External risk intelligence

PLC Authentication Algorithm Bypass Affects Confidentiality Integrity and Availability.

CVE advisorySeverity: CRITICAL (CVSS 9.2)

CVE-2026-3869

The vulnerability affects Programmable Logic Controllers (PLCs). PLCs are typically deployed within isolated industrial control networks or internal operational technology environments and are not designed to be exposed directly to the public internet.

Halo Surface Signal: 2 out of 5 — less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A security vulnerability has been identified in certain programmable logic controllers, specifically related to an incorrect implementation of an authentication algorithm. This flaw could potentially lead to unauthorized access and compromise the confidentiality, integrity, and availability of the system's functions. The primary concern is to confirm the relevance and exposure of this vulnerability within our operational technology environments.

Authentication flaw in programmable logic controllers.
Confirms relevance and exposure in operational technology.
Assess potential impact on industrial control systems.

Attack Path

How an attacker could exploit the issue

An attacker could potentially compromise a programmable logic controller (PLC) by exploiting a flaw in its authentication algorithm. This could happen if an application project with a lower security level is running on the PLC, allowing an attacker to gain unauthorized access. Successful exploitation could lead to a loss of confidentiality, integrity, and availability of the PLC's functions.

Entry condition: Network access required.
Trigger point: Lower-level application project on PLC.
Resulting risk: Loss of confidentiality, integrity, availability.

Live Threat

Current exploitation, exposure, and threat context

When an application project with a lower security level is running on a PLC, an incorrect implementation of an authentication algorithm could affect the confidentiality, integrity, and availability of the PLC. This could allow unauthorized access to or modification of industrial control processes.

PLC data and system integrity.
Authentication bypass when supported.
Disruption of industrial operations.

Operational Fix

Recommended remediation, mitigation, and detection steps

The owner of the affected Programmable Logic Controller (PLC) application project should lead the assessment. The first practical step is to identify all PLC deployments running an application project, confirm their network reachability and criticality, and then determine the accountable owner for each instance before planning remediation.

Application owners and infrastructure teams
Verify PLC application project reachability and criticality.
Plan coordinated remediation with vendor.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is a Programmable Logic Controller (PLC) in this context?

A Programmable Logic Controller is an industrial computer used to automate manufacturing processes and control machinery. These ruggedized systems are designed to operate continuously in harsh environments, managing critical tasks like assembly lines or energy grids by monitoring inputs and making real-time decisions based on programmed logic.

What does CWE-303 mean for CVE-2026-3869?

CWE-303 refers to the Incorrect Implementation of an Authentication Algorithm. In this vulnerability, the mechanism intended to verify the identity of a user or system is flawed. Because the authentication logic does not work as expected, an attacker can bypass these security controls to interact with the device as if they were an authorized user, leading to a compromise of the system's data and operational functions.

How is CVE-2026-3869 triggered?

The vulnerability is triggered when a PLC is running an application project configured with a lower security level. If this specific project condition is met, the flawed authentication algorithm becomes accessible. Note that simply having the device powered on is not enough; the bug relies on the presence of this less-secure application configuration to function.

Is my device at risk based on Halo Surface Signal?

Halo Surface Signal indicates that while this is a network-based vulnerability, the risk is often mitigated by the physical architecture of industrial environments. PLCs are typically kept within isolated operational technology networks rather than on the public internet. If your PLC is properly segmented away from external access, the likelihood of an attacker reaching this trigger point is significantly reduced.

What should I do if I manage affected PLCs?

Start by identifying every PLC in your environment that is currently running an application project. Once you have a complete inventory, verify the network configuration for each unit to determine if it is exposed to broader networks. Finally, coordinate with your infrastructure team to review the security levels of these application projects and plan the necessary updates with the device vendor.

References

download.se.com/files?p_Doc_Ref=SEVD-2026-251-04&p_enDocType=Security…

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.