Web hosting provider Network Solutions has been cleaning up customer websites after a second consecutive round of attacks via its servers that began over the weekend and injected malicious code onto the sites.

A Network Solutions spokesperson did not return phone calls, but one security firm that has been following the attacks says "a few thousand" sites were infected as a result of a few of its shared servers getting hacked. David Dede, founder of Sucuri Security, says no additional sites have been hacked as of today.

In a blog post yesterday, Network Solutions said it had "identified the issue" and was working on deploying updates to fix it. "Our teams are proactively cleaning any malicious code from affected files," blogged NSI's Shashi Bellamkonda.

The company also was experiencing email service disruptions today due to "irregular network traffic," according to NSI's Twitter feed. It's unclear whether the email issue is related to the hacks, but NSI was telling its customers it is still working on the infected website problems as well as the email service problems.

Bellamkonda in a blog post on Sunday said the company had been getting reports of malware spreading on customers' websites. "At this time since anything we say in public may help the perpetrators, we are unable to provide details. It may not be accurate to categorize this as a single issue such as 'file permissions,'" he blogged.

This is the second revelation in a week of a hack of NSI's servers. Hundreds of NSI-hosted WordPress blogs were hit with malicious iFrames that would automatically infect visitors to the blog sites and, in some cases, spread fake antivirus software. The attackers pilfered blogger credentials, which had been stored in plain text in the WordPress database.

The latest attack also employed malicious iFrames, according to security researchers who have analyzed malware found on infected NSI customer websites. And unlike the previously reported attack, this time it isn't just WordPress, but Joomla and "simple HTML sites," according to Sucuri Security.

Eddie Schwartz, chief security officer with NetWitness, says hacks like that of NSI are all too common. "We see these sorts of attacks -- they are basically a very common occurrence," Schwartz says. "The fact is that there's a vibrant community of people downloading COTS [commercial off-the-shelf] versions of Zeus and other malware kits, looking for legitimate Websites where they can install iFrame exploits."

According to another research group called StopMalvertising, the malicious iFrame used in the latest NSI attack pointed to corpadsinc.com, which then downloads Adobe exploits onto victims' machines.

Whether this latest attack is related to the first is unclear. But the hacks raise an issue increasingly being faced by website owners: what's the responsibility of the ISP or service or cloud provider to provide more application-layer security, NetWitness' Schwartz says. "If people are understanding that the adversaries are injecting iFrames and JavaScript and other code and crimeware at the application layer, potentially on a massive scale, is there any responsibility on the part of the ISP or service provider?" he says. "You can't expect your customers to have the technical capability" to address this, he says.

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