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 communication packets that trigger an out-of-bounds read (CWE-125). This memory access flaw can cause the affected service to crash, resulting in a denial of service (DoS) condition. The vulnerability does not allow for data leakage or code execution but impacts system availability. The CVSS v3.1 base score is 5.9, reflecting medium severity, with attack vector being network (AV:N), high attack complexity (AC:H), no privileges required (PR:N), no user interaction (UI:N), and scope unchanged (S:U). No known public exploits exist yet, and no patches have been released at the time of publication. The vulnerability affects industrial control systems using CODESYS, a widely deployed runtime for programmable logic controllers (PLCs) in automation environments. The flaw's exploitation requires precise timing to beat the race condition, which increases the attack complexity. However, the lack of authentication requirement and network accessibility of the communication server increase the risk profile. The vulnerability was reserved in April 2025 and published in December 2025 by CERTVDE.

For European organizations, particularly those operating critical infrastructure and industrial automation systems, this vulnerability poses a risk of service disruption. CODESYS is widely used in manufacturing, energy, and utilities sectors across Europe. An attacker exploiting this flaw could cause denial of service on PLCs or control systems, potentially halting production lines or critical processes. While the vulnerability does not compromise data confidentiality or integrity, the availability impact can lead to operational downtime, financial losses, and safety risks in industrial environments. The medium CVSS score reflects moderate risk, but the potential for disruption in critical sectors elevates its importance. Organizations with exposed or poorly segmented control networks are especially vulnerable. The lack of known exploits reduces immediate risk, but the presence of a publicly known vulnerability increases the likelihood of future exploit development. The impact is more severe in environments where redundancy and failover mechanisms are limited or where PLC downtime directly affects safety or regulatory compliance.

To mitigate CVE-2025-41739, European organizations should implement the following specific measures: 1) Immediately restrict network access to the CODESYS PLCHandler communication ports by applying strict firewall rules and network segmentation to isolate PLCs from untrusted networks. 2) Monitor network traffic for unusual or malformed socket communication patterns targeting the PLCHandler service, using intrusion detection systems (IDS) or industrial protocol analyzers. 3) Employ rate limiting or connection throttling on the communication servers to reduce the likelihood of successful race condition exploitation. 4) Coordinate with CODESYS vendors and subscribe to security advisories to obtain and apply patches promptly once released. 5) Conduct regular security assessments and penetration testing focused on industrial control system components to identify exposure. 6) Implement redundancy and failover mechanisms in critical control systems to minimize operational impact in case of service disruption. 7) Train operational technology (OT) staff to recognize signs of exploitation attempts and respond accordingly. These targeted actions go beyond generic advice by focusing on network-level controls, monitoring, and operational resilience specific to the nature of this vulnerability.