CVE-2025-53476 is a vulnerability classified under CWE-775, indicating a missing release of file descriptors or handles after their effective lifetime in the OpenPLC_v3 ModbusTCP server. OpenPLC is an open-source industrial control system platform used for programmable logic controller (PLC) applications. The vulnerability specifically affects the ModbusTCP server component, which handles network communication using the Modbus protocol over TCP/IP. The issue stems from the server not properly closing or releasing TCP connections or associated file descriptors after they are no longer needed. An attacker can exploit this by establishing a specially crafted sequence of TCP connections to the server, causing it to exhaust available file descriptors or socket handles. Once exhausted, the server becomes unable to process new Modbus requests, effectively resulting in a denial of service (DoS). The vulnerability requires no authentication or user interaction and can be triggered remotely over the network. The CVSS v3.1 score is 5.3 (medium severity), reflecting the network attack vector, low complexity, no privileges required, no user interaction, and impact limited to availability. No patches or fixes have been published at the time of disclosure, and no known exploits have been observed in the wild. This vulnerability could disrupt industrial control processes relying on OpenPLC_v3, potentially halting automation or control operations that depend on ModbusTCP communication.

For European organizations, especially those operating industrial control systems (ICS) or critical infrastructure that utilize OpenPLC_v3 with ModbusTCP, this vulnerability poses a risk of service disruption. Denial of service in PLC environments can lead to halted manufacturing lines, interrupted utility services, or compromised safety monitoring. Although the vulnerability does not allow data theft or manipulation, the loss of availability can have significant operational and financial consequences. Sectors such as manufacturing, energy, water treatment, and transportation in Europe that rely on OpenPLC-based systems could experience downtime or degraded performance. The risk is heightened in environments where network segmentation or connection limits are not enforced, allowing attackers to flood the server with TCP connections. Given the lack of patches, organizations may face prolonged exposure until mitigations are applied. The absence of known exploits reduces immediate risk but does not eliminate the potential for future attacks.

European organizations should implement specific mitigations beyond generic advice: 1) Enforce strict network segmentation to isolate OpenPLC devices from untrusted networks and limit access to ModbusTCP ports. 2) Deploy network-level rate limiting and connection throttling to restrict the number of simultaneous TCP connections to the OpenPLC ModbusTCP server, preventing resource exhaustion. 3) Monitor network traffic for unusual patterns indicative of connection flooding or DoS attempts targeting ModbusTCP services. 4) Where possible, configure OpenPLC instances to use updated or alternative versions that address resource management issues, or apply custom patches if available. 5) Employ intrusion detection/prevention systems (IDS/IPS) tuned to detect excessive connection attempts or anomalies in ModbusTCP traffic. 6) Maintain an inventory of OpenPLC deployments and prioritize risk assessments for critical systems. 7) Engage with OpenPLC community or vendors for updates or patches addressing this vulnerability. 8) Prepare incident response plans for DoS scenarios affecting industrial control systems to minimize downtime impact.