CVE-2021-20596 in MELSEC-F FX3U-ENET
Summary
by MITRE • 07/22/2021
NULL Pointer Dereference in MELSEC-F Series FX3U-ENET firmware version 1.14 and prior, FX3U-ENET-L firmware version 1.14 and prior and FX3U-ENET-P502 firmware version 1.14 and prior allows a remote unauthenticated attacker to cause a DoS condition in communication by sending specially crafted packets. Control by MELSEC-F series PLC is not affected and system reset is required for recovery.
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Analysis
by VulDB Data Team • 07/27/2021
This vulnerability exists within the MELSEC-F series FX3U-ENET firmware versions 1.14 and earlier across multiple product variants including FX3U-ENET, FX3U-ENET-L, and FX3U-ENET-P502 devices. The flaw represents a null pointer dereference condition that occurs when the firmware processes specially crafted network packets. This type of vulnerability falls under the CWE-476 category of NULL Pointer Dereference, which is a common software defect that can lead to system instability and denial of service conditions. The vulnerability is particularly concerning because it allows remote unauthenticated attackers to exploit the flaw without requiring any credentials or prior access to the system.
The technical implementation of this vulnerability involves the firmware's network protocol handling logic where it fails to properly validate incoming packet structures before attempting to dereference pointers within the packet processing code. When an attacker sends malformed packets that trigger this condition, the firmware crashes and enters a state where it cannot properly handle subsequent network communications. The vulnerability specifically affects the Ethernet communication modules of the FX3U-ENET series PLCs, which are designed to provide network connectivity for industrial control systems. The affected devices operate within the industrial control environment where network communication is essential for proper system operation and where disruptions can lead to significant operational impacts.
The operational impact of this vulnerability extends beyond simple denial of service conditions as it can potentially disrupt critical industrial processes that rely on these PLC communication modules. While the primary effect is a denial of service requiring system reset for recovery, the broader implications include potential operational downtime and the need for unplanned maintenance activities. The vulnerability affects the communication capabilities of the PLCs but does not compromise the actual control functions of the system, meaning that the physical control mechanisms remain intact. However, the network communication disruption can prevent operators from monitoring or controlling processes through network-based interfaces, which may be critical for system management and diagnostics.
Organizations should implement immediate mitigations including firmware updates to versions that address this vulnerability, network segmentation to limit exposure, and monitoring of network traffic for suspicious packet patterns. The ATT&CK framework categorizes this type of vulnerability under the T1499.004 sub-technique for Network Denial of Service, indicating that attackers can leverage such flaws to disrupt network services. Security teams should also consider implementing network access controls to limit which systems can communicate with these PLC devices, particularly in environments where the devices are not directly accessible from untrusted networks. The vulnerability highlights the importance of securing industrial control systems and demonstrates how even seemingly minor software flaws can have significant operational consequences in industrial environments where reliability is paramount.