CVE-2015-6991 in Mac OS X
Summary
by MITRE
FontParser in Apple iOS before 9.1 and OS X before 10.11.1 allows remote attackers to execute arbitrary code or cause a denial of service (memory corruption) via a crafted font file, a different vulnerability than CVE-2015-6976, CVE-2015-6977, CVE-2015-6978, CVE-2015-6990, CVE-2015-6993, CVE-2015-7008, CVE-2015-7009, CVE-2015-7010, and CVE-2015-7018.
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Analysis
by VulDB Data Team • 11/19/2024
The vulnerability identified as CVE-2015-6991 represents a critical memory corruption flaw within Apple's FontParser component that affects iOS versions prior to 9.1 and OS X versions prior to 10.11.1. This vulnerability resides in the font parsing functionality that processes various font file formats including TrueType, OpenType, and other embedded font types used across Apple's operating systems. The flaw manifests when the system encounters specially crafted malicious font files that exploit buffer overflow conditions during the parsing process, potentially leading to arbitrary code execution or system crashes. This issue is particularly concerning as font files are commonly encountered in email attachments, web downloads, and document processing scenarios where users may unknowingly trigger the vulnerable parsing code.
The technical implementation of this vulnerability stems from insufficient input validation and memory management within the font parsing routines. When a malformed font file is processed, the FontParser fails to properly bounds-check array accesses and memory allocations, creating opportunities for attackers to manipulate memory layout and execute malicious code with the privileges of the affected process. The vulnerability operates at the kernel level within the graphics rendering subsystem, making it particularly dangerous as it can be triggered through multiple attack vectors including web browsing, email processing, and document viewing operations. This flaw specifically aligns with CWE-121, which describes stack-based buffer overflow conditions, and CWE-125, which covers out-of-bounds read vulnerabilities that can lead to memory corruption.
From an operational perspective, this vulnerability enables remote code execution attacks that can compromise the entire system without requiring user interaction beyond the initial triggering of the vulnerable font parsing. Attackers can craft malicious font files that, when opened or previewed by vulnerable systems, will automatically execute malicious payloads. The impact extends beyond simple exploitation as the memory corruption can also result in denial of service conditions that render systems unstable or unusable. According to ATT&CK framework categorization, this vulnerability maps to T1059.007 for command and scripting interpreter usage and T1203 for exploitation for privilege escalation. The vulnerability is particularly dangerous in enterprise environments where users may encounter malicious font files through phishing campaigns, compromised websites, or malicious documents that leverage the font parsing functionality.
Mitigation strategies for CVE-2015-6991 primarily involve applying the official security patches released by Apple as part of iOS 9.1 and OS X 10.11.1 updates. System administrators should prioritize immediate deployment of these patches across all affected devices, particularly in enterprise environments where the risk of targeted attacks is higher. Additional defensive measures include implementing strict email filtering policies to block suspicious font file attachments, disabling automatic font preview features in web browsers, and employing network-based intrusion detection systems to monitor for exploitation attempts. Organizations should also consider implementing application whitelisting policies that restrict execution of font processing utilities and regularly audit system logs for unusual font processing activities that might indicate exploitation attempts. The vulnerability demonstrates the critical importance of font parsing security in operating system design and highlights the need for comprehensive input validation across all system components that handle user-supplied data.