CVE-2025-21291 is a high-severity vulnerability identified in Microsoft Windows 10 Version 1809 (build 10.0.17763.0), specifically within the Windows DirectShow component. The vulnerability is classified as a double free (CWE-415), which occurs when the software attempts to free the same memory location twice. This can lead to memory corruption, potentially allowing an attacker to execute arbitrary code remotely. The vulnerability is triggered through the DirectShow interface, which is used for multimedia streaming and processing. Exploitation requires no privileges (PR:N) but does require user interaction (UI:R), such as opening a specially crafted media file or visiting a malicious website that leverages DirectShow. The attack vector is network-based (AV:N), meaning the attacker can exploit the vulnerability remotely without local access. The CVSS v3.1 base score is 8.8, indicating a high severity with impacts on confidentiality, integrity, and availability (all rated high). The scope is unchanged (S:U), meaning the vulnerability affects only the vulnerable component without impacting other system components. No known exploits are currently reported in the wild, and no official patches have been linked yet. However, given the nature of the vulnerability and its potential for remote code execution, it represents a significant security risk, especially for systems still running the older Windows 10 1809 version, which is past mainstream support and may not receive timely updates. Attackers could leverage this flaw to gain control over affected systems, execute arbitrary code, install malware, or disrupt system operations.

For European organizations, this vulnerability poses a substantial risk, particularly for those still operating legacy Windows 10 Version 1809 systems. The ability to execute remote code with no privileges required means attackers can compromise systems by tricking users into interacting with malicious content, potentially leading to data breaches, ransomware infections, or espionage. Confidentiality is at risk as attackers could access sensitive data; integrity is compromised through unauthorized code execution; and availability could be disrupted by system crashes or malware payloads. Critical infrastructure, government agencies, healthcare, financial institutions, and enterprises with legacy systems are especially vulnerable. The lack of a patch and the requirement for user interaction mean phishing or social engineering campaigns could be effective attack vectors. The threat also increases the risk of lateral movement within networks if initial footholds are established, amplifying potential damage.

European organizations should prioritize identifying and inventorying all systems running Windows 10 Version 1809. Immediate mitigation steps include: 1) Restricting or disabling DirectShow usage where feasible, especially in environments where multimedia processing is not essential. 2) Implementing strict network-level protections such as web filtering and email gateway defenses to block access to malicious media files or URLs. 3) Enhancing user awareness training to reduce the likelihood of users interacting with malicious content that could trigger the vulnerability. 4) Employing application whitelisting and endpoint detection and response (EDR) solutions to detect and block exploitation attempts. 5) Applying any available security updates or workarounds from Microsoft as soon as they are released. 6) Considering upgrading affected systems to a supported Windows version that receives security patches. 7) Monitoring network and system logs for unusual activity indicative of exploitation attempts. These targeted measures go beyond generic advice by focusing on the specific attack vector and affected component.