A Microsoft Internet Explorer exploit represented the highest number of hacking attacks in the second quarter, according to figures from ScanSafe.
The London-based security company said that the No. 1 hack was Exploit.HTML.Mht, which attempts to download and install a malicious program on a computer by using a security breach in Microsoft's IE browser software. The exploit was used to target almost twice as many organizations as other exploits, ScanSafe said.
"One of the things we've been surprised at is the growth rate of threats," said Roy Tuvey, director of ScanSafe. "There's been a 15 percent rise every quarter, and the threat is really rising. The first thing exploited are browser vulnerabilities."
Twenty-one percent of virus attacks occurred on Wednesdays and 6 percent at the weekend, ScanSafe found. The managed Web security company said the reason was that most viruses were launched at weekends and spread during the week.
Researchers at ScanSafe said that while Trojan horses and worms posed a significant threat, exploits accounted for 19 percent of all attacks they saw. But spyware only made up 12 percent of the total.
The company also found that 10 percent of attacks occurred on e-mail Web sites.
Dan Ilett of ZDNet UK reported from London.