CWE-54: Path Equivalence: 'filedir\' (Trailing Backslash)

What is Path Equivalence: 'filedir\' (Trailing Backslash)?

• Overview: Path Equivalence with a trailing backslash vulnerability occurs when a software product accepts input paths with a trailing backslash ('filedir') and does not properly validate them. This can lead to ambiguous path resolution, allowing attackers to traverse the file system to unintended locations or access arbitrary files.

• Exploitation Methods:

• Attackers can manipulate input paths using a trailing backslash to bypass security checks and access restricted directories or files.
• Common attack patterns include directory traversal attacks, where attackers append or prepend specific path inputs to navigate the file system outside of the intended directory structure.

• Security Impact:

• Direct consequences include unauthorized access to sensitive files or directories, potentially leading to data breaches.
• Potential cascading effects involve further exploitation of the system, such as privilege escalation or code execution, if sensitive configuration files or executables are accessed.
• Business impact can be severe, including data theft, loss of customer trust, legal repercussions, and financial losses due to unauthorized data exposure.

• Prevention Guidelines:

• Specific code-level fixes involve implementing strict input validation and canonicalization to ensure that paths are resolved to their absolute form without ambiguity.
• Security best practices include using libraries or APIs that handle path operations securely, avoiding manual path concatenation, and employing whitelisting techniques to restrict permissible path inputs.
• Recommended tools and frameworks include static analysis tools to detect path traversal vulnerabilities and security frameworks that provide built-in protection against such issues.

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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(user_input_path):
    # Vulnerable code: accepts user input without validation, allowing path traversal
    with open(user_input_path, 'r') as file:
        return file.read()

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

Explanation:

• Lack of Validation: The code accepts a file path from user input without any validation or sanitization.
• Path Traversal Risk: An attacker could input a path like ../etc/passwd\\ to access unauthorized files, exploiting path traversal vulnerabilities.
• Trailing Backslash Bypass: Inputting paths with trailing backslashes could potentially bypass simplistic validation checks.

How to fix Path Equivalence: 'filedir' (Trailing Backslash)?

To mitigate this vulnerability, implement the following measures:

1. Normalize the Path: Use os.path.normpath() to standardize the path, removing redundant separators and up-level references.
2. Validate Against Base Directory: Ensure the path remains within a designated safe directory by checking its prefix.
3. Convert to Absolute Path: Transform the path to an absolute path and verify it against a whitelist of allowed directories.
4. Remove Trailing Slashes: Explicitly strip trailing slashes or backslashes from the input before further processing.

Fixed Code Example

import os

def read_file(user_input_path):
    # Secure code: normalizes and validates the path
    base_dir = '/safe/directory'  # Define a base directory for file access
    normalized_path = os.path.normpath(user_input_path)
    abs_path = os.path.abspath(normalized_path)  # Convert to absolute path

    # Check if the absolute path starts with the base directory
    if not abs_path.startswith(os.path.abspath(base_dir)):
        raise ValueError("Access denied: Path traversal detected.")
    
    # Remove any trailing slashes or backslashes
    abs_path = abs_path.rstrip('/\\')

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

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

Explanation:

• Normalization and Validation: The input path is normalized using os.path.normpath() and converted to an absolute path to ensure consistent path representation.
• Base Directory Verification: The code checks if the absolute path starts with a predefined base directory, effectively preventing directory traversal attacks.
• Trailing Slash Removal: Stripping trailing slashes or backslashes ensures the path does not inadvertently bypass security checks.
• These improvements ensure the application securely accesses only files within the designated directory, mitigating the risk of path traversal vulnerabilities.