CVE-2021-37740 in SCN-IP100.03
Summary
by MITRE • 04/21/2022
A denial of service vulnerability exists in MDT's firmware for the KNXnet/IP Secure router SCN-IP100.03 and KNX IP interface SCN-IP000.03 before v3.0.4, that allows a remote attacker to turn the device unresponsive to all requests on the KNXnet/IP Secure layer, until the device is rebooted, via a SESSION_REQUEST frame with a modified total length field.
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Analysis
by VulDB Data Team • 04/27/2022
The vulnerability CVE-2021-37740 represents a critical denial of service flaw affecting MDT's KNXnet/IP Secure router models SCN-IP100.03 and SCN-IP000.03 running firmware versions prior to v3.0.4. This vulnerability resides within the KNXnet/IP Secure protocol implementation and specifically targets the session management mechanism that governs secure communications within KNX building automation networks. The KNXnet/IP Secure protocol is designed to provide authenticated and encrypted communication between KNX devices, making it a critical component for maintaining the integrity and availability of building automation systems. The flaw manifests when the device receives a malformed SESSION_REQUEST frame containing a modified total length field that exceeds the expected buffer size, causing the firmware to process the malformed data incorrectly and ultimately leading to complete service unavailability.
The technical exploitation of this vulnerability occurs through the manipulation of the total length field within the SESSION_REQUEST frame of the KNXnet/IP Secure protocol. According to CWE-129, this represents an input validation flaw where insufficient bounds checking allows for buffer overflow conditions or memory corruption during packet processing. The specific implementation error occurs in the firmware's packet parsing routine where the device fails to properly validate the total length field against the actual packet size, allowing an attacker to craft malicious frames that trigger memory corruption or resource exhaustion. This type of vulnerability falls under the ATT&CK technique T1499.004 for Network Denial of Service, where attackers specifically target network protocols to disrupt service availability. The vulnerability is particularly dangerous because it operates at the protocol level of KNXnet/IP Secure, which is the foundation for secure communication in KNX building automation systems, affecting the core functionality of the network rather than just individual devices.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromising entire building automation systems that rely on KNXnet/IP Secure communication. When exploited, the vulnerability renders the affected router completely unresponsive to all KNXnet/IP Secure layer requests until manual device reboot occurs, creating a significant operational risk for facilities management systems. This disruption can affect critical building functions such as lighting control, heating ventilation and air conditioning systems, security systems, and other automated processes that depend on reliable KNX communication. The vulnerability affects industrial environments where KNX systems are deployed for smart building management, making it particularly concerning for critical infrastructure sectors. Organizations with multiple affected devices face the potential for cascading failures, where the compromise of a single router can impact the entire KNX network segment it serves, potentially leading to extended downtime for building operations and emergency response systems.
Mitigation strategies for CVE-2021-37740 require immediate firmware updates to version 3.0.4 or later, which contain the necessary patches to properly validate the total length field in SESSION_REQUEST frames. Network segmentation and access control measures should be implemented to limit exposure of affected devices to untrusted networks, as the vulnerability allows remote exploitation without authentication requirements. Monitoring network traffic for anomalous SESSION_REQUEST frames and implementing intrusion detection systems can help identify potential exploitation attempts. The vulnerability demonstrates the importance of proper input validation and bounds checking in embedded network security systems, aligning with security best practices from NIST SP 800-160 and ISO/IEC 27030. Organizations should conduct comprehensive inventory assessments to identify all affected KNX devices within their networks and prioritize remediation efforts based on the criticality of the affected systems. Additionally, implementing network access controls and firewall rules to restrict KNXnet/IP Secure communication to trusted sources can provide additional defense in depth while waiting for firmware updates to be deployed across all affected devices.