CVE-2011-0543 in fuse
Summary
by MITRE
Certain legacy functionality in fusermount in fuse 2.8.5 and earlier, when util-linux does not support the --no-canonicalize option, allows local users to bypass intended access restrictions and unmount arbitrary directories via a symlink attack.
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Analysis
by VulDB Data Team • 11/18/2021
The vulnerability identified as CVE-2011-0543 resides within the fuse (Filesystem in Userspace) implementation, specifically in the fusermount utility version 2.8.5 and earlier. This flaw represents a critical access control bypass that exploits a race condition in how the system handles symbolic link resolution during mount operations. The vulnerability manifests when the util-linux package lacks support for the --no-canonicalize option, creating an exploitable path where local attackers can manipulate the unmount process through carefully crafted symbolic links.
The technical implementation of this vulnerability stems from improper handling of symbolic links in the fusermount utility's interaction with the underlying mount system. When a user attempts to unmount a filesystem, the fusermount utility performs path resolution operations that do not adequately validate the target directory structure. This weakness allows attackers to create a symbolic link that points to a directory outside the intended scope, effectively enabling them to unmount filesystems that they should not have access to. The flaw operates through a classic symlink attack pattern where the attacker places a symbolic link in a strategic location, and the vulnerable code follows this link without proper canonicalization checks.
The operational impact of this vulnerability extends beyond simple privilege escalation as it fundamentally undermines the security model of the filesystem mounting system. Local users can leverage this vulnerability to unmount arbitrary directories, potentially disrupting system services, accessing restricted data, or creating denial of service conditions. The attack requires local system access but does not need elevated privileges, making it particularly dangerous in multi-user environments where users might have legitimate access to the system but should not be able to manipulate critical filesystem operations. This vulnerability aligns with CWE-367, which addresses Time-of-Check to Time-of-Use (TOCTOU) race conditions, and represents a clear example of improper privilege management in system utilities.
Mitigation strategies for CVE-2011-0543 require immediate patching of the fuse package to version 2.8.6 or later, which includes proper canonicalization checks in the fusermount utility. System administrators should also implement monitoring of mount and unmount operations to detect suspicious activity patterns. The vulnerability demonstrates the importance of proper input validation and canonicalization in system utilities, particularly those involved in filesystem operations. Organizations should consider implementing additional controls such as restricting access to the fusermount utility, monitoring for unusual symlink creation patterns, and ensuring that all system components support the necessary --no-canonicalize option. This vulnerability serves as a reminder of the critical security implications of legacy code in system utilities and the necessity of thorough security reviews of core system components, particularly those operating with elevated privileges. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting the use of system utilities for unauthorized access and modification of system resources.