CVE-2026-21517 is a vulnerability identified in Microsoft Windows App for Mac version 11.0.0, categorized under CWE-59: Improper Link Resolution Before File Access ('Link Following'). This vulnerability arises when the application improperly resolves symbolic or hard links before accessing files, allowing an attacker with local authorized access to manipulate file paths to escalate privileges. Specifically, an attacker with limited privileges can create or modify links to trick the application into accessing or modifying files it should not, thereby gaining higher privileges on the system. The vulnerability requires local access and has a high attack complexity, meaning exploitation is non-trivial and likely requires specific conditions or knowledge. No user interaction is needed once local access is obtained. The CVSS v3.1 base score is 4.7 (medium severity), reflecting no impact on confidentiality or availability but a high impact on integrity. No known exploits are currently reported in the wild, and no patches have been released at the time of publication. The vulnerability is significant because Windows App for Mac is used to run Windows applications on macOS, often in enterprise environments, and privilege escalation can lead to broader system compromise. The flaw highlights the risks associated with improper handling of filesystem links, which can be exploited to bypass security controls and elevate privileges.

For European organizations, this vulnerability presents a moderate risk primarily to the integrity of systems running Windows App for Mac. Successful exploitation could allow an attacker with local access to escalate privileges, potentially leading to unauthorized changes in system files or execution contexts. This could facilitate further attacks such as installing persistent malware, modifying security configurations, or accessing sensitive data indirectly. Although confidentiality and availability are not directly impacted, the integrity compromise can undermine trust in system operations and lead to significant operational disruptions. Organizations with Mac-heavy environments that utilize Windows App for Mac for cross-platform compatibility are particularly at risk. The requirement for local access limits remote exploitation but insider threats or compromised endpoints could leverage this vulnerability. Given the lack of patches, organizations face a window of exposure until Microsoft releases a fix. The impact is heightened in sectors with strict compliance requirements or critical infrastructure relying on these systems.

To mitigate this vulnerability, European organizations should implement the following specific measures: 1) Restrict local user permissions rigorously to minimize the number of users with access to vulnerable versions of Windows App for Mac. 2) Monitor filesystem activities for unusual creation or modification of symbolic or hard links, especially in directories accessed by the application. 3) Employ endpoint detection and response (EDR) tools capable of detecting suspicious link-following behaviors or privilege escalation attempts. 4) Isolate systems running Windows App for Mac to limit lateral movement if exploitation occurs. 5) Educate users and administrators about the risks of local privilege escalation and the importance of reporting suspicious activity. 6) Maintain an inventory of systems running the affected version and prepare for rapid deployment of patches once Microsoft releases them. 7) Consider temporary workarounds such as disabling or limiting the use of Windows App for Mac in high-risk environments until a patch is available. 8) Regularly review and update local security policies to prevent unauthorized link creation or manipulation.