The ITF (International Tennis Foundation) website has since been cleaned up but the ATP website remains dirty, according to Fraser Howard, a security researcher at Sophos who's tracking the attacks. Howard explained that hackers used search engines and automated tools to hunt for vulnerable sites. The timing of the infection of the tennis websites is therefore just a coincidence.

SQL injection attacks are commonly used by hackers to booby-trap legitimate websites with exploits designed to load malware onto the PCs of vulnerable machines. This type of attack, called a drive-by download, is becoming increasingly common.

Both admins and surfers need to keep systems fully patched in order to minimise the chances of attack, Howard advised. He warned sysadmins who simply remove malicious scripts without dealing with the underlying security problems are likely to get hit again.