CVE-2025-21406 is a use-after-free vulnerability (CWE-416) identified in the Windows Telephony Service component of Microsoft Windows 10 Version 1809 (build 10.0.17763.0). This vulnerability arises when the service improperly manages memory, leading to a condition where previously freed memory is accessed, potentially allowing an attacker to execute arbitrary code remotely. The vulnerability is exploitable over the network (AV:N) without requiring any privileges (PR:N), but it does require user interaction (UI:R), such as convincing a user to accept a call or interact with a telephony-related prompt. The scope is unchanged (S:U), meaning the impact is limited to the vulnerable system. The CVSS v3.1 base score is 8.8, indicating high severity, with impacts rated high on confidentiality, integrity, and availability. Although no public exploits are known at this time, the vulnerability's nature and ease of exploitation make it a significant risk. The Windows Telephony Service is a core component used for managing telephony functions, and exploitation could lead to full system compromise, including remote code execution and potential lateral movement within networks. The vulnerability was reserved in December 2024 and published in February 2025, but no patches have been released yet, increasing the urgency for mitigation steps.

European organizations running Windows 10 Version 1809 are at risk of remote code execution attacks that could lead to complete system compromise. This includes loss of confidentiality through data theft, integrity breaches via unauthorized code execution, and availability disruptions from system crashes or denial of service. Critical sectors such as telecommunications, healthcare, finance, and government agencies are particularly vulnerable due to their reliance on telephony services and legacy Windows systems. The requirement for user interaction slightly reduces the risk but does not eliminate it, especially in environments where social engineering or phishing attacks are common. The lack of available patches increases exposure time, potentially allowing attackers to develop exploits. Compromise of telephony services could also disrupt communication infrastructure, impacting business continuity and emergency services. The vulnerability could facilitate lateral movement within networks, amplifying the impact on enterprise environments.

1. Upgrade affected systems to a supported and fully patched version of Windows 10 or later, as Windows 10 Version 1809 is out of mainstream support and unlikely to receive patches promptly. 2. Implement network-level controls to restrict inbound traffic to telephony services, using firewalls and intrusion prevention systems to block unauthorized access. 3. Employ application whitelisting and endpoint detection and response (EDR) solutions to detect and prevent exploitation attempts targeting the Telephony Service. 4. Educate users about the risks of interacting with unsolicited telephony prompts or calls, reducing the likelihood of successful user interaction exploitation. 5. Monitor network and system logs for unusual telephony service activity or crashes indicative of exploitation attempts. 6. Segment critical telephony infrastructure from general user networks to limit lateral movement if compromise occurs. 7. Stay informed on Microsoft advisories for any forthcoming patches or mitigations and apply them immediately upon release.