CVE-2018-6917 in FreeBSD
Summary
by MITRE
In FreeBSD before 11.1-STABLE, 11.1-RELEASE-p9, 10.4-STABLE, 10.4-RELEASE-p8 and 10.3-RELEASE-p28, insufficient validation of user-provided font parameters can result in an integer overflow, leading to the use of arbitrary kernel memory as glyph data. Unprivileged users may be able to access privileged kernel data.
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Analysis
by VulDB Data Team • 02/09/2021
The vulnerability identified as CVE-2018-6917 represents a critical integer overflow flaw within the FreeBSD operating system's font handling subsystem. This issue affects multiple versions including various stable and release branches, specifically targeting the kernel's font processing mechanisms that are responsible for rendering graphical elements. The flaw stems from inadequate validation of user-provided font parameters, creating a pathway for malicious input to manipulate kernel memory structures. According to CWE-190, this vulnerability maps directly to integer overflow conditions where the system fails to properly validate input data before performing arithmetic operations that could exceed maximum representable values.
The technical exploitation of this vulnerability occurs when unprivileged users provide specially crafted font parameters to the system's font rendering subsystem. The insufficient validation allows these inputs to trigger integer overflow conditions within kernel memory management routines. When the overflow occurs, it results in the kernel incorrectly calculating memory addresses for glyph data structures, effectively causing the system to use arbitrary kernel memory locations as if they contained legitimate font data. This fundamental breakdown in memory management creates a potential information disclosure scenario where sensitive kernel data becomes accessible to users who would normally not have such privileges. The ATT&CK framework categorizes this as a privilege escalation technique through kernel memory manipulation, specifically under the T1068 technique of exploiting weaknesses in the kernel.
The operational impact of CVE-2018-6917 extends beyond simple information disclosure, as it fundamentally undermines the kernel's memory protection mechanisms. An attacker could potentially extract sensitive kernel data including cryptographic keys, credential information, or other privileged system details from memory locations that should remain protected. The vulnerability affects the core graphics rendering functionality and can be exploited through various user-space applications that interact with font processing capabilities, making it particularly dangerous in multi-user environments. The integer overflow creates a predictable pattern of memory corruption that could be leveraged to achieve more sophisticated attacks including arbitrary code execution within kernel space. This vulnerability essentially allows for a form of kernel memory disclosure that bypasses normal access controls and privilege boundaries.
Mitigation strategies for CVE-2018-6917 primarily involve applying the official FreeBSD security patches that address the integer overflow conditions in font parameter validation. System administrators should immediately update to the patched versions including 11.1-STABLE, 11.1-RELEASE-p9, 10.4-STABLE, 10.4-RELEASE-p8, and 10.3-RELEASE-p28. The patch implementations typically involve strengthening input validation routines and implementing proper bounds checking for font parameter calculations. Additional defensive measures include restricting font processing capabilities in applications, implementing kernel memory protection features such as kernel page table isolation, and monitoring for suspicious font processing activities. Organizations should also consider implementing network segmentation to limit potential attack vectors and ensure that font processing is restricted to trusted applications only. The vulnerability highlights the importance of thorough input validation in kernel space operations and demonstrates how seemingly benign user inputs can be leveraged to compromise system security boundaries.