CVE-2018-1092 in Linux
Summary
by MITRE
The ext4_iget function in fs/ext4/inode.c in the Linux kernel through 4.15.15 mishandles the case of a root directory with a zero i_links_count, which allows attackers to cause a denial of service (ext4_process_freed_data NULL pointer dereference and OOPS) via a crafted ext4 image.
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Analysis
by VulDB Data Team • 09/14/2025
The vulnerability identified as CVE-2018-1092 represents a critical flaw in the Linux kernel's ext4 filesystem implementation that specifically targets the ext4_iget function located in fs/ext4/inode.c. This vulnerability affects Linux kernel versions through 4.15.15 and demonstrates a fundamental issue in how the kernel handles inode structures when processing ext4 filesystem images. The flaw manifests when the kernel encounters a root directory with a zero i_links_count value, creating a scenario where normal filesystem operations can be disrupted through carefully crafted malicious filesystem images.
The technical execution of this vulnerability involves a NULL pointer dereference condition that occurs within the ext4_process_freed_data function when the kernel attempts to process freed data structures. This particular flaw falls under the CWE-476 category of NULL Pointer Dereference, which represents a common class of software vulnerabilities where a program attempts to access a memory location pointed to by a null reference. The vulnerability exploits the improper handling of inode link count values during filesystem mount operations, where the kernel fails to properly validate the integrity of directory structures before attempting to process them.
The operational impact of CVE-2018-1092 extends beyond simple system instability, as it creates a reliable denial of service condition that can be triggered remotely through malicious filesystem images. When an attacker constructs an ext4 filesystem image containing a root directory with zero i_links_count, the kernel's ext4_iget function fails to properly handle this edge case, resulting in system crashes or OOPS messages that effectively render the affected system unusable. This vulnerability can be particularly dangerous in environments where untrusted filesystem images are mounted, such as removable storage devices, network shares, or virtual machine images, as it provides attackers with a straightforward method to disrupt system operations without requiring elevated privileges.
The attack vector for this vulnerability is primarily through filesystem image manipulation, where an attacker can create or modify ext4 filesystems to include root directories with zero link counts. This approach aligns with ATT&CK technique T1059.007 for execution through file systems and demonstrates how filesystem-level vulnerabilities can be exploited to achieve system compromise. The vulnerability's impact is particularly concerning because it operates at the kernel level, meaning that successful exploitation can result in complete system crashes or forced reboots, potentially disrupting critical services or applications that depend on stable filesystem access.
Mitigation strategies for CVE-2018-1092 primarily involve updating to patched kernel versions where the ext4_iget function properly validates inode link count values and handles edge cases appropriately. System administrators should prioritize kernel updates to versions that contain the specific fixes for this vulnerability, typically found in kernel versions 4.15.16 and later. Additionally, implementing filesystem validation checks during mount operations and restricting the ability to mount untrusted filesystem images can provide additional layers of protection. Organizations should also consider monitoring for unusual filesystem access patterns that might indicate exploitation attempts, as the vulnerability's trigger conditions can be detected through careful system auditing and log analysis. The fix implemented in the patched kernels addresses the core issue by ensuring proper validation of inode structures and preventing the NULL pointer dereference condition that leads to system crashes.