CVE-2021-30771
📋 TL;DR
This vulnerability allows arbitrary code execution via malicious font files due to an out-of-bounds write in Apple's font processing. It affects macOS, iOS, iPadOS, watchOS, and tvOS before specific security updates. Attackers can exploit this by tricking users into opening malicious font files.
💻 Affected Systems
- macOS
- iOS
- iPadOS
- watchOS
- tvOS
📦 What is this software?
Ipados by Apple
Macos by Apple
macOS is Apple's desktop and laptop operating system powering Mac computers used by millions of professionals, developers, creative professionals, and enterprise users worldwide. Built on a Unix foundation with the Darwin kernel and modern Cocoa frameworks, macOS delivers a seamless ecosystem integr...
Learn more about Macos →Tvos by Apple
Watchos by Apple
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with kernel-level privileges leading to data theft, persistence, and lateral movement across Apple ecosystem devices.
Likely Case
Malicious font files delivered via phishing or compromised websites execute arbitrary code with user privileges, enabling data exfiltration or malware installation.
If Mitigated
With proper patching and security controls, impact is limited to isolated user sessions without privilege escalation or lateral movement.
🎯 Exploit Status
Exploitation requires user interaction to process malicious font files, but no authentication is needed once the file is opened.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: macOS Big Sur 11.4, iOS 14.6, iPadOS 14.6, watchOS 7.5, tvOS 14.6
Vendor Advisory: https://support.apple.com/en-us/HT212528
Restart Required: Yes
Instructions:
1. Open System Preferences > Software Update. 2. Install available updates. 3. Restart device when prompted.
🔧 Temporary Workarounds
Restrict font file processing
allBlock or quarantine font files from untrusted sources using endpoint protection or email filtering.
🧯 If You Can't Patch
- Implement application allowlisting to prevent unauthorized font processing applications.
- Use network segmentation to isolate vulnerable devices from critical systems.
🔍 How to Verify
Check if Vulnerable:
Check system version against affected versions list. On macOS: About This Mac > Overview.Check Version:
macOS: sw_vers -productVersion; iOS/iPadOS: Settings > General > About > VersionVerify Fix Applied:
Verify system version matches or exceeds patched versions listed in vendor advisory.📡 Detection & Monitoring
Log Indicators:
- Unusual font file processing activity
- Unexpected system crashes related to font services
Network Indicators:
- Font file downloads from suspicious sources
- Unusual outbound connections after font processing
SIEM Query:
source="fontd" OR process="fontd" AND (event="crash" OR event="error")🔗 References
- https://support.apple.com/en-us/HT212528
- https://support.apple.com/en-us/HT212529
- https://support.apple.com/en-us/HT212532
- https://support.apple.com/en-us/HT212533
- https://support.apple.com/en-us/HT212528
- https://support.apple.com/en-us/HT212529
- https://support.apple.com/en-us/HT212532
- https://support.apple.com/en-us/HT212533
