CVE-2025-25567 is a critical buffer overflow vulnerability identified in SoftEther VPN version 5.02.5187, specifically within the Internat.c source file in the UniToStrForSingleChars function. Buffer overflow vulnerabilities occur when a program writes more data to a buffer than it can hold, potentially overwriting adjacent memory and leading to arbitrary code execution, crashes, or privilege escalation. This vulnerability has been assigned a CVSS v3.1 score of 9.8, indicating a critical severity level. The vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) reveals that the vulnerability is remotely exploitable over the network without any privileges or user interaction, and it impacts confidentiality, integrity, and availability to a high degree. However, the supplier disputes the severity, claiming that the behavior only allows a local user to attack themselves through the UI, which would limit the exploitability and impact. Despite this, the official CVSS rating suggests a remote attack vector with no required privileges or user interaction, implying a serious risk if the vulnerability is exploitable remotely. The vulnerability is categorized under CWE-120 (Classic Buffer Overflow), a well-known and dangerous class of software bugs. No patches or known exploits in the wild have been reported at the time of publication (March 12, 2025). The lack of patch links indicates that a fix may not yet be available, increasing the urgency for organizations to monitor updates and consider mitigation strategies. SoftEther VPN is an open-source, multi-protocol VPN solution widely used for secure remote access, making this vulnerability particularly concerning for organizations relying on it for secure communications.

For European organizations, the impact of this vulnerability could be severe. SoftEther VPN is used by enterprises, educational institutions, and government agencies to provide secure remote access to internal networks. A critical buffer overflow vulnerability exploitable remotely without authentication could allow attackers to execute arbitrary code, potentially leading to full system compromise, data breaches, and disruption of VPN services. This would jeopardize the confidentiality of sensitive communications, integrity of data transmitted over the VPN, and availability of remote access infrastructure. Given the increasing reliance on VPNs for remote work and secure connectivity in Europe, exploitation could disrupt business operations, expose personal data protected under GDPR, and damage organizational reputation. The absence of known exploits in the wild currently reduces immediate risk, but the high CVSS score and lack of patches mean organizations must act proactively to prevent exploitation. Additionally, if the supplier's claim that exploitation is limited to local users attacking themselves is accurate, the impact might be less severe; however, the official assessment suggests a broader threat that cannot be ignored.

European organizations should take the following specific mitigation steps: 1) Immediately inventory all SoftEther VPN deployments to identify affected versions, particularly version 5.02.5187. 2) Monitor official SoftEther channels and trusted vulnerability databases for patches or security advisories addressing CVE-2025-25567 and apply updates promptly once available. 3) Until patches are released, consider disabling or restricting access to vulnerable SoftEther VPN servers, especially those exposed to untrusted networks. 4) Implement network-level controls such as firewall rules to limit VPN server access to trusted IP ranges and use intrusion detection/prevention systems to monitor for anomalous activity targeting VPN services. 5) Enforce strong authentication and multi-factor authentication on VPN access to reduce risk from potential exploitation vectors. 6) Conduct internal security assessments to verify whether local user access can be exploited and educate users about the risks of running untrusted code or commands locally. 7) Maintain up-to-date backups and incident response plans to quickly recover from potential compromise. These steps go beyond generic advice by emphasizing immediate inventory, access restriction, and monitoring tailored to the VPN infrastructure and the specific nature of this vulnerability.