Security experts are warning about a new threat to Web surfers: malicious Web sites that use international domain names to spoof the Web addresses of legitimate sites.

The new trick is a variation of a known technique called the "homograph attack" and takes advantage of loopholes in the way some popular Web browsers display domain names that use non-English characters. It could allow malicious hackers and online identity theft groups to trick unsuspecting users into divulging sensitive personal information, according to advisories from The Shmoo Group, a hacker collective, and Secunia.

The warning was published after a demonstration of the new kind of homograph attacks at ShmooCon, a hacker convention in Washington, D.C. Secunia, of Copenhagen, issued advisories on the new issue for users of affected browsers and declared the issue "moderately critical."

Homograph attacks are a well-known trick in which character resemblance, for example, between the letter "O" and the number "0" is used to fool users into thinking that a bogus Web site actually belongs to a legitimate company. For example, malicious hackers might register the domain www.pcw0rld.com and design it to mimic the popular computer news Web site.

The latest threat was first described by Evgeniy Gabrilovich and Alex Gontmakher, computer science students at Technion, the Israel Institute of Technology. The attack takes advantage of changes supported by Internet standards bodies such as the Internet Engineering Task Force (IETF) to allow domain names to be registered in national alphabets using non-English characters. The new Internationalized Domain Name (IDN) program makes it easier for non-English speakers to use the Web but also creates opportunities for malicious hackers, Gabrilovich and Gontmakher wrote.

For example, attackers could register a Web domain bloomberg.com, which looks identical to the popular business news Web site, but in which the letters "o" and "e" have been substituted with identical-looking substitutes from the Cyrillic alphabet, used in the Russian language, creating a new domain, the authors said (see here .) In another example, the authors registered the domain www.microsoft.com, in which the English letters "c" and "o" in that domain were substituted with their Cyrillic counterparts.

The IDG News Service is a Network World affiliate.