CWE-30: Path Traversal: '\dir\..\filename'

What is Path Traversal: '\dir..\filename'?

• Overview: CWE-30, a path traversal vulnerability, occurs when software constructs a file path using untrusted input without properly sanitizing it, allowing attackers to access files outside the intended directory by exploiting sequences like '\dir..\filename'.

• Exploitation Methods:

• Attackers can input crafted strings containing '..' to bypass directory restrictions and access unauthorized files.
• Common attack patterns include inserting '..' sequences to exploit path validation checks that only look for directory traversal at the input's start.

• Security Impact:

• Direct consequences include unauthorized access to sensitive files or directories.
• Potential cascading effects include privilege escalation, data leakage, and system compromise.
• Business impact involves data breaches, loss of customer trust, and potential legal liabilities.

• Prevention Guidelines:

• Specific code-level fixes include validating and sanitizing all file path inputs and canonicalizing paths before use.
• Security best practices involve implementing strict input validation and using whitelisting for allowed paths.
• Recommended tools and frameworks include static code analysis tools to detect path traversal vulnerabilities and libraries that securely handle file paths.

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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_name):
    base_directory = "/var/www/files/"
    # Vulnerable to path traversal attacks as it directly uses user input
    file_path = os.path.join(base_directory, file_name)
    with open(file_path, "r") as file:
        return file.read()

Explanation:

• Direct User Input Usage: The code constructs a file path directly using user input (file_name) without any validation, making it vulnerable to path traversal attacks.
• Potential Exploit: An attacker could pass input like ../etc/passwd, allowing them to read files outside the intended directory, leading to unauthorized file access.

How to fix Path Traversal: '\dir..\filename'?

To prevent path traversal vulnerabilities, always validate and sanitize user inputs. Approaches include:

1. Sanitizing Input: Remove or neutralize any suspicious path traversal sequences from user input.
2. Restricting Access: Use secure libraries or functions to ensure the path remains within a designated directory.
3. Canonicalization: Resolve the path to its absolute form and ensure it starts with the expected base directory.

Fixed Code Example

import os

def read_file(file_name):
    base_directory = "/var/www/files/"
    
    # Resolve the absolute path and ensure it remains within the base directory
    file_path = os.path.abspath(os.path.join(base_directory, file_name))
    
    # Check if the resolved path starts with the base directory to prevent traversal
    if not file_path.startswith(os.path.abspath(base_directory) + os.sep):
        raise ValueError("Access Denied: Attempt to access unauthorized file path.")
    
    # Securely open the file if validation passes
    with open(file_path, "r") as file:
        return file.read()

Explanation:

• Absolute Path Resolution: The use of os.path.abspath() provides a normalized absolute path of the file, which is crucial for verification.
• Path Verification: The check using startswith() ensures that the resolved path is within the allowed base directory. The addition of os.sep ensures that partial directory names do not bypass the check.
• Exception Handling: Raises a ValueError if the path validation fails, effectively preventing unauthorized access attempts and providing feedback on access denial.