CVE-2024-20671 is a medium-severity vulnerability affecting the Microsoft Windows Defender Antimalware Platform, specifically version 4.0.0.0. The vulnerability is categorized under CWE-276, which relates to Incorrect Default Permissions. This means that certain files, directories, or resources within the Windows Defender Antimalware Platform are configured with permissions that are too permissive by default, potentially allowing unauthorized users or processes with limited privileges to access or modify these resources. The vulnerability is described as a security feature bypass, indicating that an attacker with low privileges (PR:L) on the local system could exploit this flaw to interfere with or disable components of Windows Defender, thereby bypassing its protective mechanisms. The CVSS v3.1 base score is 5.5 (medium severity), with the vector indicating local attack vector (AV:L), low attack complexity (AC:L), requiring low privileges (PR:L), no user interaction (UI:N), unchanged scope (S:U), no impact on confidentiality or integrity (C:N/I:N), but high impact on availability (A:H). This suggests that exploitation does not compromise data confidentiality or integrity but can significantly impact system availability by disabling or impairing antimalware functions. No known exploits are currently reported in the wild, and no patches or mitigation links are provided yet. The vulnerability was publicly disclosed on March 12, 2024, and has been enriched by CISA, indicating recognition by US cybersecurity authorities. The incorrect default permissions could allow an attacker to modify or disable Windows Defender components, reducing endpoint protection and increasing the risk of malware persistence or further compromise. Since Windows Defender is a core security component on Windows systems, this vulnerability could be leveraged in multi-stage attacks where local access is already obtained, to maintain persistence or evade detection.

For European organizations, this vulnerability poses a risk primarily to endpoint security and system availability. If exploited, attackers with limited local privileges could disable or impair Windows Defender, leaving systems vulnerable to malware infections, ransomware, or other malicious activities without detection or prevention. This could lead to widespread disruption of business operations, data loss, or compliance violations, especially for organizations relying heavily on Windows Defender as their primary antimalware solution. The impact is heightened in sectors with strict regulatory requirements for cybersecurity, such as finance, healthcare, and critical infrastructure. Additionally, since the vulnerability requires local access, it may be exploited in scenarios involving insider threats, compromised user accounts, or lateral movement within networks. The lack of confidentiality or integrity impact reduces the risk of direct data breaches from this vulnerability alone, but the availability impact and potential for security feature bypass increase overall organizational risk. European organizations with large Windows environments and reliance on Microsoft Defender will need to prioritize detection and mitigation to prevent exploitation.

Given the absence of an official patch or update at the time of disclosure, European organizations should implement the following specific mitigations: 1) Review and harden permissions on Windows Defender related files and directories manually, ensuring that only SYSTEM and trusted administrators have write access, to prevent unauthorized modification. 2) Employ endpoint detection and response (EDR) tools to monitor for suspicious local privilege escalation or attempts to modify Defender components. 3) Enforce strict local user privilege management to minimize the number of users with local write access and limit the use of privileged accounts. 4) Use application whitelisting and integrity monitoring to detect unauthorized changes to Defender binaries or configuration files. 5) Implement network segmentation and strong access controls to reduce the risk of lateral movement that could enable local access exploitation. 6) Stay alert for Microsoft security advisories and deploy patches immediately upon release. 7) Conduct regular security audits and penetration testing focused on local privilege escalation and Defender bypass techniques. These targeted actions go beyond generic advice by focusing on permission hardening, monitoring, and privilege management specific to the Defender platform.