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Unknown attackers have been targeting a previously unknown vulnerability in Internet Explorer to take control of machines running the Microsoft browser, security watchers warned on Wednesday.

The exploits were hosted on a page of an unidentified website that had been breached without the owner's knowledge, according to antivirus provider Symantec, which discovered the attacks a few days ago. The perpetrators then sent emails that lured a select group of people in targeted organizations to the booby-trapped page, causing those who used IE versions 6 and 7 to be infected with a backdoor trojan.

“Looking at the log files from this exploited server we know that the malware author had targeted more than a few organizations,” they wrote. “The files on this server had been accessed by people in lots of organizations in multiple industries across the globe.”

In an encouraging sign, few of the visitors were affected because they weren't using a vulnerable browser, they added.

Version 8 of IE may also be vulnerable, but a security protection known as DEP, or data execution prevention – which is turned on by default – causes the browser to crash rather than to remotely execute the malicious code, Microsoft said. DEP, which was first added to IE 7, is designed to lessen the damage of such attacks by preventing data loaded into memory from being executed. While hackers have figured out ways to bypass the technology, so-called heap-spraying attacks don't work well with this particular bug.

The security flaw resides in a part of IE that handles CSS, or Cascading Style Sheets, tags. As a result, the browser under-allocates memory, allowing data to be overwritten in memory vtable pointers. By spraying memory with special data, an attacker can cause IE to execute code.

The report is the latest reminder of the benefits of moving to the latest version of IE – or to a different browser altogether. Those who must use IE versions 6 or 7, should consider augmenting it with EMET, Microsoft's tool for locking down older applications. It can be used to add DEP and other security mitigations to a variety of programs, including IE and Adobe Reader.

Microsoft didn't say when it planned to patch the vulnerability, but Jerry Bryant, a spokesman for Microsoft response, indicated the bug probably didn't warrant a release outside of the company's normal update cycle. That means the earliest we're likely to see a fix is December 14.

Microsoft has more details here, here and here. ®

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