CWE-621: Variable Extraction Error

What is Variable Extraction Error?

• Overview: This vulnerability occurs when the software uses external input to determine variable names for extracting information without validating those names. This can lead to overwriting variables unintentionally, affecting program behavior.

• Exploitation Methods:

• Attackers can exploit this by providing manipulated input to control variable names.
• Common techniques include using functions like PHP's extract() to overwrite critical variables or superglobals.

• Security Impact:

• Direct consequences include altered application state or logic due to overwritten variables.
• Potential cascading effects include unauthorized access to application data or functionality.
• Business impact may involve data breaches, service disruptions, or unauthorized transactions.

• Prevention Guidelines:

• Avoid using functions like extract() with untrusted input.
• Implement strict input validation and sanitization practices.
• Use secure coding frameworks and tools that emphasize input validation and variable scope control.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: PHP

Affected Technologies: Not specified

Vulnerable Code Example

Certainly! Below is the improved content with the necessary corrections and enhancements:

<?php
// Vulnerable code demonstrating CWE-621: Variable Extraction Error
// The extract function is used here without validating the keys of the input array
function processUserData(\$userInput) {
    // Assuming \$userInput is an associative array from an external source
    extract(\$userInput); // This line can introduce security issues
    
    // The rest of the code assumes certain variables are set, like \$username
    echo "Welcome, \$username!";
}

\$userData = \$_GET; // External input coming from URL parameters
processUserData(\$userData);
?>

Explanation:

• In this example, the extract() function is used on potentially unsafe external input (\$_GET array).
• This function imports variables into the local symbol table from an associative array, which could overwrite existing variables.
• An attacker could manipulate the URL parameters to set unexpected values or overwrite important variables, leading to security vulnerabilities.

How to fix Variable Extraction Error?

To fix this vulnerability, avoid using extract() on untrusted data. Instead, manually assign variables from the input array after validating and sanitizing them. This ensures that only the expected variables are created and prevents the possibility of overwriting important variables unintentionally.

Fixed Code Example

<?php
// Secure code demonstrating the fix for CWE-621: Variable Extraction Error
function processUserData(\$userInput) {
    // Manually assign variables from the array after validation
    \$username = isset(\$userInput['username']) ? htmlspecialchars(\$userInput['username']) : 'Guest';
    
    // Further input validation and sanitization can be done here
    echo "Welcome, \$username!";
}

\$userData = \$_GET; // External input
processUserData(\$userData);
?>

Key Fixes:

• Manual Assignment: Instead of using extract(), manually assign each variable from the input array. This prevents arbitrary variables from being created.
• Input Validation and Sanitization: Use functions like htmlspecialchars() to sanitize inputs, protecting against XSS and ensuring only safe characters are used.
• Default Values: Provide default values to handle cases where expected keys are missing from the input.

By applying these practices, you can prevent variable extraction errors and enhance the security of your PHP applications.