CWE-39: Path Traversal: 'C:dirname'

What is Path Traversal: 'C:dirname'?

• Overview: This vulnerability occurs when software improperly handles input that includes a drive or volume letter (e.g., 'C:dirname'), which can lead to unintended access to files or directories.

• Exploitation Methods:

• Attackers can exploit this vulnerability by providing crafted input that manipulates path resolution, potentially accessing unauthorized files or directories.
• Common techniques include using relative paths with drive letters to bypass intended directory restrictions.

• Security Impact:

• Direct consequences include unauthorized file access, data leaks, or modification of sensitive files.
• Potential cascading effects could involve privilege escalation or further compromise of the system.
• Business impact may include data breaches, loss of user trust, legal penalties, and financial losses.

• Prevention Guidelines:

• Implement strict input validation to reject any input containing drive or volume letters unless explicitly required.
• Use canonicalization to resolve and sanitize file paths before use.
• Recommended tools and frameworks include static analysis tools to detect potential path traversal vulnerabilities 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

Python Example

import os

def read_file(file_path):
    # This function takes a file path as input and reads its contents
    # Vulnerable to path traversal attacks: an attacker can supply a path like '../etc/passwd'
    # or 'C:dirname' to access arbitrary files on the server
    with open(file_path, 'r') as file:
        return file.read()

# Example usage
user_input = input("Enter the file path to read: ")
print(read_file(user_input))

Explanation

In this vulnerable code example, the read_file function directly uses the file_path provided by the user to open a file. This approach is susceptible to path traversal attacks, where an attacker can input paths like ../etc/passwd or C:dirname to access sensitive files outside the intended directory. The code lacks any validation or sanitization of the input path, making it insecure.

How to fix Path Traversal: 'C:dirname'?

To mitigate this vulnerability, ensure that the file paths are restricted to a specific directory and do not allow traversal to unintended locations. Use libraries that handle path normalization and validation to prevent access to unauthorized files. In Python, the os.path module provides utilities like os.path.abspath and os.path.commonpath to enforce directory constraints.

Fixed Code Example

import os

def read_file(file_path):
    # Define a safe root directory for file access
    SAFE_ROOT = os.path.abspath('/safe_directory')
    
    # Normalize the path to prevent traversal outside the safe directory
    full_path = os.path.abspath(file_path)
    
    # Check if the resolved path is within the safe root directory
    if not os.path.commonpath([SAFE_ROOT, full_path]) == SAFE_ROOT:
        raise ValueError("Access to the file path is not allowed.")
    
    # Safely read the file if validation passes
    with open(full_path, 'r') as file:
        return file.read()

# Example usage
user_input = input("Enter the file path to read: ")
try:
    print(read_file(user_input))
except ValueError as e:
    print(e)

Explanation

In the fixed code example, we define a SAFE_ROOT directory and ensure that all file operations are confined within this directory. We use os.path.abspath to normalize the user-supplied path and os.path.commonpath to verify that the resolved path is indeed within the SAFE_ROOT directory. This approach prevents unauthorized access to files outside the designated directory, effectively mitigating the path traversal vulnerability.