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The massive attack that has infected PCs by tricking users into clicking links in fake messages from CNN.com shows little sign of ending soon, security researchers say.

According to MX Logic Inc. , spam posing as CNN.com Top 10 lists peaked at close to 11 million messages per hour early Thursday, but remained at high volumes throughout the day Friday. The Colorado security vendor said it had been tracking an average of 8 million messages per hour since midnight.

MX Logic's vice president of information security, Sam Masiello , called the trend "a very slow, but steady decline" from the 11 a.m. Mountain Time peak the day before.

Masiello also said that the spam has changed since attacks were first launched on Tuesday. "We've also seen several morphs of this spam over the past couple of days," he said in an entry posted on the MX Logic blog Friday . Where the messages once trumpeted " CNN .com Daily Top 10" in the subject heading and linked to a single filename on malware-hosting sites, now the spam sports a subject reading "CNN Alerts: My Custom Alert" and uses a variety of filenames in the malicious URL.

"This is likely in response to all of the media attention and awareness that has been brought up over the past couple of days," Masiello speculated.

Also on Friday, Websense Inc. reported that its researchers had seen the attack mutating, with the spam subject heading not only touting "CNN Alerts: My Custom Alert," but also using legitimate news stories culled from CNN to make the messages more convincing.

Users who clicked on the "FULL STORY" link in the message were redirected to a fake CNN site, where they were told they needed to download an update to Flash Player, Adobe System Inc. 's popular Internet media player, to view a video clip from CNN.

Websense also said it had spotted traces of the campaign in blog spam.

If users agreed to download the bogus Flash update, they were trapped in an endless loop, where clicking "Cancel" in the initial dialog produced a second pop-up. Clicking "Cancel" there returned the user to the first pop-up. The only options at that point were for users to shut down the browser or give in and install the malware.

MX Logic added that it had seen the URLs in the spam lead to legitimate domains that had probably been compromised, and named a U.K.-based roofing company as an example.

Earlier this week, Bulgarian security researcher Dancho Danchev had found more than 1,000 compromised domains being used to serve up the fake Flash. In a follow-up e-mail, Danchev said that in most cases, he couldn't find any characteristics shared by the hacked sites, such as all being hosted by a single Internet service provider.

"My assumption is that they took the time and effort to do some reconnaissance of sites which are vulnerable to remote file inclusion, or other type of remotely exploitable flaw within their Web applications that would allow someone to locally host all the malicious files," Danchev said. "I wouldn't be surprised if I find out that someone has basically went through all keylogged Cpanel passwords he obtained through his botnet, or through the access to a botnet that he temporarily rented."

Cpanel, a popular server control panel program, has been targeted by password thieves in the past because of the access those passwords provide to sites.