The amount of web-based malware blocked by IT security firm ScanSafe has jumped by 278 per cent in the first six months of this year.

The security firm found that the number of legitimate websites being compromised exploded in June, accounting for 66 per cent of all malware blocked.

"The mass compromise of websites poses particular challenges to corporate users," said Mary Landesman, senior security researcher at ScanSafe.

"The affected sites are typically known, legitimate and trusted sites with a business purpose. These are sites that users visit frequently, and the attacks are so stealthy and unobtrusive that most visitors don't know that they've been infected."

ScanSafe's latest Global Threat Report said that this widespread compromise of legitimate websites is largely the result of automated attack tools which became freely available in the last months of 2007.

The research found that SQL injection attacks are by far the most popular form of website compromise, outpacing others by 212 per cent and accounting for 76 per cent of all compromised sites.

When it comes to payloads, most of the compromises attempt to install password stealers and Trojans. This category of malware increased from four per cent of malware in January to 27 per cent in June.