CWE-31: Path Traversal: 'dir\..\..\filename'

What is Path Traversal: 'dir....\filename'?

• Overview: The CWE-31 vulnerability involves improperly handling path traversal inputs such as 'dir....\filename', allowing attackers to break out of a restricted directory and access unauthorized files or directories.

• Exploitation Methods:

• Attackers manipulate inputs to include sequences like '..' to navigate the directory structure upwards and access sensitive files outside the restricted directory.
• Common attack patterns include supplying multiple '..' sequences or positioning them in unexpected parts of the pathname to bypass simple validation checks.

• Security Impact:

• Direct consequences include unauthorized access to system files or data, potentially leading to the disclosure of sensitive information.
• Potential cascading effects include the ability to modify or delete files, execute unauthorized commands, or further exploit the system.
• Business impact might include data breaches, legal liabilities, loss of customer trust, and financial losses due to system compromise.

• Prevention Guidelines:

• Specific code-level fixes involve validating and sanitizing all path inputs, ensuring they cannot resolve to paths outside the intended directory.
• Security best practices include using APIs or libraries designed to handle file paths securely and avoiding concatenating user input directly into file paths.
• Recommended tools and frameworks can include static analysis tools to detect path traversal vulnerabilities and frameworks that provide secure 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

Python Example

import os

def read_user_file(filename):
    base_path = '/var/www/data/'
    # Vulnerable to path traversal: an attacker can use '../' or '..\\' to escape the base path
    file_path = os.path.join(base_path, filename)
    with open(file_path, 'r') as file:
        return file.read()

# Example of a vulnerable call
user_input = '../../etc/passwd'
print(read_user_file(user_input))

Explanation of Vulnerability

The above code is vulnerable to a Path Traversal attack. By using os.path.join, the input filename can include path traversal characters (../ or ..\\) to access files outside the intended directory. This allows an attacker to read arbitrary files on the server, potentially exposing sensitive information.

How to fix Path Traversal: 'dir....\filename'?

To mitigate this vulnerability, you must validate and sanitize the input path to ensure it does not resolve to a location outside the intended directory. This can be achieved by:

1. Normalizing the path to remove any redundant or malicious path components.
2. Checking if the resolved path starts with the intended base directory.
3. Optionally, restricting the filenames to a predefined pattern or list to further ensure safety.

Fixed Code Example

Python Example

import os

def read_user_file(filename):
    base_path = '/var/www/data/'
    # Normalize the path to prevent path traversal attempts
    file_path = os.path.realpath(os.path.join(base_path, filename))
    # Ensure the file path is within the intended directory
    if not file_path.startswith(os.path.realpath(base_path) + os.sep):
        raise ValueError("Invalid file path")
    with open(file_path, 'r') as file:
        return file.read()

# Example of a secure call
user_input = 'user_data.txt'
print(read_user_file(user_input))

Explanation of Fix

• Normalization: The os.path.realpath function is used to resolve the absolute path, which collapses any .. or symbolic links. This ensures the path is fully expanded and canonical.
• Validation: The code checks if the resolved path starts with the base directory path, using os.path.realpath(base_path) to ensure both paths are fully resolved and canonical. The addition of os.sep ensures that partial matches (e.g., /var/www/data2) are not incorrectly validated.
• Sanitization: By allowing only specific filenames or patterns, the risk of path traversal can be further reduced. This example does not implement this step, but it is recommended for additional security.

This improved version ensures that the code is more robust against path traversal attacks by correctly validating the resolved file path.