What security vulnerability is mentioned in relation to the login routine in the PHP code?

The security vulnerability mentioned in relation to the login routine in the PHP code is the risk of SQL injection attacks. This vulnerability occurs when user input is not properly sanitized before being used in SQL queries, allowing malicious users to manipulate the query and potentially access or modify sensitive data in the database. To prevent SQL injection attacks, it is important to use prepared statements with parameterized queries to securely handle user input. 

// Implementing prepared statements to prevent SQL injection attacks
$stmt = $pdo->prepare('SELECT * FROM users WHERE username = :username AND password = :password');
$stmt->execute(array('username' => $username, 'password' => $password));

// Check if the user exists
if($stmt->rowCount() > 0) {
    // User authenticated successfully
    // Add code here to handle successful login
} else {
    // Invalid username or password
    // Add code here to handle failed login attempt
}

Keywords

SQL injection password hashing cross-site scripting session hijacking authentication vulnerability

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