The RTSP handler in libalias rewrote outgoing packets into a fixed-length stack buffer without checking whether the rewritten data fit in the buffer, or whether the result fit back in the original packet.
A host sending crafted RTSP traffic from inside a NAT gateway using libalias can overflow a stack buffer, potentially achieving remote code execution in the kernel (when using ipfw(4) NAT) or in the natd(8) process (which generally runs as the root user).