Browser vulnerabilities pose the biggest threat to the security of firms' internet users by allowing exploits such as worms, viruses and spyware, according to new research by web and virus filtering specialist ScanSafe. ScanSafe issued its warning after analysing scans of over one billion web requests made over its network.

ScanSafe found that web threats this quarter increased by 15 percent compared with last quarter. This growth shows criminals can quickly react to exploit newly-identified weaknesses, said ScanSafe.

Spyware, a relatively new problem, accounted for 12 percent of the threats. The survey found Wednesday was the most likely day to be attacked, and Sunday was the safest day.

The research found Internet Explorer (IE) was responsible for most weaknesses. This is unsurprising given that IE accounts for over 90 percent of web browsers. However, some exploits are particularly significant, the worst - Exploit.HTML.Mht - infected twice as many firms as its closest rival.

ScanSafe said Trojan horses and worms generally pose the biggest threats to firms, and both exploit browser flaws. These accounted for 19 percent of threats, and ScanSafe said they are likely to increase.

Firms are advised to think carefully about the security of their browsers and regularly update and patch them. In a statement John Edwards, technical director at ScanSafe, said, "We forecast that exploits driven by browser vulnerabilities will become an increased threat to enterprises. As vulnerabilities continue to emerge in Microsoft's IE and other browsers, and administrators struggle to update patches, attackers will be quick to take advantage."