CVE-2025-43276 is a medium-severity vulnerability affecting Apple's macOS operating system, specifically related to the iCloud Private Relay feature. The issue stems from a logic error in the handling of iCloud Private Relay activation when multiple users are logged into the same macOS system simultaneously. iCloud Private Relay is designed to enhance user privacy by routing internet traffic through multiple relays to obscure user IP addresses and browsing activity. However, due to this logic flaw, the feature may fail to activate under multi-user login conditions, potentially leaving user traffic unprotected and exposed to network-level observation or tracking. The vulnerability is classified under CWE-367, which relates to time-of-check/time-of-use (TOCTOU) race conditions or improper synchronization leading to inconsistent system states. The flaw was addressed by Apple through improved error handling in macOS Sequoia 15.6, which ensures that iCloud Private Relay activates correctly even when more than one user is logged in concurrently. The CVSS v3.1 base score is 5.3, indicating a medium severity level. The vector string (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L) shows that the vulnerability is remotely exploitable without privileges or user interaction, but it only impacts availability (iCloud Private Relay not activating), without compromising confidentiality or integrity. No known exploits are currently reported in the wild, and affected versions are unspecified but presumably include macOS versions prior to 15.6.

For European organizations, the primary impact of this vulnerability lies in the potential degradation of privacy protections for users of macOS devices, particularly in environments where multiple users share the same machine (e.g., shared workstations, labs, or kiosks). Since iCloud Private Relay is intended to protect user internet traffic from network surveillance and tracking, failure to activate this feature could expose user browsing data to interception or profiling by network operators or malicious actors. This could be especially concerning for organizations handling sensitive or regulated data, such as those in finance, healthcare, or government sectors, where privacy compliance is critical. However, since the vulnerability does not lead to direct data compromise or system control, the impact is limited to reduced availability of a privacy feature rather than a direct breach. The lack of required privileges or user interaction for exploitation means that any network attacker could potentially cause the feature to fail, but the scope is limited to the availability of the privacy service rather than broader system compromise.

European organizations should ensure that all macOS devices are updated promptly to macOS Sequoia 15.6 or later, where the fix for this vulnerability is implemented. IT administrators should audit multi-user login scenarios on shared macOS systems and verify that iCloud Private Relay activates correctly for all users. Where possible, limit the use of shared user sessions on macOS devices to reduce the risk of this issue impacting privacy protections. Additionally, network monitoring should be employed to detect unusual traffic patterns that might indicate unprotected browsing sessions. Organizations should educate users about the importance of verifying that privacy features like iCloud Private Relay are active, especially in shared environments. For environments requiring strict privacy guarantees, consider alternative or supplementary privacy tools that do not rely on iCloud Private Relay. Finally, maintain vigilance for any future advisories or patches from Apple related to this vulnerability or similar issues.