Websites affiliated with U.S. broadcaster NBC were hacked for several hours on Thursday, serving up malicious software intended to steal bank account details.

On its own technology blog, NBC released a statement saying, "We've identified the problem and are working to resolve it. No user information has been compromised."

Sites such as NBC.com are a frequent target for hackers since the high volume of visitors offers a chance to infect many people in a short period of time.

Several computer security companies said the main NBC.com website had been modified to serve up an iframe, which is a way to load content into a website from another domain.

The iframe loaded an exploit kit called Redkit, which tries to see if a visitor is running unpatched software, according to a blog post from Securi, a computer security company based in Menifee, California. The style of attack is known as a drive-by download and can infect a computer when a user merely views a website.

NBC.com was temporarily blacklisted by Google after the attack. Facebook also stopped directing users to NBC.com. Securi wrote that other NBC sites, including ones for TV talk show hosts Jimmy Fallon and Jay Leno, were also affected.

The hack follows the release of a report this week from security vendor Mandiant about a long-running hacking campaign allegedly based in Shanghai that targeted U.S. corporations, although it did not immediately appear connected with the problems at NBC.com.

Another computer security firm, SurfRight, wrote on its HitmanPro blog that the NBC attack loaded exploits that look for vulnerabilities in Oracle's Java programming framework and Adobe's PDF products. Oracle and Adobe have both released critical updates for their products this month, but hackers hope to hit users who have not updated their computers.

If the attack is successful, one of two malicious software programs is delivered, called Citadel or ZeroAccess. Citadel is a trojan designed to collect account credentials for banks including Bank of America, Wells Fargo, Chase and others, according to Fox-IT, a Dutch computer forensics company.

The version of Citadel analyzed by Fox-IT showed it was being detected by only three of 46 products on VirusTotal, a website where malicious software can be tested for detection against many of the popular security suites.

According to Symantec, ZeroAccess is an advanced rootkit, or a piece of malicious software that hides at a low level in a computer's operating system. ZeroAccess, detected by Symantec in July 2011, can create its own hidden file system and download other malware to a computer.

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