CWE-180: Incorrect Behavior Order: Validate Before Canonicalize

What is Incorrect Behavior Order: Validate Before Canonicalize?

• Overview: CWE-180 refers to a security vulnerability where input validation occurs before the data is canonicalized. This means that the system checks the data for validity before converting it to a standard format, potentially allowing malicious inputs to bypass validation and be accepted as valid after canonicalization.

• Exploitation Methods:

• Attackers can exploit this vulnerability by supplying input that appears valid before canonicalization but becomes malicious afterward.
• Common attack patterns include injection attacks, such as SQL injection or command injection, where the payload bypasses initial validation due to improper sequence of operations.

• Security Impact:

• Direct consequences of successful exploitation include unauthorized access, data breaches, and execution of arbitrary code.
• Potential cascading effects could include further system compromise or lateral movement within a network.
• Business impact could involve loss of sensitive data, damage to reputation, compliance violations, and financial loss.

• Prevention Guidelines:

• Ensure canonicalization of input data occurs before validation to guarantee checks are performed on the final, standardized data.
• Follow security best practices by adopting a 'validate-canonicalize-validate' approach, where input is validated again after canonicalization.
• Use recommended tools and frameworks that support secure input handling and provide automatic 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

import os

def process_file(file_path):
    # Validate the file path to ensure it doesn't contain any illegal characters
    if ".." in file_path or "~" in file_path:
        raise ValueError("Invalid file path provided!")
    
    # Canonicalize the file path
    safe_path = os.path.realpath(file_path)
    
    # Open and process the file
    with open(safe_path, 'r') as file:
        data = file.read()
    return data

Vulnerability Explanation:

• Issue: The code attempts to validate the file path for illegal characters such as ".." and "~" before canonicalizing it. This sequence is vulnerable because the validation is done on the initial path, which might not reflect the true path after canonicalization.
• Risk: An attacker could exploit this by using symbolic links or relative paths to bypass the validation checks, potentially leading to unauthorized file access.

How to fix Incorrect Behavior Order: Validate Before Canonicalize?

To fix this vulnerability, the canonicalization step should be performed first to resolve the actual path, followed by validation. This ensures that the validation checks are applied to the true file path.

Fixed Code Example

import os

def process_file(file_path):
    # Canonicalize the file path first
    safe_path = os.path.realpath(file_path)
    
    # Validate the canonicalized file path to ensure it doesn't point outside the allowed directory
    if ".." in safe_path or "~" in safe_path:
        raise ValueError("Invalid file path provided!")
    
    # Open and process the file
    with open(safe_path, 'r') as file:
        data = file.read()
    return data

Fix Explanation:

• Canonicalization First: The os.path.realpath() function is used first to resolve the true canonical path of the file, addressing any symbolic links or relative path segments.
• Validation After Canonicalization: The validation checks are applied to the canonicalized path, ensuring they accurately reflect the actual file location and prevent unauthorized access.
• Security: This order of operations mitigates the risk of path traversal attacks by ensuring the path is both correct and safe before any file operations occur.