CVE-2017-11401 in Tofino Xenon Security Appliance
Summary
by MITRE
An issue has been discovered on the Belden Hirschmann Tofino Xenon Security Appliance before 03.2.00. Improper handling of the mbap.length field of ModBus packets in the ModBus DPI filter allows an attacker to send malformed/crafted packets to a protected asset, bypassing function code filtering.
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Analysis
by VulDB Data Team • 12/09/2019
The Belden Hirschmann Tofino Xenon Security Appliance represents a critical network security device designed to protect industrial control systems from cyber threats through deep packet inspection and protocol filtering capabilities. This particular vulnerability affects firmware versions prior to 03.2.00, exposing a fundamental flaw in how the appliance processes Modbus protocol traffic. The vulnerability resides within the Modbus DPI (Deep Packet Inspection) filter component, which is responsible for analyzing and filtering Modbus communication packets to prevent unauthorized access to industrial assets. The device operates as a network security appliance that monitors and controls traffic flowing between industrial networks and external connections, making it a crucial component in protecting critical infrastructure from cyber attacks.
The technical flaw manifests in the improper handling of the mbap.length field within Modbus packets, a critical element of the Modbus Application Protocol that specifies the length of the following data portion of the message. When an attacker crafts malicious Modbus packets with manipulated length fields, the appliance fails to properly validate these values before processing the packet content. This vulnerability specifically affects the function code filtering mechanism, which is designed to prevent execution of potentially dangerous Modbus function codes such as those used for reading or writing registers, controlling devices, or accessing sensitive system information. The improper validation allows attackers to bypass these security controls by creating packets that appear legitimate to the protocol parser but contain malicious payloads that would normally be blocked by the function code filtering rules.
The operational impact of this vulnerability extends beyond simple protocol bypassing, as it creates a pathway for attackers to gain unauthorized access to industrial control systems that the appliance is designed to protect. Attackers can exploit this weakness to send crafted packets that bypass security controls, potentially enabling them to perform unauthorized operations on connected industrial devices, manipulate process control variables, or gather sensitive operational data. The vulnerability particularly affects environments where the Tofino appliance is used to protect critical infrastructure such as power grids, water treatment facilities, manufacturing plants, or other industrial control systems where Modbus protocol communication is prevalent. Given that Modbus is widely used in industrial environments for communication between programmable logic controllers and various industrial devices, this vulnerability could potentially allow attackers to compromise entire industrial control networks, leading to operational disruptions, safety hazards, or even physical damage to equipment.
This vulnerability aligns with CWE-129, Improper Validation of Array Index, as the system fails to properly validate the length field before processing array-based data structures within the Modbus packet. The flaw also relates to ATT&CK technique T1071.001, Application Layer Protocol: Web Protocols, though adapted for industrial protocols, where attackers leverage protocol parsing weaknesses to bypass security controls. Additionally, this issue connects to T1059.005, Command and Scripting Interpreter: Visual Basic, through potential exploitation methods that could involve crafting malicious scripts or commands that bypass filtering. The vulnerability demonstrates the critical importance of proper input validation in security appliances, as the failure to validate protocol fields can create complete bypass mechanisms for security controls. Organizations using this appliance should immediately implement firmware updates to version 03.2.00 or later, which contain fixes for the Modbus packet validation issues. Network administrators should also consider implementing additional monitoring and anomaly detection measures to identify potential exploitation attempts, while conducting thorough security assessments of their industrial control systems to identify other potential protocol-based vulnerabilities. The incident highlights the necessity of robust protocol validation mechanisms in security appliances and the importance of regular firmware updates to address emerging threats in industrial cybersecurity environments.