CWE-37: Path Traversal: '/absolute/pathname/here'

What is Path Traversal: '/absolute/pathname/here'?

• Overview: This vulnerability occurs when a software application accepts and processes input in the form of an absolute path without adequate validation, allowing attackers to navigate the file system and access files or directories outside the intended scope.

• Exploitation Methods:

• Attackers can exploit this vulnerability by crafting input that includes absolute paths to sensitive files.
• Common attack patterns include injecting path traversal sequences to access configuration files, user data, or system files.

• Security Impact:

• Direct consequences include unauthorized access to sensitive files and data leakage.
• Potential cascading effects include privilege escalation, service disruption, and further system compromise.
• Business impact may involve data breaches, legal liabilities, and damage to reputation.

• Prevention Guidelines:

• Implement input validation to ensure only expected file paths are processed.
• Use libraries and frameworks that handle path normalization and restrict access to specific directories.
• Recommended tools and frameworks include security-focused libraries that sanitize file paths and enforce directory constraints.

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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_file(filename):
    # Vulnerable to Path Traversal: An attacker can provide an absolute path
    # or use '../' to access unauthorized files on the server.
    with open(filename, 'r') as file:
        return file.read()

# Example usage
user_input = "/etc/passwd"  # Dangerous user input
data = read_file(user_input)

Explanation

In the vulnerable code example, the read_file function directly uses the filename provided by the user to open a file. This approach is risky because:

• An attacker can supply an absolute path to access sensitive files outside the intended directory.
• By using path traversal sequences like ../, an attacker can navigate to parent directories and access unauthorized files.

How to fix Path Traversal: '/absolute/pathname/here'?

To fix the Path Traversal vulnerability, it's crucial to validate and sanitize user input. The main goal is to ensure that the input does not contain any path traversal sequences like ../ that allow access to unauthorized directories. This can be achieved by:

1. Using a Whitelist Approach: Restrict access to a predefined set of directories or files.
2. Path Normalization: Convert the path to an absolute path and ensure it stays within a designated directory.
3. Avoiding Directly Using User Input in File Paths: Concatenate user input only after validation and within a controlled environment.

Fixed Code Example

import os

def read_file_safe(filename):
    # Define a safe base directory
    base_dir = "/safe/directory"

    # Normalize the path to prevent path traversal
    safe_path = os.path.join(base_dir, os.path.normpath(filename))
    
    # Ensure the final path is within the intended directory
    if not os.path.commonpath([os.path.abspath(base_dir), os.path.abspath(safe_path)]) == os.path.abspath(base_dir):
        raise ValueError("Attempted path traversal attack detected")

    with open(safe_path, 'r') as file:
        return file.read()

# Example usage
user_input = "file.txt"  # User input is now relative to the safe directory
data = read_file_safe(user_input)

Explanation

In the fixed code, the read_file_safe function:

• Defines a Safe Base Directory: All file accesses are restricted to a specific directory.
• Normalizes the Path: Uses os.path.normpath to clean the path and remove any traversal sequences.
• Validates the Path: Checks that the resolved path is within the intended directory using os.path.commonpath, which compares the base directory and the resolved path to ensure no path traversal is possible.
• Raises an Exception: If the path is outside the intended directory, a ValueError is raised to prevent unauthorized access.

This approach ensures that the application is protected against path traversal attacks by validating and controlling file path inputs effectively.