CVE-2025-41739 is a vulnerability identified in the CODESYS PLCHandler component of the CODESYS Control runtime system, specifically version 3.5.21.0 running on Linux and QNX platforms. The vulnerability arises from a race condition in the communication servers that handle socket connections. An unauthenticated remote attacker can exploit this race condition by sending specially crafted socket communications that trigger an out-of-bounds read (CWE-125). This out-of-bounds read can cause the affected process to crash or behave unpredictably, resulting in a denial of service (DoS) condition. The attack vector is network-based (AV:N), requires high attack complexity (AC:H), no privileges (PR:N), and no user interaction (UI:N). The scope remains unchanged (S:U), and the impact affects availability only (A:H), with no confidentiality or integrity loss. The vulnerability was reserved in April 2025 and published in December 2025, with no known exploits or patches available at the time of reporting. The CODESYS Control runtime system is widely used in industrial automation for programmable logic controllers (PLCs), making this vulnerability relevant to operational technology (OT) environments. Exploitation could disrupt industrial processes, potentially causing operational downtime and safety risks. The lack of authentication requirement increases the risk surface, but the high attack complexity and absence of known exploits somewhat mitigate immediate threat levels. However, the vulnerability demands attention due to the critical nature of affected systems and potential for denial of service in industrial environments.

For European organizations, especially those in manufacturing, energy, utilities, and critical infrastructure sectors relying on CODESYS Control runtime systems, this vulnerability poses a risk of operational disruption through denial of service. Industrial control systems (ICS) and programmable logic controllers (PLCs) affected by this flaw could be forced offline or experience instability, impacting production lines, energy distribution, or safety systems. The unavailability of these systems can lead to significant financial losses, safety hazards, and regulatory compliance issues under frameworks like NIS2. Since the vulnerability requires no authentication, attackers can attempt exploitation from the network perimeter if systems are exposed, increasing risk. However, the high attack complexity and absence of known exploits reduce immediate widespread impact. Still, European organizations with remote access to PLCHandler services or insufficient network segmentation are more vulnerable. The impact is primarily on availability, with no direct confidentiality or integrity compromise reported. Disruptions in critical sectors could have cascading effects on supply chains and essential services across Europe.

1. Monitor vendor communications closely and apply patches or updates as soon as they become available to address CVE-2025-41739. 2. Implement strict network segmentation to isolate CODESYS Control runtime systems and PLCHandler services from general IT networks and the internet. 3. Restrict access to PLCHandler communication ports using firewalls and access control lists (ACLs) to allow only trusted management and control systems. 4. Deploy intrusion detection/prevention systems (IDS/IPS) with signatures or heuristics to detect anomalous socket communication patterns indicative of exploitation attempts. 5. Conduct regular security audits and vulnerability assessments on industrial control systems to identify exposed or outdated CODESYS versions. 6. Employ network-level authentication and VPNs for remote access to OT environments to reduce exposure to unauthenticated attacks. 7. Train OT security teams to recognize signs of denial of service or instability potentially caused by this vulnerability. 8. Develop and test incident response plans specific to ICS disruptions to minimize downtime and safety risks. 9. Consider implementing application-layer filtering or protocol whitelisting for CODESYS communication to block malformed packets. 10. Collaborate with CODESYS support and cybersecurity communities to share threat intelligence and mitigation best practices.