What is SQL Injection
SQL injection refers to the act of someone inserting a MySQL statement to be run on your database without your knowledge. Injection usually occurs when you ask a user for input, like their name, and instead of a name they give you a MySQL statement that you will unknowingly run on your database.
SQL Injection Example
Below is a sample string that has been gathered from a normal user and a bad user trying to use SQL Injection. We asked the users for their login, which will be used to run a SELECT statement to get their information.MySQL & PHP Code:
// a good user's name
$name = "timmy";
$query = "SELECT * FROM customers WHERE username = '$name'";
echo "Normal: " . $query . "<br />";
// user input that uses SQL Injection
$name_bad = "' OR 1'";
// our MySQL query builder, however, not a very safe one
$query_bad = "SELECT * FROM customers WHERE username = '$name_bad'";
// display what the new query will look like, with injection
echo "Injection: " . $query_bad;
Display:
Normal: SELECT * FROM customers WHERE username = 'timmy'Injection: SELECT * FROM customers WHERE username = '' OR 1''
The normal query is no problem, as our MySQL statement will just select everything from customers that has a username equal to timmy.
However, the injection attack has actually made our query behave differently than we intended. By using a single quote (') they have ended the string part of our MySQL query
- username = ' '
- username = ' ' OR 1
More Serious SQL Injection Attacks
Although the above example displayed a situation where an attacker could possibly get access to a lot of information they shouldn't have, the attacks can be a lot worse. For example an attacker could empty out a table by executing a DELETE statement.
MySQL & PHP Code:
$name_evil = "'; DELETE FROM customers WHERE 1 or username = '";
// our MySQL query builder really should check for injection
$query_evil = "SELECT * FROM customers WHERE username = '$name_evil'";
// the new evil injection query would include a DELETE statement
echo "Injection: " . $query_evil;
Display:
SELECT * FROM customers WHERE username = ' '; DELETE FROM customers WHERE 1 or username = ' 'If you were run this query, then the injected DELETE statement would completely empty your "customers" table. Now that you know this is a problem, how can you prevent it?
Injection Prevention - mysql_real_escape_string()
Lucky for you, this problem has been known for a while and PHP has a specially-made function to prevent these attacks. All you need to do is use the mouthful of a function mysql_real_escape_string.What mysql_real_escape_string does is take a string that is going to be used in a MySQL query and return the same string with all SQL Injection attempts safely escaped. Basically, it will replace those troublesome quotes(') a user might enter with a MySQL-safe substitute, an escaped quote \'.
Lets try out this function on our two previous injection attacks and see how it works.
MySQL & PHP Code:
//NOTE: you must be connected to the database to use this function!
// connect to MySQL
$name_bad = "' OR 1'";
$name_bad = mysql_real_escape_string($name_bad);
$query_bad = "SELECT * FROM customers WHERE username = '$name_bad'";
echo "Escaped Bad Injection: <br />" . $query_bad . "<br />";
$name_evil = "'; DELETE FROM customers WHERE 1 or username = '";
$name_evil = mysql_real_escape_string($name_evil);
$query_evil = "SELECT * FROM customers WHERE username = '$name_evil'";
echo "Escaped Evil Injection: <br />" . $query_evil;
Display:
Escaped Bad Injection:SELECT * FROM customers WHERE username = '\' OR 1\''
Escaped Evil Injection:
SELECT * FROM customers WHERE username = '\'; DELETE FROM customers WHERE 1 or username = \''
Notice that those evil quotes have been escaped with a backslash \, preventing the injection attack. Now all these queries will do is try to find a username that is just completely ridiculous:
- Bad: \' OR 1\'
- Evil: \'; DELETE FROM customers WHERE 1 or username = \'
