CVE-2024-43390: Phoenix Contact mGuard RS4000 DoS Flaw

CVE-2024-43390 Overview

CVE-2024-43390 affects Phoenix Contact mGuard network security appliances across the FL MGUARD and TC MGUARD product families. A low-privileged remote attacker can manipulate the FW_NAT.IN_IP environment variable to perform unauthorized configuration changes to firewall services. These changes include altering packet forwarding rules and Network Address Translation (NAT) behavior. Successful exploitation leads to a denial-of-service (DoS) condition on the affected firewall.

The flaw is tracked under CWE-94: Improper Control of Generation of Code. Because mGuard devices are commonly deployed at the perimeter of industrial control system (ICS) and operational technology (OT) networks, disruption of these appliances can interrupt segmentation between IT and OT zones.

Critical Impact
A remote attacker with low privileges can disrupt firewall packet forwarding and NAT enforcement, causing denial of service on network security appliances protecting industrial environments.

Affected Products

Phoenix Contact FL MGUARD series (2102, 2105, 4302, 4305, PCI4000, PCIE4000, CENTERPORT VPN-1000, CORE TX, DELTA TX/TX, GT/GT, RS2000, RS2005, RS4000, RS4004, SMART2)
Phoenix Contact TC MGUARD RS2000 series (3G VPN, 4G VPN, 4G ATT VPN, 4G VZW VPN) firmware
Phoenix Contact TC MGUARD RS4000 series (3G VPN, 4G VPN, 4G ATT VPN, 4G VZW VPN) firmware

Discovery Timeline

2024-09-10 - CVE-2024-43390 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-43390

Vulnerability Analysis

The vulnerability resides in how the mGuard firmware processes the FW_NAT.IN_IP environment variable within its firewall service. An authenticated user with low privileges can influence this variable to modify runtime firewall configuration parameters. Because the affected code path controls packet forwarding and NAT behavior, attacker-controlled input translates directly into changes to how traffic is filtered and rewritten by the device.

Once manipulated, the firewall service can enter a state where legitimate traffic is dropped or misrouted. This produces a denial-of-service condition affecting all network segments protected by the appliance. In OT and ICS deployments, this loss of connectivity can interrupt communication between engineering workstations, PLCs, and SCADA components.

Root Cause

The root cause is improper control of code or configuration generation from environment variable input [CWE-94]. The firewall service accepts the FW_NAT.IN_IP value without sufficient validation or privilege enforcement, allowing a low-privileged authenticated user to reach configuration-changing logic that should be restricted to administrators.

Attack Vector

Exploitation requires network access to the mGuard management interface and valid low-privileged credentials. No user interaction is required. The attacker submits crafted input through the interface that consumes FW_NAT.IN_IP, causing the firewall service to apply unintended configuration changes. The result is disruption of packet forwarding and NAT, degrading integrity and availability of the appliance.

Refer to the VDE Security Advisory VDE-2024-039 for vendor-supplied technical details.

Detection Methods for CVE-2024-43390

Indicators of Compromise

Unexpected changes to firewall packet forwarding rules or NAT configuration on mGuard appliances.
Sudden loss of network connectivity through mGuard devices without a corresponding administrative change record.
Authentication events from low-privileged accounts followed by configuration-modification events in device logs.

Detection Strategies

Enable and centralize syslog forwarding from all mGuard devices, then alert on any modification to firewall or NAT configuration made by non-administrative accounts.
Baseline the normal set of firewall configuration change events per account role and flag deviations from that baseline.
Correlate device configuration changes with change-management tickets to surface unauthorized modifications.

Monitoring Recommendations

Monitor authentication logs on mGuard management interfaces for logins from unexpected sources or at unusual times.
Watch for repeated failed or successful low-privileged sessions that immediately precede configuration-change events.
Track availability metrics of traffic traversing mGuard appliances so that DoS conditions are detected quickly.

How to Mitigate CVE-2024-43390

Immediate Actions Required

Apply the firmware updates published by Phoenix Contact and referenced in VDE-2024-039 as soon as maintenance windows permit.
Restrict network access to the mGuard management interface to a dedicated management VLAN and known administrator hosts only.
Audit all local and remote user accounts on affected devices and remove or disable any unnecessary low-privileged accounts.

Patch Information

Phoenix Contact and CERT@VDE published coordinated remediation guidance in advisory VDE-2024-039. Administrators should consult the advisory for the specific fixed firmware versions applicable to each affected mGuard model and upgrade all deployed units accordingly.

Workarounds

Place mGuard management interfaces behind a jump host or VPN so that only authenticated administrators can reach the configuration service.
Rotate credentials for all mGuard accounts and enforce strong, unique passwords to reduce the risk of low-privileged account compromise.
Implement network access control lists on upstream switches to block management-plane access from operational network segments.

bash

# Example: restrict mGuard management access at an upstream router
# Replace <MGMT_HOST> and <MGUARD_IP> with your environment values
access-list 110 permit tcp host <MGMT_HOST> host <MGUARD_IP> eq 443
access-list 110 permit tcp host <MGMT_HOST> host <MGUARD_IP> eq 22
access-list 110 deny   tcp any host <MGUARD_IP> eq 443
access-list 110 deny   tcp any host <MGUARD_IP> eq 22
access-list 110 permit ip any any