Recently, the glibc library had a bug that could
be exploited over the network using a DNS request.
This bug can be fixed on debian by updating the libc6 library:
apt-get install libc6
After the update, the computer should be restarted, since a lot of
services and libraries use the libc6.
It is also possible to restart all services, but missing one service
is easily possible, so better be on the safe side.