CVE-2024-43393: Phoenix Contact mGuard Firewall DoS Flaw

CVE-2024-43393 Overview

CVE-2024-43393 affects Phoenix Contact mGuard firewall appliances across the FL MGUARD and TC MGUARD product families. A low-privileged remote attacker can inject configuration changes into firewall services by manipulating specific environment variables. The affected variables include FW_INCOMING.FROM_IP, FW_INCOMING.IN_IP, FW_OUTGOING.FROM_IP, FW_OUTGOING.IN_IP, FW_RULESETS.FROM_IP, and FW_RULESETS.IN_IP. Successful exploitation allows tampering with packet filtering, packet forwarding, network access control, and NAT rules. The impact leads to denial of service on the firewall appliance and the networks it protects. This weakness is tracked under [CWE-94] Improper Control of Generation of Code.

Critical Impact
A remote authenticated attacker with low privileges can alter core firewall rule sets, disrupt traffic filtering, and cause a denial of service on industrial network perimeter devices.

Affected Products

Phoenix Contact FL MGUARD family firewalls (including FL MGUARD 2102/2105, FL MGUARD 4302/4305, FL MGUARD 4102 PCI/PCIe, FL MGUARD CENTERPORT VPN-1000, and the FL MGUARD CORE, DELTA, GT, PCI4000, PCIE4000, RS2000, RS2005, RS4000, RS4004, and SMART2 variants)
Phoenix Contact TC MGUARD family firewalls (including TC MGUARD RS2000 3G/4G VPN, TC MGUARD RS2000 4G ATT/VZW VPN, TC MGUARD RS4000 3G/4G VPN, and TC MGUARD RS4000 4G ATT/VZW VPN)
Corresponding firmware images for all affected mGuard hardware SKUs listed above

Discovery Timeline

2024-09-10 - CVE-2024-43393 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2024-43393

Vulnerability Analysis

The vulnerability resides in how the mGuard firewall processes user-influenced input that is later consumed as configuration data for firewall rule generation. Environment variables tied to firewall ingress, egress, and rule-set definitions are not adequately validated before being applied to packet filter, forwarding, access control, and NAT engines. An authenticated attacker with low privileges can supply crafted values for FW_INCOMING.FROM_IP, FW_INCOMING.IN_IP, FW_OUTGOING.FROM_IP, FW_OUTGOING.IN_IP, FW_RULESETS.FROM_IP, or FW_RULESETS.IN_IP. These values propagate into the firewall configuration state and modify the runtime rule set. The resulting rule modifications can drop legitimate traffic, remove enforcement paths, or destabilize the firewall service. Because mGuard devices commonly sit at the perimeter of industrial and operational technology networks, service disruption at this layer cascades into downstream availability loss.

Root Cause

The root cause is improper control of code or configuration generation from external input, mapped to [CWE-94]. Environment variable values that should be validated and constrained are used to construct firewall rule directives. The device treats attacker-controllable strings as trusted configuration input for security-critical rule pipelines.

Attack Vector

Exploitation requires network access to the management interface and a low-privileged account on the mGuard device. No user interaction is needed. The attacker submits configuration input containing manipulated environment variable content. The device applies the manipulated values to the firewall rule engine, integrity of the rule set is broken, and availability of filtered traffic paths is lost. Refer to the VDE Security Advisory VDE-2024-039 for vendor-provided technical detail.

// No verified public proof-of-concept code is available for CVE-2024-43393.
// See the VDE-CERT advisory linked above for authoritative technical details.

Detection Methods for CVE-2024-43393

Indicators of Compromise

Unexpected changes to FW_INCOMING, FW_OUTGOING, or FW_RULESETS entries in mGuard configuration exports or backups.
Firewall rule sets containing malformed IP values or unusual characters in FROM_IP or IN_IP fields.
Sudden loss of legitimate traffic across a segment protected by an mGuard device, correlating with a recent configuration write event.
Authentication events from low-privileged mGuard accounts immediately followed by configuration change events.

Detection Strategies

Baseline the mGuard configuration and diff exported configurations on a scheduled cadence to catch unauthorized rule modifications.
Alert on any write operation to firewall rule set variables initiated by non-administrative accounts.
Correlate management-interface authentications with configuration change timestamps in SIEM to identify anomalous change patterns.

Monitoring Recommendations

Forward mGuard audit logs and configuration change events to a centralized log platform for retention and correlation.
Monitor availability of protected network segments and alert on drop-rate anomalies that coincide with configuration events.
Track successful and failed logins to the mGuard web and CLI management interfaces, focusing on low-privileged account activity.

How to Mitigate CVE-2024-43393

Immediate Actions Required

Apply the fixed firmware release from Phoenix Contact as identified in VDE Security Advisory VDE-2024-039.
Restrict network reachability of mGuard management interfaces to a hardened administrative VLAN or jump host.
Audit all mGuard user accounts and remove or downgrade any accounts that are not required for operations.
Export and archive current firewall configurations before patching to enable rollback and post-patch comparison.

Patch Information

Phoenix Contact has coordinated remediation through VDE-CERT. Consult the VDE Security Advisory VDE-2024-039 for the list of fixed firmware versions applicable to each affected mGuard hardware model and upgrade guidance.

Workarounds

Limit management access to the mGuard device to trusted administrator workstations using network access control lists on upstream infrastructure.
Enforce the principle of least privilege for all mGuard accounts, ensuring configuration write capability is reserved for administrators only.
Monitor configuration integrity by comparing periodic configuration exports against a known-good baseline.

bash

# Example: restrict mGuard management access with an upstream ACL
# Replace <mgmt_subnet> and <mguard_ip> with values from your environment
access-list MGUARD_MGMT permit tcp <mgmt_subnet> host <mguard_ip> eq 443
access-list MGUARD_MGMT permit tcp <mgmt_subnet> host <mguard_ip> eq 22
access-list MGUARD_MGMT deny   ip any host <mguard_ip> log