virtualenv.py in virtualenv before 1.5 allows local users to overwrite arbitrary files via a symlink attack on a certain file in /tmp/.
Virtualenv versions prior to 1.5 are vulnerable to a symlink attack, allowing local users to overwrite arbitrary files. This vulnerability can lead to privilege escalation or system compromise by manipulating the creation of files within the virtual environment's temporary directory, potentially allowing attackers to inject malicious code or overwrite critical system files.
Step 1: Target Selection: The attacker identifies a target file that they want to overwrite. This could be a configuration file, a script, or any other file that, when modified, could lead to privilege escalation or system compromise.
Step 2: Symlink Creation: The attacker creates a symbolic link (symlink) in the /tmp/ directory, pointing to the target file. The attacker must create this symlink before the virtualenv script attempts to create a file with the same name in /tmp/.
Step 3: Virtualenv Execution: The attacker triggers the execution of the vulnerable virtualenv script, typically by creating or activating a virtual environment.
Step 4: File Creation Race: The virtualenv script attempts to create a file with a predictable name in the /tmp/ directory. Because the symlink already exists, the file creation operation writes to the target file instead.
Step 5: File Overwrite: The attacker's crafted data is written to the target file, overwriting its original contents. This could include malicious code, configuration changes, or other data designed to compromise the system.
The vulnerability stems from a race condition in how virtualenv handles file creation within the /tmp/ directory. Specifically, the script creates temporary files and directories during the virtual environment setup process. An attacker can exploit this by creating a symlink pointing to a target file before virtualenv attempts to create a file with the same name in /tmp/. When virtualenv creates the file, it actually writes to the target file pointed to by the symlink, allowing the attacker to overwrite it. The root cause is the lack of proper validation and sanitization of file paths and the absence of checks to prevent symlink attacks during the file creation process. The use of a predictable temporary directory and file naming scheme further exacerbates the vulnerability, making it easier for attackers to predict and target specific files.