This also occurs using POST (obviously) despite it not being mentioned. Sanitize all input you are taking in that can cause this issue.
If you are passing $_GET parameters to your queries, make sure that they are cast to strings first. Users can insert associative arrays in GET 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's 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.
MapReduce ignore the scope field of MongoCode, but there is a scope option on the command that can be used instead.
// don't do this!
$username = $_POST['username'];
// don't do this!
// $username is set to "'); db.users.drop(); print('"
$scope = array("user" => $username);
$db->execute(new MongoCode("print('Hello, '+user+'!');", $scope));
<?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.