CVE-2024-43391: Phoenix Contact TC mGuard Firewall DoS Vulnerability

CVE-2024-43391 Overview

CVE-2024-43391 affects Phoenix Contact mGuard firewall and VPN appliances used in industrial and operational technology (OT) environments. A low-privileged remote attacker can manipulate the FW_PORTFORWARDING.SRC_IP environment variable to alter firewall service configurations. The affected functions include packet filtering, packet forwarding, network access control, and Network Address Translation (NAT). Successful exploitation leads to denial-of-service (DoS) conditions and integrity loss across firewall policy enforcement. The issue is tracked under [CWE-94] Improper Control of Generation of Code and applies to a wide range of FL mGuard and TC mGuard product families.

Critical Impact
An authenticated remote attacker with low privileges can modify firewall rules and cause a denial-of-service condition on Phoenix Contact mGuard devices protecting industrial networks.

Affected Products

Phoenix Contact FL mGuard series (2102, 2105, 4302, 4305, PCI4000, PCIe4000, GT/GT, Delta, Core TX, RS2000/RS2005/RS4000/RS4004, Smart2, Centerport VPN-1000)
Phoenix Contact TC mGuard RS2000/RS4000 3G and 4G VPN variants (ATT, VZW, standard)
All firmware variants listed in VDE Security Advisory VDE-2024-039

Discovery Timeline

2024-09-10 - CVE-2024-43391 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-43391

Vulnerability Analysis

The vulnerability resides in how the mGuard firmware handles the FW_PORTFORWARDING.SRC_IP environment variable. Instead of treating the variable as inert input, the firmware processes its contents in a context that influences firewall service configuration generation. A remote attacker holding low-privilege credentials can inject values that reconfigure packet filter rules, forwarding logic, access control lists, or NAT mappings. The result is a loss of integrity in the security enforcement layer and, ultimately, a denial-of-service condition on the appliance or the network segments it protects.

Because mGuard devices are commonly deployed as perimeter and internal segmentation firewalls in industrial control system (ICS) and OT networks, disruption of firewall services can cascade into production outages. The vulnerability is classified under [CWE-94] Improper Control of Generation of Code.

Root Cause

The root cause is insufficient validation and sanitization of the FW_PORTFORWARDING.SRC_IP environment variable before it is consumed by configuration-generation routines. Input flowing through this variable is trusted with more authority than the caller's privilege level warrants, allowing configuration state to be altered outside the intended administrative workflow.

Attack Vector

Exploitation requires network access to the management interface and valid low-privileged credentials on the device. The attacker submits crafted input through the port-forwarding configuration path, injecting content into FW_PORTFORWARDING.SRC_IP. The firmware then applies these values during firewall service reconfiguration, altering packet filter, forwarding, access control, or NAT behavior. No user interaction is required, and the attack completes over the network. Refer to the VDE Security Advisory VDE-2024-039 for vendor-supplied technical details.

Detection Methods for CVE-2024-43391

Indicators of Compromise

Unexpected changes to firewall rule sets, port-forwarding entries, or NAT tables on mGuard devices
Configuration changes originating from non-administrative user accounts
Unexplained loss of network connectivity or blocked traffic between OT segments protected by mGuard appliances
Audit log entries referencing modifications to FW_PORTFORWARDING parameters from low-privileged sessions

Detection Strategies

Baseline mGuard firewall configurations and continuously compare running configurations against the approved baseline
Alert on any modification to port-forwarding rules initiated by accounts that lack the admin role
Ingest mGuard syslog output into a centralized SIEM or data lake and correlate configuration change events with authentication events

Monitoring Recommendations

Forward mGuard management-plane logs to a centralized logging platform for retention and correlation
Monitor northbound and southbound traffic flows for sudden policy shifts that indicate firewall rule tampering
Track privileged session activity on mGuard web and CLI interfaces, flagging low-privileged users who touch firewall configuration paths

How to Mitigate CVE-2024-43391

Immediate Actions Required

Apply the firmware updates referenced in VDE Security Advisory VDE-2024-039 to all affected FL mGuard and TC mGuard devices
Restrict management-interface access to a dedicated administrative network using ACLs or jump hosts
Audit local and remote user accounts on mGuard devices and remove unused low-privileged accounts
Rotate credentials for any accounts that may have been exposed prior to patching

Patch Information

Phoenix Contact and VDE CERT have coordinated the disclosure and patch release for this issue. Consult the VDE Security Advisory VDE-2024-039 for the exact firmware versions that remediate CVE-2024-43391 across the mGuard product family. Apply the fixed firmware to every affected model listed in the advisory before re-enabling remote administrative access.

Workarounds

Limit access to the mGuard management interface to trusted administrative hosts only
Enforce least privilege by removing configuration rights from any account that does not require them
Segment mGuard management traffic onto an isolated VLAN with strict inbound filtering
Enable and review configuration change auditing on all mGuard devices until firmware updates are deployed

bash

# Example: restrict mGuard web management to a trusted admin subnet
# (apply through the mGuard configuration UI or via the device CLI)
mgmt.access.allowed_networks = 10.10.100.0/24
mgmt.access.protocols = https
users.role.default = readonly