Skip to main content

SQL injection

note

This page forms part of the Moodle security guidelines.

What is the danger?​

Suppose your code in /course/view.php?id=123 does something like:

SELECT FROM mdl_course WHERE id = $id;

where the $id = 123 has come from the URL. Suppose that your code does not bother to clean that parameter properly.

Along comes Evil Hacker, and edits the URL to be /course/view.php?id=123;DELETE+FROM+mdl_user I will let you work out why that is a very, very bad thing.

Of course, depending on exactly what the database query is, the malicious input needs to be constructed appropriately, but that is just a matter of trial and error for Evil Hacker.

How Moodle avoids this problem​

Once again, it is a case of being very suspicious of any input that came from outside Moodle. In the example above, $id should clearly have been cleaned by passing PARAM_INT to required_param.

It is more tricky with a query like

UPDATE mdl_user SET lastname = '$lastname' WHERE id = $id;

What happens when $lastname is O'Brian? Well, you have to escape the ' like this: O\\'Brian.

  • In Moodle 1.9, addslashes is applied automatically to all input you get via required_param or optional_param.
  • Moodle 2.0 onwards, completely avoid the dangerous process of building SQL by concatenating strings. In Moodle 2.0 the SQL would look like
UPDATE mdl_user SET lastname = ? WHERE id = ?;

and then we would pass an array of values [$lastname, $id] to the database along with the SQL.

What you need to do in your code​

  • Use higher level dmllib methods, like get_record, whenever possible, so you do not have to create SQL yourself.
  • When you have to insert values into SQL statements, use place-holders to insert the values safely.
  • Test your code by using a tool like sqlmap, or by manually trying tricky inputs like:
    < > & \&lt; \&gt; \&amp; ' \\' 碁 \ \\

In Moodle 1.9 or earlier:

  • Data from required_param and optional_param have already had addslashes applied, ready to be used in database queries, but make sure you put single quotes round each value.
  • If you have loaded some data from the database, and then want to re-insert it, then apply addslashes or addslashes_object to it first.

What you need to do as an administrator​

  • This is not something that administrators can do anything about (other than keeping your Moodle up-to-date).

See also​