CVE-2025-21407 is a heap-based buffer overflow vulnerability classified under CWE-122, affecting the Windows Telephony Service component in Microsoft Windows 10 Version 1809 (build 10.0.17763.0). The flaw allows remote attackers to execute arbitrary code on vulnerable systems by sending specially crafted network packets to the Telephony Service. The vulnerability does not require the attacker to have any privileges (PR:N), but it does require user interaction (UI:R), such as the user initiating or accepting a telephony-related operation. The vulnerability impacts confidentiality, integrity, and availability (C:H/I:H/A:H), meaning an attacker could fully compromise the affected system. The CVSS v3.1 base score is 8.8, indicating a high severity level. The attack vector is network-based (AV:N), and the attack complexity is low (AC:L), making exploitation feasible in many environments. Although no known exploits are currently reported in the wild, the potential for remote code execution makes this a critical risk for unpatched systems. The vulnerability is particularly relevant for organizations still running Windows 10 Version 1809, which is an older, out-of-mainstream-support version, often found in legacy or specialized environments. The lack of available patches at the time of reporting means organizations must rely on interim mitigations until official updates are released.

For European organizations, this vulnerability poses a significant risk, especially for those operating legacy Windows 10 Version 1809 systems in critical infrastructure sectors such as telecommunications, government, healthcare, and finance. Exploitation could lead to full system compromise, data breaches, disruption of telephony services, and lateral movement within networks. The requirement for user interaction may limit automated exploitation but does not eliminate risk, particularly in environments where users frequently engage with telephony features or remote communication tools. The absence of known exploits in the wild currently reduces immediate risk but also means organizations may be unprepared if exploit code emerges. Legacy systems in Europe, often retained due to compatibility or regulatory reasons, increase the attack surface. The vulnerability could also impact managed service providers and enterprises with mixed OS environments, potentially affecting supply chains and service continuity.

Organizations should prioritize identifying and inventorying all Windows 10 Version 1809 systems within their environment. Until patches are available, consider disabling the Windows Telephony Service if it is not essential to business operations, as this will mitigate the attack vector. Employ network segmentation and firewall rules to restrict access to the Telephony Service ports from untrusted networks. Enhance user awareness training to reduce risky interactions that could trigger exploitation. Monitor network traffic for anomalous activity targeting telephony services. Once Microsoft releases patches, apply them promptly following thorough testing to avoid operational disruptions. Additionally, consider upgrading legacy systems to supported Windows versions to reduce exposure to similar vulnerabilities. Implement endpoint detection and response (EDR) tools capable of detecting exploitation attempts related to heap-based buffer overflows and remote code execution.