The opus_packet_get_samples_per_frame function in client in Mumble 1.2.4 and the 1.2.3 pre-release snapshots allows remote attackers to cause a denial of service (crash) via a crafted length prefix value, which triggers a NULL pointer dereference or a heap-based buffer over-read (aka "out-of-bounds array access").
http://www.debian.org/security/2014/dsa-2854
http://mumble.info/security/Mumble-SA-2014-001.txt
http://lists.opensuse.org/opensuse-updates/2014-02/msg00063.html