CVE-2019-6856 in Modicon M580
Summary
by MITRE
A CWE-754: Improper Check for Unusual or Exceptional Conditions vulnerability exists in Modicon M580, Modicon M340, Modicon Quantum, Modicon Premium (see security notification for specific versions) which could cause a Denial of Service when writing specific physical memory blocks using Modbus TCP.
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Analysis
by VulDB Data Team • 05/29/2026
The vulnerability described in CVE-2019-6856 represents a critical weakness in several Modicon programmable logic controller models including the M580, M340, Quantum, and Premium series. This issue stems from an improper check for unusual or exceptional conditions, classified under CWE-754, which fundamentally compromises the robustness of industrial control systems. The flaw manifests when specific physical memory blocks are written using Modbus TCP protocol, creating a pathway for malicious actors to disrupt normal system operations through carefully crafted requests that trigger unexpected behaviors in the affected controllers.
The technical implementation of this vulnerability involves the controllers' failure to properly validate or handle exceptional conditions during memory write operations. When Modbus TCP requests attempt to write to specific physical memory locations, the system does not adequately verify the legitimacy of these operations or handle edge cases that might occur during the write process. This lack of proper validation creates a scenario where malformed or unexpected memory write requests can cause the controller to enter an unstable state, ultimately leading to system disruption. The vulnerability specifically targets the Modbus TCP implementation within these industrial controllers, exploiting weaknesses in how the system processes memory write commands that should normally be handled gracefully.
From an operational perspective, this vulnerability presents a significant risk to industrial environments where these controllers are deployed, particularly in critical infrastructure sectors such as manufacturing, energy, and water treatment facilities. The potential for denial of service means that legitimate operations could be interrupted, leading to production downtime, safety concerns, or even cascading failures in connected systems. The attack surface is particularly concerning because Modbus TCP is widely used in industrial environments, making this vulnerability potentially exploitable by attackers who have network access to the affected systems. The impact extends beyond simple service interruption as the affected controllers may become unresponsive or require manual intervention to restore normal operations.
Organizations should implement multiple layers of mitigation strategies to address this vulnerability effectively. Network segmentation and access controls should be enforced to limit who can communicate with these controllers, particularly restricting Modbus TCP traffic to authorized personnel only. Regular firmware updates should be applied as provided by the vendor to address the underlying implementation flaw. Monitoring systems should be deployed to detect unusual Modbus TCP traffic patterns that might indicate exploitation attempts. Additionally, implementing network intrusion detection systems can help identify and alert on suspicious memory write operations targeting these specific controller models. The mitigation approach should align with industrial cybersecurity frameworks such as NIST SP 800-82 and IEC 62443 standards, which emphasize the importance of securing industrial control systems through proper network architecture and access controls. This vulnerability highlights the critical need for robust exception handling in industrial control systems, as outlined in the ATT&CK framework's industrial control systems tactics, where adversaries may exploit such weaknesses to gain persistent access or cause operational disruptions.