CVE-2017-5940 in Firejail
Summary
by MITRE
Firejail before 0.9.44.6 and 0.9.38.x LTS before 0.9.38.10 LTS does not comprehensively address dotfile cases 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. NOTE: this vulnerability exists because of an incomplete fix for CVE-2017-5180.
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Analysis
by VulDB Data Team • 11/12/2022
The vulnerability identified as CVE-2017-5940 represents a critical sandbox escape flaw in Firejail, a security sandboxing tool designed to isolate applications and prevent unauthorized access to system resources. This issue affects versions prior to 0.9.44.6 and the 0.9.38.x LTS branch before 0.9.38.10 LTS, demonstrating how security fixes can sometimes introduce incomplete protections that leave systems vulnerable to sophisticated attacks. The flaw specifically targets Firejail's handling of dotfiles during its attempt to prevent access to user files when running with an effective user ID of zero, creating a pathway for malicious actors to bypass intended security boundaries.
The technical implementation of this vulnerability stems from Firejail's inadequate handling of symbolic link scenarios within its private directory creation process. When utilizing the --private option, Firejail attempts to create isolated environments by establishing private directories for applications while preventing access to sensitive user files. However, the implementation fails to properly account for dotfile cases where symbolic links can be manipulated to traverse the intended security boundaries. Attackers can exploit this by creating carefully crafted symbolic links that point to sensitive files outside the intended sandbox, effectively allowing them to access restricted resources despite the sandboxing protections.
This vulnerability operates under the attack pattern described in the ATT&CK framework under T1068 - Exploitation for Privilege Escalation, specifically leveraging the technique of privilege escalation through sandbox escape mechanisms. The flaw represents a direct violation of the principle of least privilege that Firejail is designed to enforce, allowing local users to potentially access files and resources that should remain isolated within the sandboxed environment. The incomplete fix for CVE-2017-5180, which was the previous vulnerability in this series, demonstrates how security patches can sometimes address symptoms rather than root causes, leaving residual attack vectors that can be exploited by determined adversaries.
The operational impact of CVE-2017-5940 extends beyond simple information disclosure, as it provides attackers with the ability to potentially execute arbitrary code or escalate privileges within the compromised environment. When combined with other attack vectors, this vulnerability could enable attackers to gain unauthorized access to sensitive data, modify system files, or establish persistent access to systems running vulnerable versions of Firejail. The local nature of this attack means that any user with access to the system can potentially exploit this flaw, making it particularly dangerous in multi-user environments or systems where privilege separation is critical. Organizations using Firejail for application isolation and security hardening are particularly at risk, as this vulnerability undermines the fundamental security model that the tool is designed to provide.
Mitigation strategies for CVE-2017-5940 require immediate patching of affected Firejail versions to the latest stable releases that contain the complete fix for this vulnerability. System administrators should also implement additional monitoring and logging of Firejail usage to detect potential exploitation attempts. The CWE database categorizes this issue under CWE-284 - Improper Access Control, which highlights the core problem of inadequate privilege management within the sandboxing mechanism. Organizations should also consider implementing additional security controls such as mandatory access controls, file integrity monitoring, and privilege separation mechanisms to reduce the impact of such vulnerabilities in their environments. Regular security assessments and vulnerability scanning should be conducted to identify and remediate similar issues in other sandboxing tools and security mechanisms within the organization's infrastructure.