CWE-66: Improper Handling of File Names that Identify Virtual Resources

What is Improper Handling of File Names that Identify Virtual Resources?

• Overview: This vulnerability occurs when a software product incorrectly handles file names that point to "virtual" resources, which are not actual files within the directory structure. These virtual resources appear like normal files but are aliases for other types of resources.

• Exploitation Methods:

• Attackers can exploit this vulnerability by tricking the application into treating a virtual resource as a file, leading to unintended file operations.
• Common attack patterns include using specially crafted file names that reference virtual resources to manipulate the application’s behavior or access unauthorized resources.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized access to resources and the execution of unintended operations.
• Potential cascading effects may involve further compromise of system integrity and confidentiality if the virtual resources are linked to sensitive data or operations.
• Business impact could include data breaches, loss of trust, and potential legal liabilities if sensitive information is exposed.

• Prevention Guidelines:

• Specific code-level fixes involve validating and sanitizing file names to ensure they do not reference unauthorized virtual resources.
• Security best practices include implementing strict access controls and ensuring robust input validation mechanisms.
• Recommended tools and frameworks include static analysis tools to identify improper handling of file names and security libraries that provide safe file handling functions.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Vulnerable Code Example

import os
from flask import Flask, request, send_file

app = Flask(__name__)

@app.route('/download', methods=['GET'])
def download_file():
    # Vulnerable code: Directly using user input without validation
    filename = request.args.get('filename')  # User input is directly used
    filepath = os.path.join('/var/www/files', filename)  # Constructs path without validation
    return send_file(filepath)  # Sends the file without checking path safety

if __name__ == "__main__":
    app.run()

Explanation

In this code, the file to be downloaded is determined by a user-provided filename parameter. This implementation is vulnerable to path traversal attacks, where an attacker can craft a filename like ../../etc/passwd to access sensitive files outside the intended directory. The code does not validate the filename, allowing attackers to potentially access any file on the server.

How to fix Improper Handling of File Names that Identify Virtual Resources?

To fix this vulnerability, we should implement validation to ensure that the filename is within the expected directory. This includes checking that no path traversal sequences (like ..) are present and verifying that the resolved path is within the allowed directory. Additionally, maintaining a whitelist of allowed filenames or using secure libraries for file access can further mitigate risks.

Fixed Code Example

import os
from flask import Flask, request, send_file, abort

app = Flask(__name__)
BASE_DIR = '/var/www/files'

@app.route('/download', methods=['GET'])
def download_file():
    # Secure code: Validate and resolve the file path
    filename = request.args.get('filename')
    if not filename or '..' in filename or filename.startswith('/'):  # Check for invalid patterns
        abort(400, "Invalid filename")  # Reject potentially harmful input
    
    filepath = os.path.join(BASE_DIR, filename)
    # Ensure the final path is within the BASE_DIR
    if not os.path.abspath(filepath).startswith(os.path.abspath(BASE_DIR)):  # Validate path safety
        abort(403, "Access denied")  # Prevent access if path is outside the allowed directory

    return send_file(filepath)

if __name__ == "__main__":
    app.run()

Explanation

In the fixed code, we perform checks to ensure the filename does not contain path traversal patterns and that it does not start with a slash, which could indicate an absolute path. We then verify that the resolved path is within the BASE_DIR. Using os.path.abspath() ensures we're comparing absolute paths, which helps in accurately determining if the path is safe. This prevents attackers from accessing files outside the designated directory, effectively mitigating the path traversal vulnerability.