CVE-2016-7118 in linux-image package
Summary
by MITRE
fs/fcntl.c in the "aufs 3.2.x+setfl-debian" patch in the linux-image package 3.2.0-4 (kernel 3.2.81-1) in Debian wheezy mishandles F_SETFL fcntl calls on directories, which allows local users to cause a denial of service (NULL pointer dereference and system crash) via standard filesystem operations, as demonstrated by scp from an AUFS filesystem.
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Analysis
by VulDB Data Team • 04/05/2019
The vulnerability described in CVE-2016-7118 represents a critical kernel-level flaw within the Advanced Union File System (AUFS) implementation in the Linux kernel version 3.2.81-1. This issue specifically affects the Debian wheezy operating system and stems from improper handling of file control operations on directory objects within AUFS filesystems. The vulnerability manifests when the kernel processes F_SETFL fcntl calls on directories, creating a scenario where a NULL pointer dereference can occur during standard filesystem operations. This flaw demonstrates how seemingly routine file system interactions can escalate into system-wide crashes, fundamentally compromising system stability and availability.
The technical root cause of this vulnerability lies in the flawed implementation of the aufs 3.2.x+setfl-debian patch within the fs/fcntl.c kernel source file. When an AUFS filesystem encounters a directory object and receives an F_SETFL fcntl operation, the kernel's handling mechanism fails to properly validate the target object type before attempting to process the file control flags. This oversight creates a condition where the kernel attempts to dereference a NULL pointer when working with directory objects, leading to an immediate system crash. The vulnerability is particularly insidious because it operates at the kernel level, bypassing typical user-space protections and security boundaries. The flaw specifically affects the file descriptor control mechanism, which is fundamental to Unix-like operating systems for managing file access permissions and flags, making it a critical component for system stability.
The operational impact of this vulnerability extends beyond simple system crashes, creating significant risks for systems relying on AUFS for storage management or containerization environments. Local users can exploit this vulnerability to cause deliberate denial of service attacks against systems running affected kernel versions, potentially disrupting critical services or applications that depend on AUFS filesystems. The demonstration using scp from an AUFS filesystem highlights how common administrative tasks can trigger this exploit, making it particularly dangerous in production environments where such operations are routine. The vulnerability essentially allows an unprivileged local user to crash the entire kernel, which can result in complete system downtime and potential data loss. This type of vulnerability directly impacts the availability aspect of the CIA triad and can be categorized under CWE-476 as NULL pointer dereference, representing a classic kernel-level memory safety issue.
Mitigation strategies for CVE-2016-7118 primarily involve immediate system updates and patches to address the underlying kernel vulnerability. Organizations should prioritize upgrading to kernel versions that contain the proper fix for the aufs implementation, specifically targeting versions that eliminate the NULL pointer dereference in the fs/fcntl.c file. System administrators should also implement monitoring to detect unusual patterns of system crashes or kernel panics that might indicate exploitation attempts. Additionally, the vulnerability can be mitigated by avoiding the use of AUFS filesystems where possible, particularly in environments where local users might have access to system resources. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving privilege escalation and denial of service, as it allows local users to perform system-level operations that should be restricted to privileged processes. The vulnerability also demonstrates how filesystem-level implementations can create unexpected attack surfaces that bypass traditional security controls, emphasizing the importance of kernel-level security reviews and proper input validation across all system components.