CWE-67: Improper Handling of Windows Device Names

What is Improper Handling of Windows Device Names?

• Overview: Improper handling of Windows device names occurs when software does not correctly process pathnames containing special device names (e.g., AUX, CON, PRN). This oversight can lead to denial of service or information exposure as the application mistakenly treats these device names as regular files.

• Exploitation Methods:

• Attackers can exploit this vulnerability by injecting virtual filenames into URLs or file paths.
• Common attack patterns include manipulating input fields or URLs to include device names, causing unexpected application behavior or errors.

• Security Impact:

• Direct consequences include denial of service, where the application crashes or becomes unresponsive.
• Potential cascading effects include exposing sensitive information through error messages or log files.
• Business impact may involve service downtime, loss of customer trust, or regulatory compliance issues.

• Prevention Guidelines:

• Implement strict input validation to filter out or reject Windows device names in user inputs.
• Use whitelist-based validation to ensure only legitimate filenames and paths are accepted.
• Recommended tools and frameworks include input validation libraries that automatically handle special characters and reserved names.

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Technical Details

Likelihood of Exploit: High

Affected Languages: Not Language-Specific

Affected Technologies: Not specified

Historically, there was a bug in the Windows operating system that caused a blue screen of death. Even after that issue was fixed DOS device names continue to be a factor.

Vulnerable Code Example

import os

def open_file(user_input):
    # Vulnerable: Directly using user input to construct file paths
    file_path = os.path.join("C:\\documents", user_input)
    try:
        with open(file_path, 'r') as file:
            data = file.read()
        return data
    except FileNotFoundError:
        return "File not found."
    except Exception as e:
        return str(e)

Explanation of the Vulnerability

The above code is vulnerable to CWE-67 (Improper Handling of Windows Device Names) because it directly uses user input to construct a file path. If a user inputs a Windows device name like CON, AUX, or NUL, it could lead to errors or unwanted behavior since these are reserved device names in Windows. This can result in denial of service or information exposure if these names are processed as regular files. Additionally, the code does not handle directory traversal attacks, which could allow access to files outside the intended directory.

How to fix Improper Handling of Windows Device Names?

To fix this vulnerability, sanitize the user input to ensure it does not contain any reserved Windows device names. Implement a validation check that compares user input against a list of known reserved names and reject or modify input that matches any of these names. Additionally, apply comprehensive path sanitization to prevent directory traversal or other path-related vulnerabilities.

Fixed Code Example

import os

# List of reserved Windows device names
RESERVED_NAMES = {"CON", "PRN", "AUX", "NUL", "COM1", "COM2", "COM3", "COM4", "COM5", 
                  "COM6", "COM7", "COM8", "COM9", "LPT1", "LPT2", "LPT3", "LPT4", "LPT5", 
                  "LPT6", "LPT7", "LPT8", "LPT9"}

def open_file(user_input):
    # Fix: Check if the user input is a reserved device name
    if user_input.upper() in RESERVED_NAMES:
        return "Invalid file name."

    # Further sanitize the input to prevent directory traversal
    user_input = os.path.basename(user_input)

    # Construct the file path in a secure manner
    file_path = os.path.join("C:\\documents", user_input)
    
    try:
        with open(file_path, 'r') as file:
            data = file.read()
        return data
    except FileNotFoundError:
        return "File not found."
    except Exception as e:
        return str(e)

Explanation of the Fix

1. Reserved Names Check: Added a list of reserved Windows device names and checked the user input against this list. If the input matches any reserved names, the function returns an error message, preventing further processing of potentially harmful input.

2. Input Sanitization: Used os.path.basename() to strip any directory components from the user input, which helps prevent directory traversal attacks. This ensures that the input is treated as a simple file name rather than a path.

3. Secure Path Construction: The file path is constructed using os.path.join() after sanitizing the input, ensuring that the path is correctly formed and safe for use.

4. Improved Error Handling: The error message for invalid file names is clear and immediate, preventing further processing of potentially harmful input.