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.

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. MapReduce ignore the scope field of MongoCode, but there is a scope option 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: execute("print('Hello, $username!');"); ?> ]]> However, what if a malicious user passes in some JavaScript? 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: $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: $jsShellInput); $db->execute(new MongoCode("eval(input);", $scope)); ?> ]]> Always use scope and never allow the database to execute user input as code.