CVE-2021-30853: Apple macOS Gatekeeper Bypass Vulnerability

CVE-2021-30853 Overview

CVE-2021-30853 is a security bypass vulnerability in Apple macOS that allows a malicious application to bypass Gatekeeper checks. Gatekeeper is a critical security feature in macOS designed to ensure that only trusted software runs on Mac systems by verifying code signatures and checking for known malware. This vulnerability undermines that protection, potentially allowing unsigned or malicious applications to execute without proper security validation.

Critical Impact

Malicious applications can bypass macOS Gatekeeper security checks, potentially allowing untrusted or malicious code to execute on affected systems without user awareness.

Affected Products

• Apple macOS (versions prior to Big Sur 11.6)

Discovery Timeline

• 2021-08-24 - CVE CVE-2021-30853 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-30853

Vulnerability Analysis

This vulnerability stems from insufficient validation checks in the macOS Gatekeeper security mechanism. Gatekeeper is designed to prevent the execution of unsigned or malicious applications by verifying code signatures and checking against Apple's notarization database before allowing an application to run.

The flaw allows attackers to craft malicious applications that can evade these security checks entirely. When exploited, the malicious application appears legitimate to Gatekeeper, bypassing the security prompts that would normally warn users about potentially dangerous software. This type of security bypass is particularly dangerous because it circumvents a core macOS defense mechanism that many users rely on for protection against malware.

The attack requires local access and user interaction, meaning an attacker would need to convince a user to download and attempt to run the malicious application. However, once executed, the application runs with the permissions granted to it, potentially compromising the integrity of the affected system.

Root Cause

The root cause of this vulnerability is improper input validation (CWE-787) in the Gatekeeper checking mechanism. The security checks implemented were insufficient to properly validate all attributes of an application, creating a bypass condition that attackers could exploit. Apple addressed this issue by implementing improved validation checks in the affected code paths.

Attack Vector

The attack vector for CVE-2021-30853 requires local access and user interaction. An attacker must first distribute the malicious application to the target system, typically through social engineering techniques such as phishing emails, malicious downloads, or compromised websites. Once the user attempts to open or execute the crafted application, the vulnerability is triggered, bypassing Gatekeeper's protective measures. This allows the malicious code to execute without the expected security warnings or blocks that would normally alert the user to the potential threat.

Detection Methods for CVE-2021-30853

Indicators of Compromise

• Unusual application executions that bypass normal Gatekeeper prompts
• Applications running without proper code signatures or notarization
• Unexpected processes or applications appearing in Activity Monitor without user initiation
• System logs showing applications executed without passing Gatekeeper validation

Detection Strategies

• Monitor system logs for evidence of applications bypassing Gatekeeper checks
• Implement endpoint detection and response (EDR) solutions to identify anomalous application behavior
• Review spctl --assess outputs for applications to identify unsigned or improperly validated binaries
• Enable and monitor com.apple.security.quarantine extended attributes on downloaded files

Monitoring Recommendations

• Configure SentinelOne agents to monitor for Gatekeeper bypass attempts and unauthorized application executions
• Enable audit logging for application launches and security policy violations
• Monitor for modifications to quarantine flags on downloaded applications
• Implement network monitoring to detect downloads of suspicious or unsigned applications

How to Mitigate CVE-2021-30853

Immediate Actions Required

• Update macOS to Big Sur version 11.6 or later immediately
• Ensure all Mac systems have the latest security updates applied
• Review recently installed applications for suspicious or unsigned software
• Enable strict Gatekeeper settings by setting spctl --master-enable on all managed systems

Patch Information

Apple has addressed this vulnerability in macOS Big Sur 11.6. The fix implements improved validation checks in the Gatekeeper mechanism to properly verify application attributes before allowing execution. Administrators should apply this update to all affected macOS systems immediately. Additional details are available in the Apple Support Article.

Workarounds

• Restrict users to only running applications from the Mac App Store where possible
• Implement application whitelisting policies to control which applications can execute
• Educate users about the risks of downloading and running applications from untrusted sources
• Consider using Mobile Device Management (MDM) solutions to enforce strict application policies on managed Mac systems

# Verify Gatekeeper is enabled and properly configured
spctl --status
# Enable Gatekeeper if disabled
sudo spctl --master-enable
# Check security assessment policy
spctl --assess --verbose /path/to/application.app