CVE-2026-1876: Mitsubishi MELSEC iQ-F FX5-ENET/IP DoS Flaw

CVE-2026-1876 Overview

CVE-2026-1876 is an Improper Resource Shutdown or Release vulnerability (CWE-404) affecting Mitsubishi Electric Corporation's MELSEC iQ-F Series FX5-ENET/IP Ethernet Module. This Industrial Control System (ICS) vulnerability allows a remote attacker to cause a denial-of-service (DoS) condition by continuously sending UDP packets to the affected products. Recovery requires a complete system reset of the impacted device.

Critical Impact
Remote attackers can disrupt industrial operations by rendering the Ethernet module unresponsive through a sustained UDP packet flood, requiring physical intervention for system recovery.

Affected Products

Mitsubishi Electric MELSEC iQ-F Series FX5-ENET/IP Ethernet Module (all versions)

Discovery Timeline

2026-03-03 - CVE-2026-1876 published to NVD
2026-03-04 - Last updated in NVD database

Technical Details for CVE-2026-1876

Vulnerability Analysis

This vulnerability stems from improper resource management within the FX5-ENET/IP Ethernet module's UDP packet handling mechanism. When the device receives a continuous stream of UDP packets, it fails to properly release or manage the associated resources, eventually leading to resource exhaustion and system unresponsiveness.

The attack can be executed remotely over the network without requiring authentication or user interaction. The impact is limited to availability—there is no compromise of data confidentiality or integrity. However, in industrial environments where these programmable logic controllers (PLCs) manage critical manufacturing or infrastructure processes, even temporary unavailability can have significant operational and safety implications.

The MELSEC iQ-F Series is commonly deployed in factory automation, process control, and building management systems, making this vulnerability particularly concerning for operational technology (OT) environments.

Root Cause

The root cause is classified as CWE-404 (Improper Resource Shutdown or Release). The FX5-ENET/IP module does not adequately manage system resources when processing incoming UDP traffic. Under sustained load conditions, the module fails to properly release resources associated with processed packets, leading to progressive resource depletion until the device becomes unresponsive.

Attack Vector

The attack exploits the network-accessible UDP services on the FX5-ENET/IP module. An attacker positioned on the same network segment—or with network access to the ICS environment—can flood the device with UDP packets. The attack requires:

Network connectivity to the target device
No authentication or special privileges
No user interaction
Sustained packet transmission to exhaust resources

The attack results in complete loss of availability, requiring manual system reset for recovery. This characteristic makes it particularly disruptive in environments where physical access to devices may be limited or where downtime impacts production schedules.

Detection Methods for CVE-2026-1876

Indicators of Compromise

Unusual volume of UDP traffic directed at FX5-ENET/IP modules
FX5-ENET/IP modules becoming unresponsive to legitimate communications
Unexpected system resets or manual recovery operations on affected devices
Network monitoring alerts indicating UDP flood patterns targeting ICS devices

Detection Strategies

Implement network traffic analysis to identify anomalous UDP packet volumes targeting ICS assets
Configure IDS/IPS rules to detect UDP flood patterns against known FX5-ENET/IP device addresses
Deploy industrial protocol-aware monitoring solutions that can distinguish legitimate automation traffic from attack patterns
Establish baseline network behavior for ICS segments and alert on deviations

Monitoring Recommendations

Enable logging on network infrastructure devices to capture traffic patterns to ICS segments
Monitor FX5-ENET/IP module availability through regular health checks and SNMP polling
Implement real-time alerting for device unresponsiveness or communication failures
Correlate ICS network events with security information and event management (SIEM) systems

How to Mitigate CVE-2026-1876

Immediate Actions Required

Segment ICS networks from corporate and external networks using firewalls and DMZ architectures
Implement strict access control lists (ACLs) limiting UDP traffic to FX5-ENET/IP modules
Deploy rate limiting on network devices to prevent UDP flood attacks from reaching ICS assets
Review and restrict network access to only authorized devices and personnel

Patch Information

Consult the official Mitsubishi Electric Vulnerability Document for the latest patch and firmware update information. Additional guidance is available through the CISA ICS Advisory ICSA-26-62-01 and the JVN Vulnerability Report.

Workarounds

Implement network segmentation to isolate ICS devices from untrusted networks
Deploy firewall rules to filter and rate-limit UDP traffic to affected devices
Use VPN connections for remote access to ICS networks rather than direct exposure
Consider deploying industrial-grade network monitoring and intrusion prevention systems

bash

# Example firewall rate limiting configuration (iptables)
# Limit UDP packets to ICS device IP to 100 per second
iptables -A INPUT -p udp -d <FX5_ENET_IP> -m limit --limit 100/sec --limit-burst 200 -j ACCEPT
iptables -A INPUT -p udp -d <FX5_ENET_IP> -j DROP