What potential pitfalls should be considered when allowing special characters like !@_-*~$& in usernames?

Allowing special characters in usernames can lead to potential security vulnerabilities such as SQL injection attacks or cross-site scripting (XSS) attacks. It can also make it harder for users to remember their usernames or for the system to properly handle and display usernames with special characters. To address this, it is recommended to restrict usernames to alphanumeric characters and possibly a few special characters that are safe to use. 

// Validate username to only allow alphanumeric characters and a few safe special characters
$username = $_POST['username'];

if (!preg_match('/^[a-zA-Z0-9_\-]+$/', $username)) {
    // Invalid username format
    echo "Username can only contain letters, numbers, underscore, and hyphen.";
} else {
    // Username is valid
    // Proceed with storing the username in the database or using it in the application
}

Keywords

SQL injection XSS attacks password hashing input validation user authentication

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