Several security analysts issued warnings today that hackers were distributing malware from the website of NBC, one of the top four TV networks.

Brian Krebs, a former Washington Post reporter and well-known internet security expert who writes the blog Krebs on Security, told The Verge that the hackers inserted code into the NBC.com homepage. This caused visiting browsers to load pages from third-party sites that were compromised.

"The compromised sites tried to foist the Citadel Trojan, a variant of the Zeus Trojan," Krebs said. The Zeus is a "sophisticated data theft tool that steals passwords and allows attackers to control machines remotely."

NBC News reported that NBC confirmed that its site had been attacked. The broadcaster released the following statement regarding the website: "We've identified the problem and are working to resolve it. No user information has been compromised."

NBC told Reuters this evening that the site was safe for visitors again. Krebs said that NBC appeared to be operating normally but he was still trying to confirm that.

The compromised sites tried to foist the Citadel Trojan

When it comes to security, this has been a trying period for many US sites. Facebook, Twitter, and Apple acknowledged this month that laptops used by a small number of their employees were recently infected with malware. A website owned by The Los Angeles Times unknowingly served browsers exploits and malware for more than a month until being removed last week, according to Krebs.

Internet security is a big subject after a report from computer security firm Mandiant was issued on Monday that detailed how China's cyber warriors have for years waged a sophisticated espionage campaign against the United States. There's no evidence that the attack on NBC had anything to do with state-sponsored hackers.

For anyone who may have visited NBC.com, Krebs recommends updating operating systems and browser plugins. He noted that the attack on NBC was similar to many that have occurred in recent years in that "the malicious sites tried to exploit vulnerabilities in Java."

Carl Franzen contributed to this report.