CVE-2024-11737: Modbus Controller DoS Vulnerability

CVE-2024-11737 Overview

CVE-2024-11737 is a critical Improper Input Validation vulnerability (CWE-20) affecting Schneider Electric industrial controllers. The vulnerability exists in the Modbus protocol handling mechanism, allowing unauthenticated attackers to send specially crafted Modbus packets to vulnerable devices over the network. Successful exploitation can result in denial of service conditions and compromise the confidentiality and integrity of the affected controller.

Critical Impact

This vulnerability enables unauthenticated remote attackers to disrupt industrial control systems, potentially causing operational outages and compromising data integrity in critical infrastructure environments.

Affected Products

• Schneider Electric Industrial Controllers (specific models detailed in SEVD-2024-345-03)
• Devices with exposed Modbus protocol interfaces
• Controllers accessible via network without proper segmentation

Discovery Timeline

• December 11, 2024 - CVE-2024-11737 published to NVD
• December 11, 2024 - Last updated in NVD database

Technical Details for CVE-2024-11737

Vulnerability Analysis

This vulnerability stems from inadequate input validation in the Modbus protocol implementation on affected Schneider Electric controllers. The Modbus protocol, commonly used in industrial control systems (ICS) and SCADA environments, is a communication standard that allows devices to exchange data. When processing incoming Modbus packets, the affected controllers fail to properly validate certain input parameters, creating an exploitable condition.

The attack requires no authentication, making it particularly dangerous in environments where Modbus services are exposed to untrusted networks. An attacker who can reach the Modbus port (typically TCP/502) can craft malicious packets that trigger the vulnerability, potentially crashing the controller or manipulating its operational data.

Root Cause

The root cause of CVE-2024-11737 is improper input validation (CWE-20) in the Modbus packet processing routine. The controller does not adequately verify the structure, length, or content of incoming Modbus frames before processing them. This allows malformed or malicious packets to bypass expected validation checks and interact with controller memory or functions in unintended ways.

Attack Vector

The attack vector is network-based, requiring the attacker to have network connectivity to the vulnerable controller's Modbus service. The attack characteristics include:

• Network accessibility - The attacker must be able to reach the Modbus port on the target device
• No authentication required - The vulnerability can be exploited without valid credentials
• No user interaction - Exploitation does not require any action from a legitimate user
• Protocol-based - Attack leverages malformed Modbus protocol messages

The attacker constructs a specially crafted Modbus packet containing malformed data fields that exploit the input validation weakness. When the vulnerable controller receives and processes this packet, it can result in denial of service (controller crash or hang), loss of confidentiality (information disclosure), or loss of integrity (unauthorized modification of controller state or data). Detailed technical information is available in the Schneider Electric Security Notice SEVD-2024-345-03.

Detection Methods for CVE-2024-11737

Indicators of Compromise

• Unusual Modbus traffic patterns or malformed packets targeting port TCP/502
• Controller crashes, restarts, or unexpected state changes without legitimate operational cause
• Anomalous network connections to Modbus services from unexpected source addresses
• Log entries indicating Modbus protocol errors or parsing failures

Detection Strategies

• Deploy network intrusion detection systems (IDS) with rules to identify malformed Modbus protocol packets
• Implement deep packet inspection (DPI) for Modbus traffic to detect anomalous message structures
• Monitor controller health metrics for unexpected crashes, restarts, or performance degradation
• Establish baseline Modbus communication patterns and alert on deviations

Monitoring Recommendations

• Enable logging on firewalls and network devices for all Modbus protocol traffic (TCP/502)
• Configure SIEM alerts for repeated connection attempts to Modbus services from untrusted sources
• Monitor controller availability and operational state through OT-specific monitoring solutions
• Implement asset inventory to track all Modbus-enabled devices and their network exposure

How to Mitigate CVE-2024-11737

Immediate Actions Required

• Apply vendor-provided patches or firmware updates from Schneider Electric as soon as they are available
• Segment industrial control networks to restrict Modbus access only to authorized systems
• Disable or block external network access to Modbus services (TCP/502) using firewalls
• Implement network access controls (NAC) to limit connectivity to critical controllers

Patch Information

Schneider Electric has released a security notice (SEVD-2024-345-03) addressing this vulnerability. Organizations should review the Schneider Electric Security Notice for specific patch availability, affected product versions, and remediation guidance. Contact Schneider Electric support for detailed upgrade procedures applicable to your deployment.

Workarounds

• Implement strict network segmentation to isolate ICS/SCADA networks from corporate and internet-facing networks
• Deploy firewalls with explicit deny rules for inbound Modbus traffic from untrusted zones
• Use VPN or other secure access methods for remote maintenance requiring Modbus connectivity
• Enable Modbus security features or consider Modbus/TCP Security (TLS) where supported

# Example firewall rule to restrict Modbus access (iptables)
# Allow Modbus only from trusted OT management subnet
iptables -A INPUT -p tcp --dport 502 -s 10.10.50.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 502 -j DROP