CVE-2011-0541 in fuse
Summary
by MITRE
fuse 2.8.5 and earlier does not properly handle when /etc/mtab cannot be updated, which allows local users to unmount arbitrary directories via a symlink attack.
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Analysis
by VulDB Data Team • 11/18/2021
The vulnerability identified as CVE-2011-0541 affects the fuse filesystem implementation version 2.8.5 and earlier, presenting a significant security risk through improper handling of mount table updates. This flaw exists within the core filesystem management mechanisms of the fuse subsystem, which is designed to allow users to create their own filesystems without writing kernel code. The vulnerability specifically manifests when the system attempts to update the /etc/mtab file, a critical component that maintains a list of currently mounted filesystems and their properties.
The technical flaw stems from insufficient validation and handling of symbolic link attacks during the mount table update process. When fuse attempts to write to /etc/mtab and encounters issues with file access or permissions, the system fails to properly validate the target of the update operation. This lack of proper validation creates an opportunity for local attackers to manipulate the mount table through carefully crafted symbolic links. The attacker can create a symlink pointing to any directory they wish to unmount, effectively bypassing normal mount table restrictions and gaining unauthorized access to filesystem operations.
The operational impact of this vulnerability is substantial as it allows local users to escalate their privileges and perform unauthorized filesystem operations. An attacker with minimal privileges can exploit this weakness to unmount arbitrary directories, potentially disrupting system services or gaining access to sensitive data that should be protected. This vulnerability particularly affects systems where fuse is used for user-space filesystem implementations, which are common in modern Linux distributions for various applications including network filesystems, compressed filesystems, and custom filesystem solutions. The attack vector is particularly dangerous because it requires only local access and can be executed without elevated privileges, making it a preferred method for privilege escalation attacks.
From a cybersecurity perspective, this vulnerability aligns with CWE-59, which describes improper handling of symbolic links, and relates to ATT&CK technique T1068, which covers exploitation of remote services or local privilege escalation. The vulnerability also demonstrates weaknesses in the principle of least privilege and proper access control mechanisms within the filesystem subsystem. Mitigation strategies should include updating to fuse version 2.8.6 or later, which contains the necessary patches to properly handle mount table updates and validate symbolic link targets. System administrators should also implement proper file permissions and access controls for /etc/mtab, regularly audit filesystem operations, and monitor for suspicious mount table modifications. Additional defensive measures include restricting local user access to fuse-related operations, implementing mandatory access controls, and ensuring proper filesystem integrity checks to prevent unauthorized modifications to critical system files.