An unpatched vulnerability in Internet Explorer could aid fraudsters in pulling off phishing scams, experts have warned.

The error could be exploited to fake the address bar in a browser window, security monitoring company Secunia said in an advisory published on Tuesday. This tactic could be used in phishing scams that attempt to trick people into believing they are on a legitimate site, when in fact they are viewing a fraudulent Web page.

Phishing is a prevalent type of online scam that seeks to pilfer personal information from unsuspecting Internet users. The scams typically combine spam e-mail with fraudulent Web sites that appear to come from a trusted source, such as a credit card company or a bank.

The flaw exists because of an error in the way the Microsoft Web browser loads Web pages and Macromedia Flash animations, according to Secunia. The company rates the issue "moderately critical" and has created a special Web page where users can test their Web browser to see if they are affected.

Secunia has confirmed that the vulnerability affects IE 6.0 on Windows XP with all current security patches. It also affects the latest IE 7 Beta release, Secunia said. Other versions may also be affected, it said.

Microsoft is investigating the newly reported flaw, a representative said in an e-mailed statement late Wednesday. "Our initial investigation has revealed that customers who have set their Internet security settings to high, or who have disabled active scripting, are at reduced risk from attack as the attack vector requires scripting," the representative said.

Additionally, Microsoft noted that it has not seen any active attacks that take advantage of this issue, which Secunia has dubbed the "Internet Explorer Window Loading Race Condition Address Bar Spoofing" flaw.

This is the fourth unpatched vulnerability for IE that has become public in the last few weeks. Microsoft plans to release a security update for the Web browser on Tuesday. At least one of the disclosed bugs will be fixed in that update, the company has said. That flaw, related to how IE handles the "createTextRange()" tag in Web pages, has been exploited in attacks to install spyware, remote-control software and Trojan horses on vulnerable PCs.