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Tuesday's edition of Microsoft's monthly bundle of security advisories features an omission that should keep online fraud artists and identity thieves happy: over one month after its discovery, there is no official patch available for a bug in Internet Explorer that lets swindlers pass off counterfeit websites as the real thing.

The bug, publicly detailed on December 9th by "Zap the Dingbat," is an easily exploited flaw in the way Internet Explorer displays URLs in the address bar: it turns out the browser is incapable of displaying the special character "%01," or anything following it, in a Web address.

That simple gaffe is tailor-made for the devious online swindle called "phishing," in which a fraudster spams the Internet with e-mail purporting to be from a reputable financial institution or e-commerce site, and urging the recipient to click on an included link to update their personal profile or carry out some transaction. The link takes the victim to a fake website designed -- with increasing sophistication -- to look like the real deal, but where any personal or financial information entered is routed directly to the scammer.

Experts have traditionally advised consumers to avoid these scams by carefully checking the address bar in their browser window to verify that they're actually on citibank.com, for example, before entering their password or account information. But the IE bug makes that advice obsolete: combined with URL obfuscation techniques already well known to phishers, IE helpfully transforms a clumsy fake like "www.citibank.com%01@211.239.150.170/login/login.htm" into the flawless counterfeit "www.citibank.com."

It took scammers a full week to begin taking advantage of the bug, says Dan Maier, director of marketing for the Anti-Phishing Working Group, but when they caught on, they were fully incorporated the dodge into their bag of tricks. PayPal and Earthlink customers the first targets of the enhanced swindle. "There's one we saw that was very sophisticated attacking Earthlink customers," says Maier. "It brought them back to an Earthlink-branded website which asked for a scary amount of personal information, and the most frightening part of it was that the URL said earthlink.net."

Now financial institutions are the target of choice. A fraudulent e-mail received at SecurityFocus last week used the hole in directing recipients to a convincing reproduction of a Bank of America site, asking for an account number and pass code for BofA's online banking service. On Monday, Citibank warned of a fresh batch of phishing e-mails tempting victims by urging them to check their account balances, also using the bug. And Barclays and Lloyds have reported scammers using the vulnerability in recent UK-focused phishing expeditions.

But Russ Cooper, moderator of the NTBugtraq list and "Surgeon General" of TruSecure Corporation, says the impact of the IE bug is easily overstated: most victims of phishing scams would have fallen for the ruse even without the added cloaking provided by the bug. "There's a limited amount of people who would be subjected to this particular attack who wouldn't already give their information to an even dumber version of this attack," says Cooper.

In a statement, a Microsoft spokesperson said the company will only release a patch "that is as well engineered and thoroughly tested as possible -- whether that [takes] a day, week, month or longer." Under Redmond's monthly schedule, the next batch of security fixes is due February 10th. But an article on Microsoft's website offers consumers an easy, if drastic, workaround in the meantime: simply abandon the whole hypertext thing altogether. "The most effective step that you can take to help protect yourself from malicious hyperlinks is not to click them," the company advises. "Rather, type the URL of your intended destination in the address bar yourself."

"Which means never follow hyperlinks on the Web," Cooper says with a laugh. "What was the Web for then?"

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