Security firm Sophos says the number of infected web pages has soared nearly six-fold since the start of the year.

The spike shows just how widespread web attacks have become, Sophos said. In June, it detected an average of almost 30,000 newly infected pages each day. Earlier in the year, the tally was as low as only 5,000 new pages daily.

The vast majority of pages serving up malicious content are hosted on legitimate websites, Sophos added. About 80% of all web based malware is on innocent, albeit compromised, sites.

Sophos said the June attacks were launched from a collection of more than 10,000 legitimate websites, the bulk of them hosted on Italian servers. The servers were compromised using an unknown vulnerability, then loaded with Mpack, a multiple exploit toolkit hackers deploy to hijack PCs visiting those sites.

"It begs the question as to why web hosts are not taking the necessary steps to properly secure their servers," said Graham Cluley, senior technology consultant at Sophos. "Simple measures such as keeping up to date with security patches will go a long way towards thwarting this problem. The fewer holes in server setups, the lower the risk of infection.”

Just over half of the infected sites are on servers powered by Apache, the open source web server software, Sophos reported. Microsoft's Internet Information Services (IIS) web server accounted for 34% of compromised or malicious systems. Both numbers are in line with web server market share, according to the internet measuring company Netcraft. Its figures put Apache at 50% of all servers, IIS at 35.5%.

"Malware is not just a Microsoft problem," Cluley said.

The Italian incident, he said, was a textbook example of a threat that targets and exploits all kinds of vulnerable sites, not just the usual suspects. "Web security solutions must go beyond blocking sites based simply on category. A gambling site may seem more of a threat, but sometimes the most innocuous sounding site can pose the greatest danger."

In a recent instance, hackers exploited this month's plane crash in Brazil to lure surfers to a malicious website that ran malicious code on their machines.