CVE-2025-43526: 
Apple Safari vulnerability analysis and mitigation

CVE-2025-43526 is a URL validation vulnerability in Apple Safari and macOS Tahoe that allows web content opened via a file URL to bypass Web API restrictions on systems with Lockdown Mode enabled. It was discovered by Andreas Jaegersberger and Ro Achterberg of Nosebeard Labs and disclosed on December 12, 2025, when Apple released patches. The vulnerability affects Safari and macOS versions prior to 26.2. It carries a CVSS v3.1 base score of 9.8 (Critical), as assessed by CISA-ADP (Apple macOS Advisory, Apple Safari Advisory).

The root cause is insufficient URL validation (CWE-601: URL Redirection to Untrusted Site / Open Redirect) in Safari's handling of file URLs. When web content is loaded via a file:// URL on a Mac with Lockdown Mode enabled, the browser fails to properly enforce the Web API restrictions that Lockdown Mode is designed to impose, allowing the content to access APIs that should be blocked. Lockdown Mode is Apple's enhanced security feature intended to protect high-risk users (e.g., journalists, activists) from sophisticated targeted attacks, making this bypass particularly significant. No public proof-of-concept or detailed technical write-up has been published as of the time of this report (Apple macOS Advisory, Apple Safari Advisory).

Successful exploitation allows web content delivered via a file URL to access restricted Web APIs on Macs running Lockdown Mode, directly undermining the enhanced security protections that mode provides. This could result in high confidentiality, integrity, and availability impacts, as the CISA-ADP CVSS assessment reflects. The vulnerability is particularly concerning for high-risk individuals who rely on Lockdown Mode as a defense against sophisticated, targeted attacks, as it nullifies a key layer of their security posture (Apple macOS Advisory, Apple Safari Advisory).

1. Identify target: Confirm the target is using a Mac with Lockdown Mode enabled and running Safari or macOS prior to version 26.2.
2. Craft malicious file: Create a web page or HTML file that invokes restricted Web APIs (e.g., camera, microphone, geolocation, or other APIs blocked by Lockdown Mode).
3. Deliver via file URL: Deliver the malicious HTML file to the target system via phishing, a shared drive, email attachment, or other means, such that it is opened using a file:// URL in Safari.
4. Trigger bypass: When the target opens the file in Safari, the insufficient URL validation fails to enforce Lockdown Mode's Web API restrictions, allowing the malicious content to invoke the restricted APIs.
5. Achieve objective: The attacker's web content gains access to restricted Web APIs, potentially enabling data exfiltration, surveillance, or further exploitation of the compromised security context (Apple macOS Advisory, Apple Safari Advisory).

• File System: Unexpected or unfamiliar HTML/web files in user-accessible directories (Downloads, Desktop, temp folders) that reference sensitive Web APIs.
• Logs: Safari or WebKit process logs showing Web API access (e.g., camera, microphone, geolocation) initiated from file:// URL contexts on systems with Lockdown Mode enabled.
• Process: Unusual Safari subprocess activity or unexpected permission prompts for restricted APIs when no browser-based web content (non-file URL) is open.
• Network: Outbound network connections from Safari initiated shortly after opening a local HTML file, potentially indicating data exfiltration via restricted API access.

Apple has released patches addressing this vulnerability in Safari 26.2 and macOS Tahoe 26.2, both released on December 12, 2025. Users should update to these versions or later immediately, with priority given to systems that have Lockdown Mode enabled, as those are the directly affected configurations. As a temporary workaround for systems that cannot be immediately patched, administrators may consider disabling file URL access in Safari or avoiding opening untrusted local HTML files until the patch is applied (Apple macOS Advisory, Apple Safari Advisory).

The CIS (Center for Internet Security) issued an advisory noting multiple vulnerabilities in Apple products that could allow for arbitrary code execution, which included CVE-2025-43526 in its scope. The vulnerability was also noted by SANS Internet Storm Center shortly after disclosure. Community attention has been moderate, with the vulnerability appearing in standard security aggregation feeds and vulnerability databases, but no major independent researcher commentary or media coverage specific to this CVE has been identified beyond routine patch reporting.