CVE-2018-6942 in FreeType 2
Summary
by MITRE
An issue was discovered in FreeType 2 through 2.9. A NULL pointer dereference in the Ins_GETVARIATION() function within ttinterp.c could lead to DoS via a crafted font file.
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Analysis
by VulDB Data Team • 02/06/2023
The vulnerability identified as CVE-2018-6942 represents a critical NULL pointer dereference flaw within the FreeType 2 font rendering library version 2.9 and earlier. This issue manifests specifically within the Ins_GETVARIATION() function located in the ttinterp.c source file, which is responsible for processing TrueType font instructions. The flaw occurs when the font processing engine encounters a malformed or maliciously crafted font file that triggers unexpected behavior in the variable variation handling code path.
The technical nature of this vulnerability stems from inadequate input validation and error handling within the font interpretation process. When FreeType attempts to process a font containing malformed variation data, the Ins_GETVARIATION() function fails to properly check for NULL pointer references before dereferencing pointers that may not have been properly initialized. This condition creates a scenario where the application crashes or becomes unresponsive when attempting to render fonts that contain crafted variation instructions, effectively creating a denial of service condition. The vulnerability is classified as a CWE-476_NULL_Pointer_Dereference, which is a well-known weakness pattern that occurs when a program attempts to access a memory location through a pointer that has not been initialized to a valid address.
The operational impact of this vulnerability extends beyond simple service disruption as it affects any application that relies on FreeType for font rendering operations. Systems utilizing this library for document processing, web browsers, desktop applications, and print services become vulnerable to exploitation through malicious font files. Attackers can craft specially designed font files that, when processed by vulnerable applications, will cause the target system to crash or become unresponsive. This makes the vulnerability particularly dangerous in environments where users might encounter untrusted font content, such as email attachments, web downloads, or shared document repositories. The attack surface is broad since FreeType is widely used across multiple operating systems and applications, including major software suites like LibreOffice, Firefox, and various PDF viewers.
Mitigation strategies for CVE-2018-6942 primarily focus on updating to FreeType version 2.9.1 or later, where the NULL pointer dereference has been resolved through proper input validation and error handling. System administrators should implement comprehensive patch management procedures to ensure all affected applications are updated promptly. Additionally, organizations can deploy application whitelisting policies that restrict font file processing to trusted sources, implement sandboxing techniques for font rendering operations, and establish monitoring protocols to detect unusual application behavior that might indicate exploitation attempts. The vulnerability demonstrates the importance of robust input validation in font processing libraries and aligns with ATT&CK technique T1203_Access_Control_Bypass through its ability to disrupt system availability. Organizations should also consider implementing defensive measures such as font file scanning, restricted font processing capabilities, and regular security assessments of font handling code paths to prevent similar vulnerabilities from emerging in other components of their software ecosystems.