CVE-2018-1095 in Linux
Summary
by MITRE
The ext4_xattr_check_entries function in fs/ext4/xattr.c in the Linux kernel through 4.15.15 does not properly validate xattr sizes, which causes misinterpretation of a size as an error code, and consequently allows attackers to cause a denial of service (get_acl NULL pointer dereference and system crash) via a crafted ext4 image.
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Analysis
by VulDB Data Team • 02/26/2023
The vulnerability identified as CVE-2018-1095 resides within the Linux kernel's ext4 file system implementation, specifically in the ext4_xattr_check_entries function located in fs/ext4/xattr.c. This flaw represents a classic case of improper input validation that can be exploited to trigger system instability. The vulnerability affects Linux kernel versions through 4.15.15, making it a significant concern for systems running these kernel versions. The issue stems from the function's failure to properly validate extended attribute sizes, creating a scenario where malformed data can be misinterpreted by the kernel's internal processing mechanisms.
The technical flaw manifests when the ext4_xattr_check_entries function encounters extended attributes with invalid size parameters. Under normal circumstances, the function should validate that attribute sizes fall within acceptable ranges and properly handle any size-related errors. However, due to inadequate validation logic, the function fails to correctly distinguish between valid size values and error codes that may be inadvertently stored in the same data fields. This misinterpretation occurs because the function treats size values as if they were error indicators, leading to a cascade of errors when the system attempts to process these malformed attributes.
The operational impact of this vulnerability extends beyond simple denial of service to potentially causing complete system crashes. Attackers can craft malicious ext4 file system images containing specially constructed extended attributes that trigger the NULL pointer dereference during get_acl operations. This particular attack vector demonstrates the severity of the flaw, as it allows adversaries to cause system instability through file system manipulation rather than requiring direct kernel-level access. The vulnerability essentially provides a path for unprivileged users to potentially crash the entire system through carefully crafted file system structures.
This vulnerability aligns with CWE-129, which addresses improper validation of array indices, and CWE-704, which covers incorrect type conversion. The attack pattern follows the typical methodology described in the MITRE ATT&CK framework under T1059 for system execution and T1499 for endpoint denial of service. The exploitation requires minimal privileges and can be executed through file system manipulation, making it particularly dangerous for environments where users have the ability to mount or create ext4 file systems. The vulnerability's impact is amplified in virtualized environments where malicious file system images could be used to compromise multiple virtual machines.
Mitigation strategies for CVE-2018-1095 include immediate kernel updates to versions that contain the patched ext4_xattr_check_entries function. System administrators should prioritize patching all affected systems, particularly those running kernel versions through 4.15.15. Additionally, implementing proper file system validation checks and monitoring for suspicious extended attribute usage patterns can provide early detection of potential exploitation attempts. Organizations should also consider implementing file system access controls to limit the ability of untrusted users to mount or modify ext4 file systems, thereby reducing the attack surface for this particular vulnerability.