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Website defacement is an attack on a website that changes the visual appearance of the site or a webpage. These are typically the work of system crackers, who break into a web server and replace the hosted website with one of their own. Defacement is generally meant as a kind of electronic graffiti, although recently it has become a means to spread messages by politically motivated "cyber protesters" or hacktivists.
Common methods 
The most common method of defacement is using SQL injections which allows gaining administrative access. Defacements usually consist of an entire page. This page usually includes the defacer's pseudonym or "Hacking Codename." Sometimes, the Website Defacer makes fun of the system administrator for failing to maintain server security. Most times, the defacement is harmless and is only done to show off a system cracker's skills or for Hacktivism, however, it can sometimes be used as a distraction to cover up more sinister actions such as uploading malware or deleting essential files from the server. Another method of defacement is through FTP once the username and password are obtained.
Common targets of defacement 
Religious and government sites are regularly targeted by hackers in order to display political or religious beliefs, whilst defacing the views and beliefs of others. Disturbing images and offensive phrases might be displayed in the process, as well as a signature of sorts, to show who was responsible for the defacement. Websites are not only defaced for political reasons, many defacers do it just for the thrill. For example, there are online contests in which hackers are awarded points for defacing the largest number of web sites in a specified amount of time. Corporations are also targeted more often than other sites on the Internet and they often seek to take measures to protect themselves from defacement or hacking in general. Web sites represent the image of a company or organisation and these are therefore suffer significant losses due to defacement. Visitors may lose faith in sites that cannot promise security and will become wary of performing online transactions. After defacement, sites have to be shut down for repairs, sometimes for an extended period of time, causing expenses and loss of profit and value.