The GHOST vulnerability is a serious weakness in the Linux glibc library. It allows attackers to remotely take complete control of the victim system without having any prior knowledge of system credentials. A buffer overflow was discovered in the __nss_hostname_digits_dots() function of the GNU C Library (glibc). This bug is reachable both locally and remotely via the gethostbyname*() functions. Web applications can also be affected by this flaw. For example, WordPress can be exploited using the XML-RPC "pingback" functionality.
To fix this issue apply a patch from your Linux vendor. The first vulnerable version of the GNU C Library affected by this is glibc-2.2. It was fixed on May 21, 2013 (between the releases of glibc-2.17 and glibc-2.18).