CWE-44: Path Equivalence: 'file.name' (Internal Dot)

What is Path Equivalence: 'file.name' (Internal Dot)?

• Overview: Path Equivalence: 'file.name' (Internal Dot) is a security vulnerability where software improperly handles input paths containing internal dots. This can result in ambiguous path resolution, potentially allowing attackers to navigate the file system to access unauthorized directories or files.

• Exploitation Methods:

• Attackers can exploit this vulnerability by crafting input paths that include internal dots to manipulate the file system navigation.
• Common attack patterns include directory traversal, where an attacker bypasses intended directory restrictions to access sensitive files.

• Security Impact:

• Direct consequences include unauthorized access to protected files or directories.
• Potential cascading effects involve further exploitation of sensitive information obtained through unauthorized access.
• Business impact could be severe, including data breaches, loss of customer trust, and potential legal repercussions.

• Prevention Guidelines:

• Specific code-level fixes involve validating and sanitizing path inputs to ensure they conform to expected formats and do not include ambiguous elements like internal dots.
• Security best practices include implementing strict input validation and employing allow-lists for permissible paths.
• Recommended tools and frameworks involve using well-established libraries that handle path validation securely and consider using static code analysis tools to detect potential vulnerabilities.

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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

def read_file(file_path):
    # Vulnerable to path traversal attacks due to lack of validation on file_path
    with open(file_path, 'r') as file:
        data = file.read()
    return data

# Example of a call that could be exploited
user_input = 'data/../secret.txt'  # Represents a path traversal attempt
print(read_file(user_input))

Explanation

In the vulnerable code above, the read_file function accepts a file path from the user and directly opens it without any validation. An attacker could exploit this by providing a path like data/../secret.txt to traverse directories and access sensitive files. This lack of validation allows for path traversal, potentially leading to unauthorized file access.

How to fix Path Equivalence: 'file.name' (Internal Dot)?

To fix this vulnerability, it's crucial to properly validate and sanitize the file path input. This can be achieved by using functions that resolve and normalize paths, such as os.path.abspath and os.path.normpath, ensuring the path stays within a designated directory. Additionally, compare the resolved path against a trusted base directory to prevent traversal attacks.

Fixed Code Example

import os

def read_file_safe(file_path):
    # Define the base directory to restrict access
    base_dir = os.path.abspath('data')
    
    # Resolve the absolute and normalized path
    full_path = os.path.abspath(os.path.normpath(file_path))
    
    # Ensure the resolved path is within the base directory
    if not full_path.startswith(base_dir + os.sep):
        raise ValueError("Access to the specified path is not allowed.")
        
    with open(full_path, 'r') as file:
        data = file.read()
    return data

# Safe example call
safe_user_input = 'data/file.txt'  # Safe path within the base directory
print(read_file_safe(safe_user_input))

Explanation

In the fixed code, several security measures are implemented:

1. Base Directory Restriction: A base_dir is defined to limit file access to a specific directory, preventing access to files outside this directory.

2. Path Normalization: os.path.abspath and os.path.normpath are used to resolve the real path, eliminating any .. sequences that could be used for directory traversal.

3. Path Validation: The resolved path is checked to ensure it starts with the base directory path (including a trailing separator to prevent prefix attacks). If it doesn't, an exception is raised, preventing unauthorized access.

These changes ensure that the file paths are securely handled, mitigating the risk of path traversal and path equivalence vulnerabilities.