There are several methods of classifying exploits. The most common is by how the exploit contacts the vulnerable software. A remote exploit works over a network and exploits the security vulnerability without any prior access to the vulnerable system. A local exploit requires prior access to the vulnerable system and usually increases the privileges of the person running the exploit past those granted by the system administrator. Exploits against client applications also exist, usually consisting of modified servers that send an exploit if accessed with a client application. Exploits against client applications may also require some interaction with the user and thus may be used in combination with the social engineering method. Another classification is by the action against the vulnerable system; unauthorized data access, arbitrary code execution, and denial of service are examples. Many exploits are designed to provide superuser-level access to a computer system. However, it is also possible to use several exploits, first to gain low-level access, then to escalate privileges repeatedly until one reaches root. Normally a single exploit can only take advantage of a specific software vulnerability. Often, when an exploit is published, the vulnerability is fixed through a patch and the exploit becomes obsolete until newer versions of the software become available. This is the reason why some black hat hackers do not publish their exploits but keep them private to themselves or other hackers. Such exploits are referred to as zero day exploits and to obtain access to such exploits is the primary desire of unskilled attackers, often nicknamed script kiddies.