CVE-2025-21306 is a high-severity heap-based buffer overflow vulnerability (CWE-122) found in the Windows Telephony Service component of Microsoft Windows 10 Version 1809 (build 10.0.17763.0). This vulnerability allows remote attackers to execute arbitrary code on affected systems without requiring prior authentication (AV:N/PR:N), although user interaction is required (UI:R). The flaw arises from improper handling of memory buffers in the Telephony Service, which can be exploited by sending specially crafted network packets or requests to the vulnerable service. Successful exploitation can lead to remote code execution with high impact on confidentiality, integrity, and availability, potentially allowing attackers to take full control of the system. The vulnerability has a CVSS 3.1 score of 8.8, reflecting its critical impact and ease of exploitation over the network with low attack complexity. Although no known exploits are currently reported in the wild, the lack of available patches at the time of publication increases the risk for unpatched systems. The vulnerability affects a legacy version of Windows 10 (1809), which is still in use in some environments, particularly in enterprise or industrial settings where upgrading is slower. The requirement for user interaction suggests that exploitation may involve social engineering or tricking users into triggering the vulnerability, possibly through malicious telephony-related communications or network interactions.

For European organizations, this vulnerability poses a significant risk, especially for those still operating Windows 10 Version 1809 in production environments. The ability for remote attackers to execute arbitrary code can lead to data breaches, ransomware deployment, espionage, or disruption of critical services. Organizations in sectors such as telecommunications, manufacturing, healthcare, and government, which may rely on legacy Windows systems for operational technology or telephony integration, are particularly vulnerable. The compromise of such systems could result in loss of sensitive personal data protected under GDPR, operational downtime, and reputational damage. Given the remote exploitability and high impact on system integrity and availability, this vulnerability could be leveraged in targeted attacks or widespread campaigns if exploit code becomes publicly available. The requirement for user interaction somewhat limits mass exploitation but does not eliminate the threat, especially in environments where users may be less security-aware or where telephony services are heavily integrated with business processes.

European organizations should prioritize upgrading or patching affected Windows 10 Version 1809 systems as soon as Microsoft releases an official security update. In the interim, organizations should implement network-level protections such as blocking or filtering traffic to and from the Telephony Service ports to reduce exposure. Employing application whitelisting and endpoint detection and response (EDR) solutions can help detect and prevent exploitation attempts. User awareness training focused on recognizing suspicious telephony communications or social engineering tactics can mitigate the risk posed by the required user interaction. Additionally, organizations should audit their asset inventories to identify legacy Windows 10 1809 systems and consider accelerated migration plans to supported Windows versions. Network segmentation to isolate vulnerable systems and strict access controls can further reduce the attack surface. Monitoring for unusual telephony service activity and anomalous network traffic patterns is also recommended to detect potential exploitation attempts early.