CWE-626: Null Byte Interaction Error (Poison Null Byte)

What is Null Byte Interaction Error (Poison Null Byte)?

• Overview: A Null Byte Interaction Error occurs when software does not correctly handle null bytes, leading to unexpected behaviors when data is transferred between different software components or languages, such as C and PHP. This can result in security vulnerabilities.

• Exploitation Methods:

• Attackers can exploit this vulnerability by injecting null bytes into input data to manipulate how strings are terminated or processed.
• Common attack patterns include path traversal attacks, where null bytes are used to bypass security checks and access unauthorized files, and manipulation of regular expressions in PHP.

• Security Impact:

• Direct consequences include unauthorized access to sensitive files or data and bypassing security mechanisms.
• Potential cascading effects involve further exploitation of the system, potentially leading to broader system compromise.
• Business impact can range from data breaches and loss of customer trust to legal and regulatory penalties.

• Prevention Guidelines:

• Specific code-level fixes include sanitizing input data to remove null bytes before processing and validating inputs rigorously.
• Security best practices involve adopting secure coding guidelines, performing regular code reviews, and using input validation libraries.
• Recommended tools and frameworks include static analysis tools that can detect null byte vulnerabilities and web application firewalls to block malicious inputs.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: PHP, Perl, ASP.NET

Affected Technologies: Not specified

Vulnerable Code Example

// file_upload.php {10-12}

// Vulnerable code with detailed comments explaining the security issue

// Function to handle file uploads
function uploadFile(\$filename) {
    // Open the file using fopen, which is susceptible to null byte injection
    \$file = fopen(\$filename, 'r'); // This line is vulnerable to null byte injection

    // Perform operations on the file...
    
    fclose(\$file);
}

// Example usage: A user input is directly used as a filename
\$filename = \$_GET['file'];
uploadFile(\$filename); // User input is directly used without validation

Explanation:

• Vulnerability: The code above does not handle null byte (\0) injection correctly. PHP's fopen() function interprets a null byte as the end of a string, which an attacker can exploit by appending a null byte to the filename. This can potentially bypass intended controls or access unauthorized files.

How to fix Null Byte Interaction Error (Poison Null Byte)?

To fix this vulnerability, always sanitize and validate user inputs. Specifically, ensure that any input used in file operations is properly validated to prevent injection of null bytes or other malicious strings. PHP provides built-in functions like filter_var() with the FILTER_SANITIZE_STRING filter, which can help in removing null bytes. Additionally, consider using basename() to strip directory information and realpath() to resolve symlinks and ensure the file path is as expected.

Fixed Code Example

// file_upload.php {10-15}

// Fixed code with comments explaining the security controls implemented

// Function to handle file uploads securely
function uploadFile(\$filename) {
    // Sanitize the filename to remove null bytes and other malicious content
    \$sanitizedFilename = filter_var(\$filename, FILTER_SANITIZE_STRING);

    // Use basename to avoid directory traversal attacks
    \$sanitizedFilename = basename(\$sanitizedFilename);

    // Use realpath to get the absolute path and confirm it's within the allowed directory
    \$filePath = '/var/www/uploads/' . \$sanitizedFilename;
    \$realPath = realpath(\$filePath);

    // Ensure the resolved path is within the intended directory
    if (\$realPath === false || strpos(\$realPath, '/var/www/uploads/') !== 0) {
        die('Invalid file path!'); // Prevent unauthorized access
    }

    // Open the file securely
    \$file = fopen(\$realPath, 'r');

    // Perform operations on the file...
    
    fclose(\$file);
}

// Example usage: Sanitize and validate user input
\$filename = \$_GET['file'];
uploadFile(\$filename); // Input is sanitized and validated before use

Explanation:

• Sanitization and Validation: The input is sanitized using filter_var() with FILTER_SANITIZE_STRING to remove null bytes and other potentially harmful characters.
• Path Handling: Using basename() ensures that only the file name is used, preventing directory traversal attacks.
• Path Validation: realpath() is used to resolve the absolute path, and the code checks if the resolved path is within the intended directory to prevent unauthorized access.
• Security Controls: The combination of these practices mitigates the risk of null byte injection and ensures that file operations are conducted in a secure manner.