CVE-2005-0990 in sharutils
Summary
by MITRE
unshar (unshar.c) in sharutils 4.2.1 allows local users to overwrite arbitrary files via a symlink attack on the unsh.X temporary file.
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Analysis
by VulDB Data Team • 06/04/2019
The vulnerability described in CVE-2005-0990 represents a classic race condition and symbolic link attack scenario that affects the unshar utility within sharutils version 4.2.1. This flaw enables local attackers to potentially overwrite arbitrary files on the system by exploiting the insecure handling of temporary files during the unshar process. The vulnerability specifically targets the unsh.X temporary file that is created during decompression operations, making it a significant concern for system security and integrity.
The technical implementation of this vulnerability stems from the improper management of temporary file creation and access permissions within the unshar utility. When the unshar command processes a shar file, it creates a temporary file named unsh.X in the current working directory without sufficient security measures to prevent symbolic link attacks. The flaw occurs because the program first creates the temporary file and then opens it for writing, creating a window where a malicious user can substitute the temporary file with a symbolic link pointing to a target file of their choice. This race condition allows attackers to manipulate the file system and overwrite critical system files or user data. The vulnerability is classified under CWE-367 which specifically addresses Time-of-Check to Time-of-Use (TOCTOU) flaws, where the state of a resource changes between the time it is checked and when it is used.
From an operational impact perspective, this vulnerability poses a serious threat to system integrity and confidentiality. Local attackers can leverage this flaw to overwrite any file that the user running the unshar command has write permissions to, potentially leading to privilege escalation, data corruption, or even system compromise. The attack vector is particularly dangerous because it requires minimal privileges and can be executed by any local user who can run the unshar utility. This vulnerability undermines the principle of least privilege and can be exploited to modify system configuration files, replace executables, or corrupt important data files, making it a critical concern for system administrators and security professionals.
The mitigation strategies for this vulnerability involve both immediate and long-term solutions to address the underlying security flaw. System administrators should immediately upgrade to a patched version of sharutils where the temporary file handling has been corrected to use secure file creation methods such as mkstemp() or similar atomic operations that prevent symbolic link attacks. Additionally, proper file permission management and directory access controls should be implemented to limit the ability of local users to manipulate temporary files in sensitive locations. The vulnerability aligns with ATT&CK technique T1059 which covers command and scripting interpreter usage, as attackers may use this vulnerability to execute malicious commands through compromised temporary files. Organizations should also implement monitoring and logging of temporary file creation activities to detect potential exploitation attempts. Regular security audits and vulnerability assessments should include checks for similar TOCTOU vulnerabilities in other system utilities, as this pattern of insecure temporary file handling has been observed in numerous other software components throughout the industry.