nbd-server.c in Network Block Device (nbd-server) 2.9.21 allows remote attackers to cause a denial of service (NULL pointer dereference and crash) by causing a negotiation failure, as demonstrated by specifying a name for a non-existent export.
Gentoo Linux Security Advisory 201206-35 - Multiple vulnerabilities were found in nbd, which could lead to remote execution of arbitrary code. Versions less than 2.9.22 are affected.