CVE-2025-43276: Apple macOS Logic Error Vulnerability

CVE-2025-43276 Overview

CVE-2025-43276 is a logic error in Apple macOS that prevents iCloud Private Relay from activating when more than one user is logged in simultaneously. Apple addressed the flaw with improved error handling in macOS Sequoia 15.6. The issue affects the availability of a privacy-enhancing network service rather than allowing code execution or data exposure. The vulnerability is tracked under [CWE-367] (Time-of-Check Time-of-Use race condition) in the National Vulnerability Database. Apple published guidance in its security advisory HT124149, and the flaw was disclosed publicly through the Full Disclosure mailing list.

Critical Impact
iCloud Private Relay silently fails to activate under multi-user sessions, exposing user traffic to conventional network observation that Private Relay is designed to prevent.

Affected Products

Apple macOS versions prior to Sequoia 15.6
Systems configured with multiple concurrently logged-in user accounts
iCloud Private Relay subscribers using affected macOS builds

Discovery Timeline

2025-07-30 - CVE-2025-43276 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-43276

Vulnerability Analysis

The defect resides in the macOS component that negotiates activation of iCloud Private Relay, Apple's dual-hop traffic anonymization service. When more than one user account is signed in on a single Mac, the activation routine encounters a logic error that prevents Private Relay from starting for the affected session. Traffic that users expect to traverse Apple's Private Relay ingress and egress nodes instead traverses the network in its normal form. The condition is categorized under [CWE-367], indicating a race between the check for session state and the use of that state during service activation. The impact is limited to availability of the privacy service, with no direct effect on confidentiality or integrity of stored data.

Root Cause

Apple's advisory attributes the flaw to insufficient error handling in the activation flow. The service does not correctly reconcile activation state across multiple simultaneous user sessions, so the negotiation fails silently rather than retrying or surfacing a clear error. Improved error handling in macOS Sequoia 15.6 corrects the state reconciliation logic.

Attack Vector

The vulnerability is exposed over the network from the perspective of the CVSS scoring, but exploitation does not involve an attacker sending malicious input. Instead, the failure mode is triggered by a benign local condition of multiple logged-in users. A network-positioned observer can then collect traffic metadata, IP addresses, and DNS queries that Private Relay would normally obscure. No user interaction or privileges are required to reach the vulnerable code path.

No verified exploit code is available. Refer to the Apple Support Advisory and the Full Disclosure Mailing List Post for technical details.

Detection Methods for CVE-2025-43276

Indicators of Compromise

Absence of Private Relay ingress hostnames such as mask.icloud.com or mask-h2.icloud.com in DNS logs originating from macOS endpoints that have Private Relay enabled
Direct outbound connections from macOS clients to origin servers that should otherwise be masked by Private Relay egress nodes
Private Relay status reporting as inactive in System Settings > Apple ID > iCloud while the subscription is confirmed active

Detection Strategies

Inventory macOS endpoints and identify systems running versions prior to Sequoia 15.6 that also have multiple active user sessions
Correlate endpoint telemetry showing multiple concurrent logged-in users with network telemetry showing non-Private-Relay egress
Query endpoint management platforms for the ProductVersion reported by sw_vers and flag hosts below 15.6

Monitoring Recommendations

Track Private Relay activation state on managed Macs using MDM configuration reporting
Alert on unexpected drops in traffic to Private Relay egress ranges from populations of macOS clients
Review authentication logs for patterns of multiple simultaneous interactive user sessions on shared workstations

How to Mitigate CVE-2025-43276

Immediate Actions Required

Update all affected Macs to macOS Sequoia 15.6 or later through Software Update or an MDM-managed update policy
Verify Private Relay activation status after the update on systems that regularly host multiple user sessions
Communicate to users that Private Relay may have been inactive on multi-user Macs prior to patching

Patch Information

Apple released the fix in macOS Sequoia 15.6. Deployment details and the full list of addressed issues are documented in the Apple Support Advisory. Administrators managing fleets should promote 15.6 in their update rings and enforce installation through configuration profiles.

Workarounds

Restrict Macs to a single active user session by signing out other accounts before enabling Private Relay
Use Fast User Switching sparingly on unpatched systems where Private Relay is required
Route sensitive traffic through an alternative privacy control such as a corporate VPN until macOS 15.6 is deployed

bash

# Verify the installed macOS version and Private Relay eligibility
sw_vers -productVersion
defaults read com.apple.networkserviceproxy | grep -i private