CVE-2020-7536 in Modicon M340
Summary
by MITRE • 12/11/2020
A CWE-754:Improper Check for Unusual or Exceptional Conditions vulnerability exists in Modicon M340 CPUs (BMXP34* versions prior to V3.30) Modicon M340 Communication Ethernet modules (BMXNOE0100 (H) versions prior to V3.4 BMXNOE0110 (H) versions prior to V6.6 BMXNOR0200H all versions), that could cause the device to be unreachable when modifying network parameters over SNMP.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 12/16/2020
The vulnerability identified as CVE-2020-7536 represents a critical weakness in the Modicon M340 series of programmable logic controllers and their associated communication modules. This flaw manifests as a CWE-754 improper check for unusual or exceptional conditions, which fundamentally compromises the reliability and availability of industrial control systems. The affected devices include BMXP340 CPU modules running firmware versions prior to V3.30, alongside various Ethernet communication modules such as BMXNOE0100(H), BMXNOE0110(H), and BMXNOR0200H models. The specific operational context involves scenarios where network parameters are modified through Simple Network Management Protocol (SNMP) interfaces, creating a dangerous condition where normal system operations can lead to complete device unreachability.
The technical implementation of this vulnerability stems from inadequate error handling within the network management subsystem of these industrial devices. When SNMP commands are issued to modify network parameters, the system fails to properly validate exceptional conditions that may occur during parameter transitions or configuration changes. This deficiency creates a scenario where legitimate network management operations can trigger unexpected system states that result in complete communication failure. The flaw essentially allows an attacker or even a well-intentioned administrator to inadvertently cause a denial of service condition by performing what should be routine network configuration modifications. The improper exception handling means that the system does not gracefully recover from edge cases or unusual parameter combinations, leading to a complete loss of connectivity that requires manual intervention or device reboot to restore functionality.
The operational impact of this vulnerability extends beyond simple availability concerns to potentially compromise industrial control system integrity and safety. In industrial environments where Modicon M340 controllers are deployed for critical process control, the sudden unreachability of network modules can lead to production halts, safety system failures, or emergency shutdown scenarios. The vulnerability affects not just individual devices but entire network segments that rely on these communication modules for data exchange and system monitoring. Network administrators who attempt to perform routine maintenance or configuration updates may unknowingly trigger this condition, resulting in unplanned downtime that can cost thousands of dollars in lost productivity and potential safety risks. The severity is amplified by the fact that these devices often operate in environments where immediate physical access is not always possible, making remote recovery complex and time-consuming.
Mitigation strategies for CVE-2020-7536 require immediate firmware updates to the affected Modicon M340 systems, specifically targeting the BMXP340 CPU modules and the designated Ethernet communication modules mentioned in the vulnerability description. Organizations should implement comprehensive patch management procedures that include thorough testing of firmware updates in controlled environments before deployment to production systems. Network segmentation and access control measures should be strengthened to limit SNMP access to only trusted administrators, reducing the attack surface for potential exploitation. The implementation of redundant communication paths and backup monitoring systems can help maintain operational continuity if one communication module fails due to this vulnerability. Additionally, system administrators should establish regular monitoring procedures to detect early signs of network instability that might indicate this vulnerability is being triggered, including logging and alerting mechanisms for unexpected communication failures. Organizations should also consider implementing the ATT&CK framework's defensive strategies for industrial control systems, particularly focusing on network security and operational technology monitoring to detect anomalous behavior that could indicate exploitation attempts.