How is this MySQL query vulnerable to SQL injection?

Question

In a comment on a previous question, someone said that the following sql statement opens me up to sql injection:

select
    ss.*,
    se.name as engine,
    ss.last_run_at + interval ss.refresh_frequency day as next_run_at,
    se.logo_name    
from 
    searches ss join search_engines se on ss.engine_id = se.id
where
    ss.user_id='.$user_id.'
group by ss.id
order by ss.project_id, ss.domain, ss.keywords

Assuming that the $userid variable is properly escaped, how does this make me vulnerable, and what can I do to fix it?

Answer 1

Every SQL interface library worth using has some kind of support for binding parameters. Don't try to be clever, just use it.

You may really, really think/hope you've escaped stuff properly, but it's just not worth the time you don't.

Also, several databases support prepared statement caching, so doing it right can also bring you efficiency gains.

Easier, safer, faster.

Answer 2

Assuming it is properly escaped, it doesn't make you vulnerable. The thing is that escaping properly is harder than it looks at first sight, and you condemn yourself to escape properly every time you do a query like that. If possible, avoid all that trouble and use prepared statements (or binded parameters or parameterized queries). The idea is to allow the data access library to escape values properly.

For example, in PHP, using mysqli:

$db_connection = new mysqli("localhost", "user", "pass", "db");
$statement = $db_connection->prepare("SELECT thing FROM stuff WHERE id = ?");
$statement->bind_param("i", $user_id); //$user_id is an integer which goes 
                                       //in place of ?
$statement->execute();