CVE-2025-21252 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 privileges (PR:N) but does require user interaction (UI:R), such as clicking a malicious link or opening a crafted file. 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 that overflow a heap buffer. Successful exploitation can lead to full compromise of confidentiality, integrity, and availability (C:H/I:H/A:H) of the system, enabling attackers to run code remotely, install malware, or disrupt system operations. The CVSS v3.1 base score is 8.8, reflecting the critical impact and relatively low attack complexity (AC:L). No known exploits are currently reported in the wild, and no official patches have been linked yet, indicating that mitigation may rely on workarounds or system hardening until a patch is released. The vulnerability affects a legacy Windows 10 version (1809), which is still in use in some environments but is no longer the latest supported version, potentially limiting exposure but also complicating patch management for organizations that have not upgraded. Given the Telephony Service's role in managing telephony and communication features, exploitation could also impact business communications and related services.

For European organizations, this vulnerability poses a significant risk, especially for those still operating legacy Windows 10 Version 1809 systems in critical infrastructure, government, healthcare, or telecommunications sectors. Successful exploitation could lead to remote code execution, allowing attackers to gain persistent access, exfiltrate sensitive data, disrupt services, or deploy ransomware. The high impact on confidentiality, integrity, and availability could cause operational downtime, data breaches, and regulatory non-compliance under GDPR. Organizations with remote or hybrid workforces may be particularly vulnerable due to increased exposure to network-based attacks and user interaction requirements. The lack of a patch increases the urgency for mitigation to prevent potential targeted attacks or opportunistic exploitation once exploit code becomes available. Additionally, the Telephony Service's involvement suggests potential impact on unified communication systems, which are critical for business continuity.

1. Immediate mitigation should include disabling or restricting the Windows Telephony Service where feasible, especially on systems that do not require telephony features. 2. Implement network-level protections such as firewall rules to block or limit access to ports and protocols used by the Telephony Service from untrusted networks. 3. Employ application whitelisting and endpoint detection and response (EDR) solutions to detect and prevent exploitation attempts. 4. Enforce strict user awareness training to reduce the risk of user interaction-based exploitation, emphasizing caution with unsolicited links or attachments. 5. Prioritize upgrading affected systems to a supported, patched version of Windows 10 or later to eliminate exposure. 6. Monitor security advisories from Microsoft for official patches and apply them promptly once available. 7. Conduct vulnerability scanning and penetration testing focused on telephony and communication services to identify potential exposure. 8. Use network segmentation to isolate legacy systems and limit lateral movement in case of compromise.