CVE-2025-43526: Apple Safari SSRF Vulnerability

CVE-2025-43526 Overview

CVE-2025-43526 is a URL validation bypass vulnerability affecting Apple Safari and macOS that allows web content opened via a file URL to access Web APIs that should be restricted when Lockdown Mode is enabled. This vulnerability represents a significant security concern as it undermines the security guarantees provided by Apple's Lockdown Mode feature, which is specifically designed to provide enhanced protection for high-risk users such as journalists, activists, and government officials.

The vulnerability stems from improper URL validation, classified as CWE-601 (URL Redirection to Untrusted Site), which enables attackers to bypass security restrictions that Lockdown Mode is intended to enforce. When exploited, malicious web content delivered through file URLs can gain access to restricted Web APIs, potentially exposing sensitive information or enabling further attacks.

Critical Impact

This vulnerability bypasses Lockdown Mode protections on macOS systems, potentially exposing high-risk users to attacks that Lockdown Mode was specifically designed to prevent. Web APIs that should be blocked can be accessed through file URL exploitation.

Affected Products

• Apple Safari versions prior to 26.2
• Apple macOS versions prior to Tahoe 26.2
• Systems with Lockdown Mode enabled are specifically impacted

Discovery Timeline

• 2025-12-17 - CVE-2025-43526 published to NVD
• 2025-12-18 - Last updated in NVD database

Technical Details for CVE-2025-43526

Vulnerability Analysis

This vulnerability exists in the URL validation mechanism within Safari and macOS. When Lockdown Mode is enabled, Apple implements additional security restrictions to limit the attack surface available to potential attackers. These restrictions include blocking certain Web APIs that could be abused for exploitation or tracking purposes.

The flaw occurs when web content is loaded through file URLs (using the file:// scheme). The URL validation logic fails to properly enforce Lockdown Mode restrictions for content accessed via this method, creating a bypass path. An attacker could craft malicious HTML content designed to be opened locally, which would then have access to Web APIs that should have been blocked under Lockdown Mode.

This type of vulnerability is particularly concerning because Lockdown Mode is specifically designed for individuals who may be targeted by sophisticated adversaries. The bypass essentially negates the additional protections these users rely on for their security.

Root Cause

The root cause is improper URL validation (CWE-601) in the handling of file URL schemes within Safari's security enforcement layer. The security boundary checks that restrict Web API access under Lockdown Mode do not properly account for the file:// URL scheme, allowing local content to bypass these restrictions.

When content is loaded from a file URL, the system fails to apply the same security policies that would be enforced for content loaded from remote URLs. This differential treatment creates an exploitable gap in the security model.

Attack Vector

The attack vector for this vulnerability is network-based, though exploitation requires user interaction to open malicious content via a file URL. An attacker could:

1. Deliver a malicious HTML file to the target through email, messaging, or other file transfer methods
2. Convince the user to open the file locally in Safari
3. The malicious content would then execute with access to restricted Web APIs
4. These APIs could be used for fingerprinting, data exfiltration, or as part of a larger exploit chain

The vulnerability bypasses Lockdown Mode restrictions specifically, meaning the attack is most impactful against users who have enabled this enhanced security feature. The ability to access restricted Web APIs could allow an attacker to perform actions such as device fingerprinting, accessing sensitive browser capabilities, or preparing the environment for additional exploitation stages.

Detection Methods for CVE-2025-43526

Indicators of Compromise

• Suspicious HTML or web content files received via email or messaging applications
• Unexpected local file access patterns in Safari browser logs
• Web API calls originating from file:// URL contexts that should be restricted
• Anomalous browser behavior when Lockdown Mode is enabled

Detection Strategies

• Monitor for HTML files being opened from local storage with subsequent Web API access patterns
• Implement endpoint detection rules to identify file URL exploitation attempts in Safari
• Review browser activity logs for restricted API calls that should be blocked under Lockdown Mode
• Deploy behavioral analysis to detect anomalous patterns in Safari when processing local content

Monitoring Recommendations

• Enable enhanced logging for Safari and WebKit processes on macOS systems
• Monitor file system access patterns for suspicious HTML content in user directories
• Track Web API usage patterns to identify unexpected access from file URL contexts
• Implement alerting for any detected Lockdown Mode bypass attempts

How to Mitigate CVE-2025-43526

Immediate Actions Required

• Update Safari to version 26.2 or later immediately
• Update macOS to Tahoe 26.2 or later on all affected systems
• Advise users with Lockdown Mode enabled to prioritize these updates
• Avoid opening untrusted HTML files locally until patches are applied

Patch Information

Apple has addressed this vulnerability with improved URL validation in the following releases:

• macOS Tahoe 26.2 - Full patch for the URL validation bypass
• Safari 26.2 - Standalone browser update with the security fix

For detailed patch information, refer to Apple Support Document #125886 and Apple Support Document #125892.

Organizations should prioritize deployment of these updates, particularly for systems where Lockdown Mode is enabled or for users who may be at elevated risk of targeted attacks.

Workarounds

• Avoid opening HTML files from untrusted sources locally in Safari until patched
• Consider using alternative browsers for viewing local HTML content temporarily
• Implement strict file handling policies to prevent execution of untrusted web content
• Enable additional endpoint protection to detect and block exploitation attempts

For systems that cannot be immediately patched, administrators should implement strict controls on file downloads and educate users about the risks of opening local HTML content from untrusted sources.