A newly discovered vulnerability in Microsoft's Internet Explorer browser could be a powerful new tool for scammers, allowing them to convincingly mask the real origin of Web pages used to trick targets into revealing sensitive information.

The glitch allows attackers to use a specially crafted URL to display a domain name, different from the Web page's actual location, in Internet Explorer's Address field, a practice known as "spoofing." Spoofing is a favorite tactic of spammers hoping to con users out of passwords and other personal details with e-mails pretending to be from banks, e-commerce sites, software vendors and other trusted institutions.

The new security hole was first publicised in a posting to BugTraq, a mailing list for discussion of security vulnerabilities. The post's author set up an illustration of the bug here.

Denmark-based security services firm Secunia issued its own advisory on the loophole Tuesday, rating it a "moderately critical" threat.

The vulnerability afflicts several versions of Internet Explorer, including a fully patched edition of the software's latest release. Several other popular browsers, including Mozilla and Opera, are not affected and correctly display the actual location of sites taking advantage of the URL hack.

Microsoft said it is investigating reports of the vulnerability. When that inquiry is complete the company will take whatever steps it deems necessary, such as issuing a new patch, a spokeswoman said.

Standard PC-protection practices, like anti-virus software and firewalls, may not help in thwarting exploitation of the new Internet Explorer bug, since it relies on social engineering rather than a technical attack. Secunia recommended in its advisory that users avoid following links from untrusted sources. Firewalls with URL-filtering capabilities may also defeat the vulnerability, the firm said.

Microsoft said it has not received any reports of the glitch being actively exploited and objected to the bug's disclosure on the BugTraq mailing list before it had been notified.

"We continue to encourage the responsible disclosure of vulnerabilities. We believe the commonly accepted practice of reporting vulnerabilities directly to a vendor serves everyone's best interests," a spokeswoman said in a written statement.

The author of the BugTraq posting detailing the vulnerability did not respond to a request for comment.