If you are passing $_GET (or $_POST)
parameters to your queries, make sure that they are cast to strings first.
Users can insert associative arrays in GET and POST requests, which could
then become unwanted $-queries.
A fairly innocuous example: suppose you are looking up a user's information
with the request http://www.example.com?username=bob.
Your application does the query
$collection->find(array("username" => $_GET['username'])).
Someone could subvert this by getting
http://www.example.com?username[$ne]=foo, which PHP
will magically turn into an associative array, turning your query into
$collection->find(array("username" => array('$ne' => "foo"))),
which will return all users not named "foo" (all of your users, probably).
This is a fairly easy attack to defend against: make sure $_GET and $_POST parameters are the type you expect before you send them to the database (cast them to strings, in this case).
Note that this type of attack can be used with any databases interation that locates a document, including updates, upserts, find-and-modifies, and removes.
Thanks to » Phil for pointing this out.
See » the main documentation for more information about SQL-injection-like issues with MongoDB.
If you are using JavaScript, make sure that any variables that cross the PHP-
to-JavaScript boundry are passed in the scope field of
MongoCode, not interpolated into the JavaScript
string. This can come up when using MongoDB::execute(),
$where clauses, MapReduces, group-bys, and any other time
you may pass JavaScript into the database.
Note:
MapReduce ignore the
scopefield of MongoCode, but there is ascopeoption on the command that can be used instead.
For example, suppose we have some JavaScript to greet a user in the database logs. We could do:
<?php
// don't do this!
$username = $_POST['username'];
$db->execute("print('Hello, $username!');");
?>
However, what if a malicious user passes in some JavaScript?
<?php
// don't do this!
// $username is set to "'); db.users.drop(); print('"
$db->execute("print('Hello, $username!');");
?>
Now MongoDB executes the JavaScript string
"print('Hello, '); db.users.drop(); print('!');".
This attack is easy to avoid: use scope to pass
variables from PHP to JavaScript:
<?php
$scope = array("user" => $username);
$db->execute(new MongoCode("print('Hello, '+user+'!');", $scope));
?>
This adds a variable user to the JavaScript scope. Now if
someone tries to send malicious code, MongoDB will harmlessly print
Hello, '); db.dropDatabase(); print('!.
Using scope helps prevent malicious input from being
executed by the database. However, you must make sure that your code does
not turn around and execute the input anyway! For example, never use the
JavaScript eval function on user input:
<?php
// don't do this!
// $jsShellInput is "db.users.drop();"
$scope = array("input" => $jsShellInput);
$db->execute(new MongoCode("eval(input);", $scope));
?>
Always use scope and never allow the database to execute
user input as code.