CVE-2018-12928 in Linux
Summary
by MITRE
In the Linux kernel 4.15.0, a NULL pointer dereference was discovered in hfs_ext_read_extent in hfs.ko. This can occur during a mount of a crafted hfs filesystem.
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Analysis
by VulDB Data Team • 02/23/2020
The vulnerability identified as CVE-2018-12928 represents a critical NULL pointer dereference flaw within the Linux kernel version 4.15.0 affecting the hfs.ko module. This issue specifically manifests in the hfs_ext_read_extent function which handles extent read operations for HFS (Hierarchical File System) filesystems. The vulnerability arises during the mounting process of a specially crafted HFS filesystem image that has been designed to trigger this particular code path. The flaw exists in the kernel's handling of filesystem metadata structures where insufficient validation occurs before dereferencing pointers that may remain uninitialized or set to NULL.
The technical implementation of this vulnerability stems from inadequate input validation within the HFS filesystem driver's extent management code. When the kernel attempts to read extent information from a malformed HFS filesystem, the hfs_ext_read_extent function fails to properly validate the extent descriptor structure before attempting to access its members. This results in a NULL pointer dereference that causes the kernel to crash and terminate the mount operation. The vulnerability is classified under CWE-476 as a NULL pointer dereference, which represents a fundamental programming error where code assumes a pointer will always contain a valid address without proper validation. The attack vector requires an attacker to create a malicious HFS filesystem image and convince a victim to mount it, making this a privilege escalation vulnerability that can lead to system instability and potential denial of service conditions.
The operational impact of CVE-2018-12928 extends beyond simple system crashes as it represents a potential pathway for privilege escalation attacks within kernel space. When the kernel encounters the malformed filesystem and attempts to dereference the NULL pointer, the system experiences a kernel oops or panic, which can be exploited to gain unauthorized access to the system's memory space. This vulnerability affects all Linux systems running kernel versions up to and including 4.15.0 that have HFS filesystem support enabled. The attack surface is particularly concerning given that HFS filesystems are commonly used on Apple systems and may be encountered during cross-platform file sharing scenarios. The vulnerability aligns with ATT&CK technique T1068 which describes the exploitation of local privilege escalation vulnerabilities, and T1499 which covers network denial of service attacks through kernel exploits.
Mitigation strategies for CVE-2018-12928 primarily involve upgrading to a patched kernel version where the NULL pointer dereference has been addressed through proper input validation and pointer checking. The Linux kernel maintainers released fixes in subsequent kernel versions that include proper validation of extent descriptors before dereferencing pointers in the hfs_ext_read_extent function. System administrators should ensure that all systems running affected kernel versions are updated immediately, particularly in environments where untrusted filesystems might be mounted. Additional defensive measures include disabling HFS filesystem support in kernel configurations when the filesystem is not required, implementing proper filesystem validation checks, and monitoring for unusual mount operations that might indicate exploitation attempts. The vulnerability also highlights the importance of thorough input validation in kernel modules and demonstrates how seemingly minor programming errors can result in significant security implications, reinforcing the need for comprehensive code review processes and automated testing frameworks that can detect such issues before they are exploited in production environments.