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."
https://jira.mongodb.org/browse/PYTHON-532
http://www.securityfocus.com/bid/60252
http://www.debian.org/security/2013/dsa-2705
http://ubuntu.com/usn/usn-1897-1
http://seclists.org/oss-sec/2013/q2/447
http://lists.opensuse.org/opensuse-updates/2013-06/msg00180.html