CVE-2017-5180 in Firejail
Summary
by MITRE
Firejail before 0.9.44.4 and 0.9.38.x LTS before 0.9.38.8 LTS does not consider the .Xauthority case during its attempt to prevent accessing user files with an euid of zero, which allows local users to conduct sandbox-escape attacks via vectors involving a symlink and the --private option.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 12/25/2024
The vulnerability identified as CVE-2017-5180 affects Firejail versions prior to 0.9.44.4 and the 0.9.38.x LTS branch before 0.9.38.8 LTS, representing a critical sandbox escape flaw that undermines the security model of this Linux sandboxing tool. Firejail is designed to provide lightweight sandboxing capabilities by isolating applications from the host system through various containment mechanisms including user namespace separation and file system restrictions. The vulnerability specifically targets the tool's handling of X11 authentication files during the sandboxing process, creating a pathway for local attackers to bypass security controls and escalate privileges.
The technical flaw stems from Firejail's inadequate handling of the .Xauthority file during sandbox initialization when the --private option is employed. This option is intended to create a private home directory for the sandboxed application, but the implementation fails to properly account for the .Xauthority file's special case when the effective user ID (euid) is zero. When Firejail attempts to prevent access to user files with euid zero, it does not adequately consider the .Xauthority file's potential for symlink manipulation, allowing malicious actors to exploit this gap in the security model. The vulnerability leverages the fact that .Xauthority files contain authentication tokens that enable X11 access, and when properly manipulated through symbolic links, they can provide unauthorized access to the host system's X11 server.
The operational impact of this vulnerability is significant as it allows local users to conduct sandbox-escape attacks that can potentially lead to full system compromise. Attackers can exploit this flaw by creating a malicious symlink in the sandboxed environment that points to sensitive host files, particularly those related to X11 authentication. This creates a scenario where a sandboxed application running with euid zero can access files that should normally be restricted, effectively breaking the isolation provided by Firejail. The attack vector specifically involves the combination of symlink manipulation and the --private option, which together create a condition where the sandbox's security boundaries are crossed, potentially allowing access to the user's X11 authentication tokens and subsequent unauthorized access to graphical sessions.
This vulnerability aligns with CWE-22 (Improper Limitation of a Pathname to a Restricted Directory) and CWE-264 (Permissions, Privileges, and Access Controls) categories, as it demonstrates both path traversal issues and improper privilege handling. From an ATT&CK framework perspective, this vulnerability maps to T1068 (Local Port Forwarding) and T1133 (External Remote Services) techniques, as it enables unauthorized access to graphical services that would normally be isolated. The flaw also relates to T1059 (Command and Scripting Interpreter) and T1078 (Valid Accounts) as attackers can leverage the compromised X11 access to execute commands or gain persistent access through graphical interfaces. The vulnerability demonstrates a fundamental flaw in the sandboxing implementation where the security model's assumptions about file access patterns are incorrect, leading to a complete bypass of the intended isolation mechanisms.
The recommended mitigations for this vulnerability include immediate upgrade to Firejail version 0.9.44.4 or the 0.9.38.8 LTS release, which contain the necessary patches to address the .Xauthority handling issue. System administrators should also implement additional monitoring for suspicious symlink creation patterns in user directories, particularly around X11-related files. The patch implementation addresses the core issue by ensuring proper handling of .Xauthority files during sandbox initialization, specifically checking for and properly managing symbolic links that could be used to escape the sandbox environment. Organizations should also review their Firejail configurations to ensure that the --private option is not being used in contexts where it could be exploited, and consider implementing additional security controls such as mandatory access controls or additional file system monitoring to detect potential exploitation attempts.