Rule Definition
UAs the db.collection.ensureIndex() is deprecated since version 3.0.0 and the db.collection.ensureIndex() is now an alias for db.collection.createIndex() it is recommended to always use db.collection.createIndex() rather than db.collection.ensureIndex() to create new indexes. Indeed Using deprecated MongoDB API could lead to security compliancy issues.

Remediation
Use db.collection.createIndex() rather than db.collection.ensureIndex() to create new indexes.

Violation Code Sample

var server = MongoServer.Create("mongodb://localhost");
var db = server.GetDatabase("myapp");

var users = db.GetCollection<User>("users");

users.EnsureIndex(new IndexKeysBuilder().Ascending("EmailAddress"));

Fixed Code Sample

var client = new MongoClient("mongodb://localhost");
var db = client.GetServer().GetDatabase("myapp");
var collection = db.GetCollection<User>("users");

collection.CreateIndex(new IndexKeysBuilder().Ascending("EmailAddress"), IndexOptions.SetBackground(true));