CVE-2020-7549 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 denial of HTTP and FTP services when a series of specially crafted requests is sent to the controller over HTTP.
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Analysis
by VulDB Data Team • 05/29/2026
The vulnerability identified as CVE-2020-7549 represents a critical weakness in the Modicon M340 series of industrial control systems, specifically affecting legacy offerings including the Modicon Quantum and Modicon Premium controllers along with their associated communication modules. This issue manifests as a CWE-754 improper check for unusual or exceptional conditions, which fundamentally undermines the system's ability to handle abnormal input sequences gracefully. The affected devices operate within industrial environments where reliability and continuous operation are paramount, making this vulnerability particularly dangerous as it can lead to complete service disruption.
The technical flaw resides in how the web server component of these industrial controllers processes incoming HTTP requests. When subjected to a sequence of specially crafted requests, the system fails to properly validate or handle exceptional conditions that should normally trigger appropriate error handling mechanisms. This improper exception handling causes the HTTP and FTP services to become unresponsive or completely terminate, resulting in denial of service conditions that can persist until manual intervention or system reboot occurs. The vulnerability exploits the controller's lack of robust input validation and error recovery procedures, creating a scenario where legitimate service operations are disrupted by seemingly innocuous but carefully constructed request patterns.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise industrial control system integrity and availability. In manufacturing and process control environments, where these controllers manage critical infrastructure, the denial of HTTP and FTP services can prevent essential maintenance operations, configuration updates, and monitoring activities. This vulnerability directly impacts the availability component of the CIA triad, as it renders critical communication channels unusable and can lead to extended downtime for industrial processes. The attack vector is particularly concerning because it requires only network access to the controller, making it accessible to remote threat actors who may exploit this weakness to disrupt operations or create conditions for more sophisticated attacks.
Organizations operating affected Modicon controllers should implement immediate mitigations including network segmentation to isolate these devices from general network access, deploying network intrusion detection systems to monitor for suspicious request patterns, and applying vendor-provided security patches or firmware updates when available. The vulnerability aligns with ATT&CK technique T1499.004 for network denial of service, and organizations should consider implementing defensive measures such as rate limiting and request validation to prevent exploitation. Additionally, regular security assessments should be conducted to identify similar improper exception handling vulnerabilities in other industrial control systems, as CWE-754 represents a common class of flaws that can lead to similar denial of service conditions in industrial environments. The incident underscores the importance of robust error handling and input validation in industrial control systems where the consequences of service disruption can be severe and far-reaching.