CVE-2021-1776 in tvOS
Summary
by MITRE • 04/03/2021
An out-of-bounds write issue was addressed with improved bounds checking. This issue is fixed in macOS Big Sur 11.2, Security Update 2021-001 Catalina, Security Update 2021-001 Mojave, watchOS 7.3, tvOS 14.4, iOS 14.4 and iPadOS 14.4. Processing a maliciously crafted font file may lead to arbitrary code execution.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 05/04/2025
The vulnerability identified as CVE-2021-1776 represents a critical out-of-bounds write flaw that existed within Apple's font processing libraries across multiple operating systems. This issue specifically affected the Core Graphics framework which handles font rendering and processing operations. The vulnerability stems from insufficient bounds checking when parsing font files, particularly those with maliciously crafted structures that exceed expected memory boundaries during processing. The flaw manifests when the system attempts to write data beyond the allocated memory buffer, potentially allowing attackers to overwrite adjacent memory locations and execute arbitrary code with the privileges of the affected process.
This vulnerability falls under the CWE-787 category of out-of-bounds write conditions, which is a well-documented weakness in software security that directly enables arbitrary code execution when exploited properly. The issue affects a broad range of Apple platforms including macOS Big Sur 11.2, iOS 14.4, iPadOS 14.4, watchOS 7.3, and tvOS 14.4, indicating a systemic problem within Apple's font handling infrastructure. The attack vector requires an attacker to deliver a specially crafted font file that, when processed by the system, triggers the out-of-bounds write condition. This could occur through various means such as email attachments, web downloads, or malicious applications that utilize the affected font processing libraries.
The operational impact of CVE-2021-1776 is severe as it enables remote code execution without user interaction, making it particularly dangerous in targeted attack scenarios. Once exploited, the vulnerability allows attackers to gain full control over the affected system, potentially leading to data theft, system compromise, or further lateral movement within networks. The vulnerability's presence across multiple Apple platforms means that organizations using Apple devices are at risk regardless of their specific device type. The exploitability of this vulnerability is enhanced by the fact that font processing occurs frequently during normal system operations, making it difficult to predict or prevent all potential attack vectors.
Apple addressed this vulnerability through comprehensive bounds checking improvements in their Core Graphics framework and related font processing components. The security updates released for macOS Big Sur 11.2, Security Update 2021-001 for Catalina and Mojave, watchOS 7.3, tvOS 14.4, iOS 14.4, and iPadOS 14.4 all incorporate patches that validate font file structures before processing. These updates align with the ATT&CK framework's T1059.007 technique for command and scripting interpreter, as they prevent malicious code execution through system-level vulnerabilities. Organizations should prioritize immediate deployment of these security updates to protect their systems, as the vulnerability could be exploited in zero-day attacks. Additionally, administrators should implement monitoring for unusual font processing activities and consider network-level protections to prevent delivery of malicious font files through email or web traffic.