The bug, according to security alerts by a bug hunter and a Danish security company, Secunia, could let hackers use a technique to display a false Web address on a fake site.
Secunia credited the bug to "Zap the Dingbat," who posted an alert to the Bugtraq security mailing list. That alert links to a demonstration of the exploit, and says Microsoft was informed of the bug Tuesday.
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Malicious hackers frequently lure victims to convincing replicas of e-commerce sites such as eBay, where they're tricked into handing over financial and other private information. The method is said to be a key tool in credit card and identity theft.
Savvy Web surfers often figure out the ruse from irregularities in the Web address. But in the method described by Secunia, IE could allow the address bar for the spoofed eBay site, for example, to read "ebay.com."
"Microsoft is investigating new public reports of a possible vulnerability in Internet Explorer," the company said in a statement. "We have not been made aware of any active exploits of the reported vulnerabilities or customer impact at this time, but we are aggressively investigating the public reports."
Microsoft did not set a timetable for its investigation, but said it may eventually release a patch to address the problem. Meanwhile, the company recommended that people follow basic security procedures, including the use of firewalls, software updates and antivirus software.
Microsoft faulted security mavens for publicizing the flaw, implying that they hadn't given Microsoft sufficient time to craft a patch.
"Microsoft is concerned that this new report of a vulnerability in Internet Explorer was not disclosed responsibly, potentially putting computer users at risk," the statement reads. "We believe the commonly accepted practice of reporting vulnerabilities directly to a vendor serves everyone's best interests, by helping to ensure that customers receive comprehensive, high-quality patches for security vulnerabilities with no exposure to malicious attackers while the patch is being developed."
Secunia was not immediately available for comment.
Secunia's advisory faulted IE for an "input validation error" that let a certain character sequence mask the actual Web address and substitute a fake one.
It recommended using a proxy server or firewall to filter the character sequence out of Web addresses, and urged people not to "follow links from untrusted sources."