bson/_cbsonmodule.c in the mongo-python-driver (aka. pymongo) before 2.5.2, as used in MongoDB, allows context-dependent attackers to cause a denial of service (NULL pointer dereference and crash) via vectors related to decoding of an "invalid DBRef."
MongoDB 2.4.x before 2.4.5 and 2.5.x before 2.5.1 allows remote authenticated users to obtain internal system privileges by leveraging a username of __system in an arbitrary database.