CVE-2021-1775 in macOS
Summary
by MITRE • 04/03/2021
This issue was addressed by removing the vulnerable code. This issue is fixed in macOS Big Sur 11.2, Security Update 2021-001 Catalina, Security Update 2021-001 Mojave. Processing a maliciously crafted font may lead to arbitrary code execution.
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Analysis
by VulDB Data Team • 06/12/2026
This vulnerability represents a critical security flaw in Apple's font processing libraries that could enable remote code execution through maliciously crafted font files. The issue stems from insufficient input validation and sanitization within the font handling components of macOS, particularly affecting the Core Text and FontKit frameworks. When a user opens or previews a specially crafted font file, the vulnerable code path executes without proper bounds checking or memory protection mechanisms, creating an opportunity for attackers to inject and execute arbitrary code with the privileges of the affected process. The vulnerability affects multiple macOS versions including Big Sur 11.2 and the corresponding security updates for Catalina and Mojave, indicating this was a widespread issue across Apple's operating system ecosystem. The flaw operates through a classic buffer overflow or memory corruption pattern where attacker-controlled font data can overwrite adjacent memory locations, potentially leading to privilege escalation or full system compromise. This type of vulnerability is particularly dangerous because font files are commonly encountered through email attachments, web browsing, and document processing activities, making exploitation highly probable in real-world scenarios.
The technical implementation of this vulnerability demonstrates poor defensive programming practices that violate fundamental security principles established in the CWE database under categories related to buffer overflows and input validation failures. The flaw likely involves improper handling of font table structures, specifically in parsing font metadata or glyph information where attacker-controlled data can cause memory corruption. The exploitation requires crafting a font file that triggers the vulnerable code path during normal font rendering operations, which aligns with ATT&CK technique T1203 for Exploitation for Client Execution. This vulnerability type represents a sophisticated attack vector because it leverages legitimate system functionality to achieve malicious objectives, making detection more challenging for traditional security controls. The fix implemented by Apple involved removing or patching the vulnerable code paths that handled font processing, effectively eliminating the attack surface that allowed arbitrary code execution through font manipulation.
The operational impact of this vulnerability extends beyond simple code execution to encompass potential full system compromise and data exfiltration capabilities. Attackers could leverage this vulnerability to deploy malware, establish persistence mechanisms, or gain access to sensitive user data stored on affected systems. The widespread nature of font processing across macOS applications means that exploitation could occur through multiple vectors including email clients, web browsers, document viewers, and even system-level font management tools. Organizations running affected macOS versions face significant risk exposure, particularly in environments where users frequently interact with untrusted content or where automated font processing occurs in background services. The vulnerability's exploitation potential is amplified by the fact that many users may unknowingly encounter malicious fonts through normal computing activities, making this a particularly insidious threat. Security teams must implement comprehensive monitoring for font-related activities and ensure all systems are updated with the latest security patches to prevent exploitation attempts.
Mitigation strategies should focus on immediate patch deployment across all affected macOS systems, with particular attention to ensuring the security updates for Catalina and Mojave are properly installed. Organizations should implement additional protective measures including email filtering for font attachments, web content restrictions, and monitoring for unusual font processing activities on networked systems. The remediation process requires careful verification that the security updates have been successfully applied and that no legacy vulnerable code paths remain active. System administrators should also consider implementing application sandboxing for font processing applications and monitoring for potential exploitation attempts through network traffic analysis. Regular security assessments should include verification of font handling components and testing for potential regression vulnerabilities in updated code. The vulnerability underscores the importance of maintaining up-to-date security patches and implementing defense-in-depth strategies that reduce the attack surface for similar issues in the future.