The sosendto function in slirp/udp.c in QEMU before 2.1.2 allows local users to cause a denial of service (NULL pointer dereference) by sending a udp packet with a value of 0 in the source port and address, which triggers access of an uninitialized socket.
http://rhn.redhat.com/errata/RHSA-2015-0349.html
http://rhn.redhat.com/errata/RHSA-2015-0624.html
https://bugzilla.redhat.com/show_bug.cgi?id=1144818
http://www.debian.org/security/2014/dsa-3044