specific flaw requires complex conditions during record processing. It is theoretically reachable from the internet if an application uses the kernel's TLS implementation to accept public traffic, but it is not a default, high-level internet-facing service or portal by design.

Linux Kernel

6.0 to before 6.1.1496.2 to before 6.6.1036.7 to before 6.12.446.13 to before 6.16.46.1711.0before 5.0

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 has been identified in the Linux kernel's handling of network traffic records, specifically within the TLS receive path. This issue could potentially allow for improper processing of data under certain conditions, impacting the integrity of communications. The main concern is to confirm if our specific systems utilize the affected kernel components and are exposed.

Issue involves incorrect handling of network data records.
Leadership should remember this for potential system integrity risks.
Confirm relevance and exposure within our environment.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by sending specially crafted TLS records to a system processing network traffic. This could lead to the improper handling of these records, potentially allowing for unauthorized access or disruption of services. There is insufficient information to determine the specific entry conditions or the exact attack vectors.

Requires network access.
Triggered by malformed TLS records.
May lead to system compromise.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability in the Linux kernel's TLS handling could allow a specially crafted zero-length record to be processed incorrectly. This could lead to subsequent TLS records being mishandled, potentially impacting the integrity and confidentiality of network communications when supported by the advisory.

Network data integrity and confidentiality.
Incorrect record processing.
Unspecified security impact.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability resides within the Linux kernel's TLS handling, suggesting that platform or infrastructure teams are primarily responsible for managing affected systems. The initial step involves identifying all instances of the vulnerable Linux kernel versions, assessing their exposure and criticality, and then coordinating with application owners to plan remediation within maintenance windows.

Own the issue and affected systems.
Verify system exposure and business criticality.
Plan coordinated remediation actions.

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 in the context of CVE-2025-39682?

The Linux kernel is the core component of the operating system that manages hardware and system resources. In this context, it includes the built-in implementation of Transport Layer Security (TLS) used for secure network communication. This vulnerability specifically affects how that kernel-level TLS component handles incoming data records, which may be utilized by various Linux distributions, such as Debian, and proprietary industrial systems like Siemens SIMATIC CN 4100 firmware.

What does CWE-754 mean regarding this Linux kernel flaw?

CWE-754 refers to an improper check for unusual or exceptional conditions. For CVE-2025-39682, this means the kernel's TLS receive process fails to correctly validate specific, unexpected record types—specifically zero-length records—when they arrive from a list of pending data. This oversight disrupts the normal logic meant to distinguish between different types of TLS records, potentially leading the system to make incorrect assumptions about how to process subsequent network data.

How is this TLS record handling vulnerability triggered?

The vulnerability is triggered when a specially crafted, zero-length TLS record is processed from the system's receive list (rx_list). It is not triggered by standard, well-formed TLS traffic. The issue arises because this unexpected empty record bypasses the logic intended to control decryption and queuing, potentially causing the kernel to misidentify the type of later records that follow.

Is my system at risk according to Halo Surface Signal?

Halo Surface Signal indicates that while this flaw exists in the kernel's TLS path, it is not a default, high-level internet-facing service. The risk is considered 'Possible' because it depends on whether your specific application uses the kernel's TLS implementation to process traffic directly from the internet. You should evaluate if your services utilize this specific kernel-level networking feature.

What are the first steps to address CVE-2025-39682?

Begin by identifying all systems in your environment running the affected Linux kernel versions listed in the advisory. Once identified, assess whether these systems utilize the kernel-level TLS functionality. If they do, coordinate with your infrastructure or platform teams to plan for patching or updates within your standard maintenance windows, following the specific guidance provided by your software or hardware vendors.

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.