BOSTON -- The Department of Homeland Security is warning that al-Qaida may be planning to attack Web sites belonging to financial institutions.

NewsCenter 5's Gail Huff reported that the financial industry was bracing for a busy day Friday, even though the warning, which came after Thursday's closing bell, was unconfirmed.

The threat was posted on a jihadist Web site a few days ago and warned of attacks to be conducted in December until "the infidel New Year." It said the attacks would be launched against stock and banking Internet sites. Called 'denial of service' attacks, they involve repeatedly bombarding a server with so many hits that they shut down.

The jihadist Web site also calls for computer-savvy attackers to use viruses that can penetrate Internet sites and destroy data.

The threat is said to be a retaliation for Muslim prisoners being held by the U.S. at Guantanamo Bay in Cuba. The U.S. Computer Emergency Readiness Team put out the warning to the cyber-security industry, meaning U.S. stock markets and banking Web sites have been notified. A similar advisory went out two years ago but the threat never materialized.

Computer experts said even if the latest threat is real, firewalls should protect against the most serious damage.