CVE-2025-21413 is a high-severity heap-based buffer overflow vulnerability (CWE-122) found in the Windows Telephony Service of Microsoft Windows 10 Version 1809 (build 10.0.17763.0). This vulnerability allows remote code execution (RCE) without requiring any privileges or authentication, although it requires user interaction. The flaw exists due to improper handling of memory buffers in the Telephony Service, which can be exploited by an attacker sending specially crafted network packets or requests to the vulnerable service. Successful exploitation could allow an attacker to execute arbitrary code in the context of the system, leading to full compromise of the affected machine. The CVSS v3.1 base score is 8.8, reflecting high impact on confidentiality, integrity, and availability, with network attack vector and low attack complexity. No known exploits are currently reported in the wild, and no patches have been linked yet, indicating that organizations should prioritize mitigation and monitoring. This vulnerability affects an older Windows 10 version (1809), which is still in use in some environments, particularly where legacy systems or software compatibility is critical.

For European organizations, this vulnerability poses a significant risk, especially in sectors relying on legacy Windows 10 1809 systems, such as industrial control, healthcare, and government agencies. Exploitation could lead to unauthorized system control, data breaches, disruption of telephony-related services, and lateral movement within networks. Given the network-based attack vector and lack of required privileges, attackers could target exposed systems remotely, increasing the risk of widespread compromise. The potential for full system compromise threatens confidentiality of sensitive data, integrity of critical services, and availability of telephony and communication infrastructure. Organizations with remote or hybrid workforces using VPNs or exposed telephony services are particularly vulnerable. The absence of known exploits currently provides a window for proactive defense, but the high severity score demands urgent attention to prevent future attacks.

1. Immediate upgrade or patching: Although no patch links are provided, organizations should monitor Microsoft security advisories closely and apply any forthcoming patches promptly. 2. Upgrade affected systems: Where feasible, migrate from Windows 10 Version 1809 to a supported, updated Windows version to eliminate exposure. 3. Network segmentation: Isolate systems running Windows 10 1809, especially those exposing telephony services, from untrusted networks and limit access to trusted users only. 4. Firewall and filtering: Block or restrict inbound traffic to the Telephony Service ports at network perimeter devices to reduce attack surface. 5. User awareness: Since exploitation requires user interaction, educate users to avoid suspicious links or network requests related to telephony services. 6. Monitoring and detection: Deploy intrusion detection/prevention systems (IDS/IPS) with signatures for anomalous Telephony Service activity and monitor logs for unusual behavior. 7. Application whitelisting and endpoint protection: Use advanced endpoint security solutions to detect and block exploitation attempts and unauthorized code execution. 8. Incident response readiness: Prepare response plans for potential exploitation scenarios, including system isolation and forensic analysis.