CVE-2020-7539 in Modicon M340
Summary
by MITRE • 12/11/2020
A CWE-754 Improper Check for Unusual or Exceptional Conditions vulnerability exists in the Web Server on Modicon M340, Legacy Offers Modicon Quantum and Modicon Premium and associated Communication Modules (see security notification for affected versions), that could cause a denial of service vulnerability when a specially crafted packet is sent to the controller over HTTP.
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Analysis
by VulDB Data Team • 12/16/2020
The CVE-2020-7539 vulnerability represents a critical security flaw in Schneider Electric's Modicon M340, Quantum, and Premium series controllers that operates under the CWE-754 classification for improper check for unusual or exceptional conditions. This vulnerability specifically affects the web server component embedded within these industrial control systems, creating a potential denial of service scenario when maliciously crafted HTTP packets are transmitted to the affected controllers. The flaw stems from inadequate error handling mechanisms within the web server implementation, where the system fails to properly validate or process exceptional network conditions that arise during HTTP communication. This weakness allows attackers to exploit the controller's web interface through carefully constructed network traffic that triggers unexpected behavior in the server's response handling logic. The vulnerability is particularly concerning in industrial environments where continuous operation is critical, as it can lead to complete service disruption of the affected controllers.
The technical implementation of this vulnerability manifests when the web server component receives malformed or specially crafted HTTP requests that fall outside normal operational parameters. The controller's web server lacks robust input validation and exception handling routines that would normally detect and gracefully manage unusual network conditions. When such packets are processed, the server enters an undefined state where it cannot properly respond to subsequent legitimate requests, ultimately leading to a denial of service condition. This improper handling of exceptional conditions creates a scenario where the controller's web interface becomes unresponsive, effectively cutting off remote access capabilities that are essential for system monitoring, configuration, and maintenance. The vulnerability does not require authentication to exploit, making it particularly dangerous in environments where these controllers are directly exposed to network traffic, and it can be triggered through simple network packet crafting techniques that leverage the controller's HTTP server implementation.
The operational impact of CVE-2020-7539 extends beyond simple service disruption to potentially compromise industrial control system availability and operational continuity. In critical infrastructure environments, such as manufacturing plants, water treatment facilities, or energy distribution systems, the denial of service condition can result in complete loss of remote access to control systems, forcing operators to rely on physical access methods for system management. This limitation significantly increases operational complexity and response times during maintenance or emergency situations. The vulnerability also creates opportunities for attackers to conduct prolonged denial of service attacks that can go unnoticed for extended periods, potentially masking more sophisticated attacks or simply disrupting operations for extended durations. Organizations running these controllers may experience production delays, increased maintenance costs, and potential safety risks if the controllers become inaccessible during critical operational phases.
Mitigation strategies for CVE-2020-7539 should focus on both immediate defensive measures and long-term architectural improvements to protect industrial control systems from similar vulnerabilities. Organizations should implement network segmentation and access controls to limit exposure of affected controllers to untrusted networks, effectively reducing the attack surface available to potential adversaries. Network-based solutions such as firewalls, intrusion detection systems, and access control lists can help filter malicious traffic before it reaches the vulnerable web server components. The most effective immediate solution involves applying official firmware updates and patches provided by Schneider Electric, which address the underlying improper exception handling in the web server implementation. Additionally, organizations should consider disabling the web server functionality on controllers when it is not required for operations, as this reduces the attack surface and eliminates the vulnerability entirely. Regular network monitoring and anomaly detection should be implemented to identify potential exploitation attempts, while security awareness training for industrial control system operators can help detect unusual access patterns that might indicate exploitation attempts. This vulnerability aligns with ATT&CK techniques related to network denial of service and system service manipulation, emphasizing the importance of robust input validation and proper error handling in industrial control system implementations.