CVE-2008-4956 in fwbuilder
Summary
by MITRE
fwb_install in fwbuilder 2.1.19 allows local users to overwrite arbitrary files via a symlink attack on a /tmp/ssh-agent.##### temporary file.
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Analysis
by VulDB Data Team • 08/03/2021
The vulnerability identified as CVE-2008-4956 resides within the fwbuilder 2.1.19 software suite, specifically within the fwb_install component that handles firewall configuration installation processes. This flaw represents a classic race condition and insecure temporary file handling vulnerability that enables local attackers to escalate privileges through a carefully crafted symbolic link attack. The vulnerability occurs when the fwb_install utility creates temporary files in the /tmp directory without proper security checks, making it susceptible to exploitation by malicious users who can manipulate the file system during the creation process.
The technical implementation of this vulnerability stems from the insecure handling of temporary files in the /tmp/ssh-agent location, which serves as a critical point of attack for privilege escalation. When fwb_install executes and attempts to create or modify files in the temporary directory, it does not validate whether the target file path is a symbolic link or if it points to a location where an attacker can control the file content. This behavior directly violates the principle of least privilege and proper file system security practices, as the application creates temporary files without ensuring their exclusivity or verifying their integrity before use. The vulnerability is classified under CWE-377 as "Insecure Temporary File" and aligns with CWE-378, which addresses "Creation of Temporary File With Insecure Permissions" and CWE-379, "Creation of Temporary File and Directory with Insecure Permissions."
The operational impact of this vulnerability extends beyond simple file overwriting, as it provides local users with a potential path to escalate privileges within the system. An attacker who gains access to a low-privilege account can exploit this weakness to overwrite critical system files or configuration data that might be owned by the root user or other privileged accounts. The attack vector is particularly concerning because it leverages the /tmp directory, which is typically world-writable and accessible to all users on Unix-like systems. This vulnerability can be exploited in various scenarios including privilege escalation attacks, data corruption, or even persistent backdoor installation if the compromised application has elevated permissions. The attack requires minimal privileges and can be executed by any local user with access to the system, making it a significant security concern for environments where multiple users share the same system.
Mitigation strategies for this vulnerability involve both immediate patching and operational security improvements. The most effective solution is to upgrade to a patched version of fwbuilder that properly handles temporary file creation with secure permissions and atomic operations. System administrators should implement proper file system permissions, ensuring that temporary directories have appropriate access controls and that applications create temporary files with secure, exclusive access modes. The use of secure temporary file creation functions such as mkstemp() or mkdtemp() should be enforced to prevent symbolic link attacks, as these functions create files with exclusive access and return an error if the file already exists. Additionally, the principle of least privilege should be enforced by running applications with minimal required permissions and by implementing proper file system auditing to detect unauthorized file modifications. This vulnerability aligns with attack techniques described in the MITRE ATT&CK framework under privilege escalation tactics, specifically targeting the T1068 technique for "Exploitation for Privilege Escalation" and T1548.001 for "Abuse Elevation Control Mechanism" through insecure file handling practices. Organizations should also consider implementing monitoring solutions that track temporary file creation and modification activities in critical system directories to detect potential exploitation attempts.