CVE-2025-21371 is a heap-based buffer overflow vulnerability classified under CWE-122, affecting the Windows Telephony Service 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 requests to the Telephony Service. The vulnerability does not require any privileges (PR:N) but does require user interaction (UI:R), such as the user opening a malicious file or link that triggers the exploit. The CVSS v3.1 base score is 8.8, indicating a high severity level, with impacts on confidentiality, integrity, and availability (all high). The attack vector is network-based (AV:N), and the attack complexity is low (AC:L), making exploitation feasible in many environments. The scope is unchanged (S:U), meaning the exploit affects only the vulnerable component without escalating privileges beyond it. Currently, there are no known exploits in the wild, and no official patches have been released, increasing the urgency for organizations to implement mitigations. The Telephony Service is a core Windows component that handles telephony-related functions, and exploitation could allow attackers to run arbitrary code remotely, potentially leading to full system compromise or lateral movement within networks.

For European organizations, the impact of CVE-2025-21371 is significant due to the potential for remote code execution leading to complete system compromise. Confidentiality could be breached by attackers accessing sensitive data, integrity compromised by unauthorized modification of system files or configurations, and availability disrupted by system crashes or ransomware deployment. Organizations relying on Windows 10 Version 1809, particularly in sectors such as telecommunications, critical infrastructure, healthcare, and government, face elevated risks. The vulnerability could be exploited to establish persistent footholds, conduct espionage, or disrupt services. Given the lack of patches and the requirement for user interaction, phishing or social engineering campaigns could be used to trigger exploitation. The widespread use of Windows 10 1809 in some European enterprises and public sector entities increases the attack surface. The absence of known exploits currently provides a window for proactive defense, but the high severity score demands urgent attention to prevent future exploitation.

1. Upgrade affected systems from Windows 10 Version 1809 to a supported and patched Windows version immediately to eliminate the vulnerability. 2. If upgrading is not immediately feasible, disable the Windows Telephony Service (TapiSrv) on affected systems to prevent exploitation, especially if telephony features are not required. 3. Implement network segmentation and firewall rules to restrict inbound traffic to the Telephony Service ports, limiting exposure to untrusted networks. 4. Educate users to recognize and avoid phishing attempts or suspicious links that could trigger the required user interaction for exploitation. 5. Monitor network and endpoint logs for unusual Telephony Service activity or crashes that could indicate exploitation attempts. 6. Prepare incident response plans specifically addressing remote code execution scenarios involving Windows Telephony Service. 7. Stay alert for official patches or security advisories from Microsoft and apply them promptly once available. 8. Employ endpoint detection and response (EDR) tools capable of detecting anomalous behavior related to heap overflows or Telephony Service misuse. 9. Conduct vulnerability scans and asset inventories to identify all systems running Windows 10 Version 1809 to prioritize remediation efforts.