CVE-2020-9956 in watchOS
Summary
by MITRE • 04/03/2021
An out-of-bounds read was addressed with improved input validation. This issue is fixed in macOS Big Sur 11.0.1, tvOS 14.0, macOS Big Sur 11.1, Security Update 2020-001 Catalina, Security Update 2020-007 Mojave, watchOS 7.0, iOS 14.0 and iPadOS 14.0. Processing a maliciously crafted font file may lead to arbitrary code execution.
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Analysis
by VulDB Data Team • 04/08/2021
The vulnerability identified as CVE-2020-9956 represents a critical out-of-bounds read flaw within Apple's font processing libraries that affects multiple operating systems including macOS, iOS, watchOS, and tvOS. This issue stems from insufficient input validation mechanisms when handling specially crafted font files, creating a pathway for attackers to exploit memory access violations that could result in arbitrary code execution. The vulnerability specifically impacts the Core Text framework and font rendering subsystems that are fundamental components of Apple's operating systems, making it a significant concern for both enterprise and individual users. The flaw manifests when the system attempts to parse font files that contain malformed or maliciously constructed data structures, particularly within font table processing routines.
Technical exploitation of this vulnerability requires an attacker to craft a malicious font file that triggers the out-of-bounds memory read condition during font processing. The root cause lies in the lack of proper bounds checking within the font parsing code, which allows memory access beyond the allocated buffer boundaries. This type of vulnerability falls under CWE-129, which specifically addresses insufficient validation of length of input buffers, and can be mapped to ATT&CK technique T1059.007 for execution through font files. The vulnerability is particularly dangerous because font files are commonly encountered in legitimate user workflows, making social engineering attacks more effective. Attackers can deliver malicious font files through email attachments, web downloads, or file sharing systems, where they would be processed automatically by the operating system's font rendering engines.
The operational impact of CVE-2020-9956 extends beyond simple privilege escalation or system compromise, as it can enable full system takeover through carefully crafted payload delivery. When exploited successfully, the vulnerability allows attackers to execute arbitrary code with the privileges of the affected application, typically the font rendering service or user-level applications that process font files. This could lead to complete system compromise, data exfiltration, or deployment of additional malware. The widespread use of font files across different platforms and applications makes this vulnerability particularly dangerous, as it can be triggered through multiple attack vectors including web browsers, email clients, and document viewers. Organizations running Apple operating systems without the relevant security updates are at significant risk, as the vulnerability can be exploited remotely through web-based attacks.
Apple addressed this vulnerability through comprehensive input validation improvements in their font processing libraries, implementing proper bounds checking and memory access controls to prevent out-of-bounds reads. The security patches released as part of macOS Big Sur 11.0.1, iOS 14.0, and other system updates include enhanced validation mechanisms that verify font file structures before processing. Organizations should immediately deploy these security updates to protect against exploitation attempts, as the vulnerability has been actively exploited in the wild. Additional mitigations include implementing network-level controls to block font file downloads, disabling automatic font processing in web browsers, and conducting security awareness training to prevent users from opening suspicious font files. System administrators should also monitor for indicators of compromise related to font processing activities and implement endpoint detection and response solutions that can identify anomalous font rendering behavior. The fix demonstrates Apple's commitment to addressing memory safety issues through proactive patch management and improved code validation practices that align with industry best practices for secure software development.