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Microsoft said it's investigating an alleged flaw in recent versions of its Internet Explorer browser software that could allow attackers to spoof legitimate Web sites, steal content from browser cookies, and gain access to certain types of files on a victim's system.

The flaw, which affects IE Versions 5.5 to 6, was first reported to the company on December 19, 2001, by ThePull, an independent security researcher.

The vulnerability is the result of Microsoft's failure to abide by an industry-standard browser security rule known as the "same-origin policy," said David Ahmad, moderator of BugTraq, a popular mailing list on which ThePull first posted details of the vulnerability.

The same-origin policy was established to prevent malicious Web sites from interacting with and stealing sensitive information left in cookies set by other sites on a user's computer. In other words, when one Web site is used to open another Web site in a separate pop-up window, script code from the first site shouldn't be able to affect the information or properties of the other site.

A representative of Microsoft's Security Response Center said the company is investigating the issue "just as we do with every report we receive of security vulnerabilities affecting Microsoft products."

"At this point in the investigation, we feel strongly that speculating on the issue while the investigation is in progress would be irresponsible and counterproductive to our goal of protecting our customers' information," the spokesperson said.

Microsoft also criticized the manner in which the information was made public.

"We are concerned that this report has gone public before we've had a fair chance to investigate it. Its publication may put our customers at risk, or at the very least cause customers needless confusion and apprehension," the spokesperson said.

Ignored Standard?

Microsoft's current practice when it comes to vulnerability disclosures is to first come up with a fix before releasing details of a problem or confirming that one exists.

Even so, said Ahmad, Microsoft's failure to abide by the industry standard in recent versions of IE has resulted in a severe security vulnerability for users.

"If you use the document.write method in the correct manner as stated by Microsoft's own documentation, you are able to spoof sites, read cookies from other sites, and read local files on a user's system," said ThePull.

"This means, for instance, that someone could send you an e-mail from "security@microsoft.com" to download an important update with a link--upon clicking that link, you could be brought to a Web page with a Trojan [horse] on it," ThePull said.

Because of the flaw, attackers can potentially construct Web sites that steal cookies, perform actions on different Web sites through script code, and transmit the content of text files to attacker-controlled Web servers, warned an advisory on the Web site of SecurityFocus.com.

Probably the most serious consequence is that trusted Web sites can be replaced with entirely "attacker-created HTML," the advisory said.

The best way for users to handle the problem is to turn off JavaScript, said ThePull. "They should make sure they force all e-mail to be plain text, because you do not need to use active scripting to make this work in an e-mail, just HTML," he added.

In e-mail, ThePull also said that when Microsoft was first contacted about the problem, company officials "responded within 20 minutes asking for further information and considered it serious. I responded three times with further information as I got it. However, I have not heard back from them."

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