mongod in MongoDB 2.6, when using 2.4-style users, and 2.4 allow remote attackers to cause a denial of service (memory consumption and process termination) by leveraging in-memory database representation when authenticating against a non-existent database.

The find prototype in scripting/engine_v8.h in MongoDB 2.4.0 through 2.4.4 allows remote authenticated users to cause a denial of service (uninitialized pointer dereference and server crash) or possibly execute arbitrary code via an invalid RefDB object.

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.