CVE-2025-21241 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 any privileges (PR:N) but does require user interaction (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 full confidentiality, integrity, and availability impact, potentially allowing attackers to take complete control of the affected system. The CVSS v3.1 base score is 8.8, reflecting the critical nature of the vulnerability due to its network attack vector (AV:N), low attack complexity (AC:L), and the fact that it does not require privileges. Although no known exploits are currently reported in the wild, the vulnerability is publicly disclosed and poses a significant risk to unpatched systems. The lack of available patches at the time of publication increases the urgency for mitigation. The vulnerability affects a legacy Windows 10 version (1809), which may still be in use in certain environments, especially in industrial or legacy systems. The requirement for user interaction suggests that exploitation may involve tricking users into performing an action, such as opening a malicious link or file that triggers the vulnerable Telephony Service functionality.

For European organizations, this vulnerability presents a serious risk, particularly for those still operating legacy Windows 10 Version 1809 systems. The potential for remote code execution without privileges means attackers could compromise systems remotely, leading to data breaches, disruption of services, or lateral movement within networks. Confidentiality, integrity, and availability of critical systems could be severely impacted, especially in sectors like finance, healthcare, manufacturing, and government where legacy systems may persist. The vulnerability could be leveraged to deploy ransomware, steal sensitive data, or establish persistent footholds. Given the lack of patches and known exploits, organizations face a window of exposure that could be exploited by advanced threat actors. The requirement for user interaction may limit mass exploitation but targeted phishing or social engineering campaigns could effectively trigger the vulnerability. European organizations with strict regulatory requirements (e.g., GDPR) must consider the compliance implications of potential data breaches resulting from exploitation.

1. Immediate mitigation should focus on reducing the attack surface by disabling or restricting the Windows Telephony Service if it is not essential for business operations. 2. Implement network-level controls such as firewall rules to block inbound traffic to ports and protocols used by the Telephony Service, limiting exposure to untrusted networks. 3. Employ endpoint detection and response (EDR) solutions to monitor for suspicious activity related to the Telephony Service and potential exploitation attempts. 4. Conduct user awareness training emphasizing the risks of interacting with unsolicited links or files, reducing the likelihood of successful user interaction exploitation. 5. Maintain strict network segmentation to isolate legacy systems running Windows 10 Version 1809 from critical infrastructure and sensitive data environments. 6. Monitor vendor communications closely for the release of official patches or workarounds and plan for prompt deployment once available. 7. Where feasible, prioritize upgrading or migrating systems off Windows 10 Version 1809 to supported versions with ongoing security updates to eliminate exposure to this and similar vulnerabilities.