New attack fells Internet Explorer

The zero-day flaw is unreliable, but Symantec expects reliable exploits in the 'near future'

A hacker has posted attack code that could be used to break into a PC running older versions of Microsoft's Internet Explorer browser.

The code was posted on Friday to the Bugtraq mailing list by an unidentified hacker. According to security vendor Symantec, the code does not always work properly, but it could be used to install unauthorised software on a victim's computer.

"Symantec has conducted further tests and confirmed that it affects Internet Explorer versions 6 and 7," the company wrote on its website Saturday. "We expect that a fully-functional reliable exploit will be available in the near future."

Security consultancy Vupen Security has also confirmed that the attack works, saying it worked on a Windows XP Service Pack 3 system running IE 6 or IE7. Neither company was able to confirm that the attack worked on Microsoft's latest browser, IE 8.

Symantec did not report that the attack is being used by cyber-criminals, but because Internet Explorer is so popular, this type of code is highly coveted by hackers. If the software does pop up in online attacks, it will put pressure on Microsoft to rush out an emergency patch, ahead of its regularly scheduled Dec. 8 security update. Microsoft could not be reached Saturday for a comment on the issue.

Together, IE 6 and IE 7 command close to 40 percent of the browser market.

The flaw lies in the way Internet Explorer retrieves certain Cascading Style Sheet (CSS) objects, used to create a standardised layout on web pages. For the attack to work, the hacker would have to lure a victim to a web page that contained maliciously encoded JavaScript, Symantec said. This technique has emerged as a favorite way for hackers to install their malicious software on computers in recent years.

"To minimise the chances of being affected by this issue, Internet Explorer users should ensure their antivirus definitions are up to date, disable JavaScript and only visit websites they trust until fixes are available from Microsoft," Symantec said.