Question

I’m trying to harden some of my PHP code and use mysqli prepared statements to better validate user input and prevent injection attacks.

I switched away from mysqli_real_escape_string as it does not escape % and _. However, when I create my query as a mysqli prepared statement, the same flaw is still present. The query pulls a users salt value based on their username. I’d do something similar for passwords and other lookups.

Code:

$db = new sitedatalayer();

if ($stmt = $db->_conn->prepare("SELECT `salt` FROM admins WHERE `username` LIKE ? LIMIT 1")) {

  $stmt->bind_param('s', $username);
  $stmt->execute();
  $stmt->bind_result($salt);
  

  while ($stmt->fetch()) { 
    printf("%s\n", $salt);
  } 
    
  $stmt->close();    

}
    
else return false;

• Am I composing the statement correctly?
• If I am what other characters need to be examined? What other flaws are there?
• What is best practice for doing these types of selects?

Thanks,

Answer 1

% is not an inherently harmful character.

The question is: why are you using a LIKE in the first place? Are there any circumstances in which you wouldn’t require an exact match for username?

The query should be simply:

SELECT `salt` FROM admins WHERE `username` = ? LIMIT 1

In that case, if I were to enter %bsmith my username would have to be (literally) “%bsmith” in order for you to find a match.