CVE-2009-0854 in dash
Summary
by MITRE
Untrusted search path vulnerability in dash 0.5.4, when used as a login shell, allows local users to execute arbitrary code via a Trojan horse .profile file in the current working directory.
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Analysis
by VulDB Data Team • 09/02/2019
The vulnerability identified as CVE-2009-0854 represents a critical untrusted search path issue within the dash shell version 0.5.4 when operating in login shell mode. This flaw resides in the shell's handling of environment initialization files, specifically the .profile file that is typically loaded during shell startup. The vulnerability stems from dash's insecure practice of searching for the .profile file in the current working directory before examining standard system locations, creating a dangerous privilege escalation vector for local attackers. When a user logs into a system using dash as their login shell, the shell attempts to locate and execute the .profile file, but due to the flawed search order, it first checks the current directory for this file. This behavior creates an opportunity for malicious actors to place a specially crafted .profile file in any directory where they have write access, thereby executing arbitrary code with the privileges of the targeted user. The vulnerability is particularly severe because it leverages the trusted nature of login shell initialization processes, which are typically considered safe and controlled components of the operating system environment. This flaw directly maps to CWE-426, which describes untrusted search path vulnerabilities where programs search for files in insecure locations, and aligns with ATT&CK technique T1068 which covers privilege escalation through insecure file permissions and search paths. The impact extends beyond simple code execution to potentially allow attackers to escalate privileges, access sensitive data, or establish persistent access to compromised systems.
The operational implications of this vulnerability are significant for system administrators and security practitioners. Any local user who can write to a directory that will be used as a login shell working directory can exploit this weakness to execute arbitrary commands. This includes scenarios where users have write permissions to shared directories, temporary filesystem areas, or any location where they might establish a login session. The vulnerability is particularly dangerous in multi-user environments where users might have access to directories they should not be able to compromise. The attack vector requires minimal privileges to exploit and can be executed silently without alerting system monitoring mechanisms. Systems using dash 0.5.4 as a login shell are at risk regardless of whether they are running in single-user or multi-user modes, as the flaw exists in the shell's fundamental file resolution mechanism. The vulnerability demonstrates a classic security principle where trusted system components become attack vectors due to improper implementation of file resolution processes, creating a fundamental weakness in the system's privilege model. This type of vulnerability is often overlooked in security audits because it appears to be a legitimate shell behavior that developers may not consider as a security risk.
Mitigation strategies for CVE-2009-0854 require both immediate remediation and long-term architectural improvements to prevent similar vulnerabilities. The primary and most effective solution involves upgrading to a newer version of dash that properly implements secure search path resolution, ensuring that system directories are always searched before current working directories. System administrators should conduct comprehensive audits to identify all systems using dash 0.5.4 as a login shell and prioritize their remediation. Additionally, implementing strict file permissions and directory access controls can help reduce the attack surface, though this approach alone does not fully address the core vulnerability. The use of secure shell initialization practices such as setting explicit paths in shell configuration files and avoiding reliance on potentially compromised current working directories provides additional defense layers. Security monitoring should include detection of suspicious .profile file modifications in user directories, particularly those that might indicate privilege escalation attempts. Organizations should also consider implementing automated patch management systems to ensure timely updates of shell components and other system utilities. The vulnerability highlights the importance of secure coding practices and the need for thorough security reviews of system components that handle user input or environment variables. This particular flaw demonstrates how seemingly innocuous shell behaviors can create significant security risks when not properly implemented with security considerations in mind. The remediation process should also include user education about the risks of executing code from untrusted directories and the importance of maintaining secure working environments.