CVE-2015-3202 in fuse
Summary
by MITRE
fusermount in FUSE before 2.9.3-15 does not properly clear the environment before invoking (1) mount or (2) umount as root, which allows local users to write to arbitrary files via a crafted LIBMOUNT_MTAB environment variable that is used by mount's debugging feature.
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Analysis
by VulDB Data Team • 11/26/2024
The vulnerability identified as CVE-2015-3202 affects the FUSE (Filesystem in Userspace) implementation prior to version 2.9.3-15, specifically within the fusermount utility. This flaw represents a critical security issue that stems from improper environment variable handling during mount and unmount operations. The vulnerability allows local attackers to escalate privileges and write to arbitrary files by manipulating the LIBMOUNT_MTAB environment variable, which is utilized by mount's debugging functionality. The root cause lies in the failure of fusermount to properly sanitize the environment before executing privileged mount or umount commands, creating a path for privilege escalation attacks.
The technical exploitation of this vulnerability occurs through the manipulation of the LIBMOUNT_MTAB environment variable, which is designed for debugging mount operations but becomes a vector for malicious file system modifications. When fusermount executes mount or umount commands, it inherits the environment variables from the calling process without proper sanitization. This allows an attacker to inject malicious values into the LIBMOUNT_MTAB variable, which then gets processed by the mount utility during debugging operations. The vulnerability is classified under CWE-276 as improper privilege management and aligns with ATT&CK technique T1068, which covers local privilege escalation through environment variable manipulation. The flaw specifically targets the security boundary between user-space and kernel-space operations within the FUSE framework.
The operational impact of this vulnerability extends beyond simple file system manipulation to encompass broader system compromise capabilities. An attacker with local access can leverage this vulnerability to modify critical system files, potentially gaining persistent access or disrupting system operations. The vulnerability affects systems where FUSE is installed and used for mounting file systems, particularly in environments where users might have access to execute mount-related commands. The privilege escalation aspect means that even users with limited permissions can potentially write to files that should be restricted to root access. This vulnerability has implications for containerized environments, virtualized systems, and any infrastructure relying on FUSE for file system operations, as it can be exploited to bypass security controls and access sensitive data.
Mitigation strategies for CVE-2015-3202 primarily focus on updating the FUSE implementation to version 2.9.3-15 or later, which includes proper environment variable sanitization. System administrators should also implement strict access controls on FUSE-related utilities and monitor for unauthorized modifications to mount-related environment variables. The solution involves ensuring that fusermount properly clears or validates environment variables before executing privileged operations, preventing malicious input from being passed to the mount utility. Organizations should conduct vulnerability assessments to identify systems running vulnerable FUSE versions and prioritize patching efforts. Additional defensive measures include implementing mandatory access controls, monitoring for suspicious environment variable usage patterns, and ensuring that only authorized users have access to FUSE mount operations. The fix addresses the underlying issue by implementing proper privilege separation and environment sanitization, aligning with security best practices for preventing privilege escalation attacks.