CVE-2016-10120 in Firejail
Summary
by MITRE
Firejail uses 0777 permissions when mounting (1) /dev, (2) /dev/shm, (3) /var/tmp, or (4) /var/lock, which allows local users to gain privileges.
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Analysis
by VulDB Data Team • 08/30/2020
The vulnerability identified as CVE-2016-10120 affects Firejail, a security sandboxing tool designed to restrict application access to system resources through Linux namespaces and seccomp-bpf. This flaw stems from Firejail's improper handling of mount operations during the sandbox initialization process, specifically when creating mount points for critical system directories including /dev, /dev/shm, /var/tmp, and /var/lock. The security issue arises from the tool's use of overly permissive 0777 file permissions during these mount operations, which creates a significant privilege escalation vector for local attackers.
The technical implementation of this vulnerability involves Firejail's failure to properly restrict permissions on mounted filesystems within the sandbox environment. When Firejail initializes a sandbox, it automatically mounts several critical system directories to provide necessary functionality while maintaining isolation. However, during this process, the tool assigns world-writable permissions to the mount points, allowing any local user to modify or replace the mounted filesystems. This misconfiguration directly violates the principle of least privilege and creates a pathway for attackers to manipulate the sandboxed environment. The 0777 permissions grant read, write, and execute access to all users, effectively nullifying the security boundaries that Firejail intends to establish.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it fundamentally undermines the security model that Firejail is designed to enforce. Local users can exploit this flaw to gain elevated privileges by replacing the mounted filesystems with malicious equivalents or by directly modifying the contents of these directories. This creates opportunities for attackers to escalate their privileges from standard user level to root access, particularly when the sandboxed applications themselves run with elevated capabilities. The vulnerability affects systems where Firejail is used to create restricted environments for applications, making it particularly dangerous in multi-user systems or environments where applications require elevated privileges for legitimate operations.
From a cybersecurity perspective, this vulnerability aligns with CWE-276, which describes improper file permissions, and represents a classic case of privilege escalation through insecure file handling. The flaw also maps to ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation," and T1548.001, covering "Abuse Elevation Control Mechanism." The vulnerability demonstrates how seemingly minor configuration issues in security tools can create significant security holes, as Firejail's design philosophy relies on creating isolated environments where such flaws can be exploited to bypass the intended security boundaries. Organizations using Firejail for application sandboxing should consider this vulnerability as a critical threat that could allow attackers to undermine the entire sandboxing strategy.
Mitigation strategies for this vulnerability involve immediate patching of Firejail to properly restrict mount permissions to appropriate levels, typically limiting access to the owning user or specific groups rather than granting universal permissions. System administrators should also implement monitoring for suspicious mount operations and file system changes in the affected directories. Additionally, organizations should review their Firejail configurations to ensure that sandboxed applications are not running with unnecessary elevated privileges and that proper access controls are maintained throughout the system. The fix typically involves modifying the mount operations to use restrictive permissions that maintain the security isolation while preserving functional requirements.