Panda Security: Downadup worm now infects 1 in every 16 PCs

The computer worm responsible for the biggest attack in years has infected at least 1 out of every 16 PCs worldwide, a security company said Wednesday, and may have managed to compromise as many as nearly 1 in 3.

According to Panda Security, almost 6% of the Windows systems scanned with its antivirus technology were found to be infected with "Downadup," a worm that began aggressive attacks just over a week ago. Panda was one of the first security firms to ring the alarm over Downadup, when it raised its security threat level Jan. 12 as reports of attacks mounted.

Using data from antivirus scans performed by its consumer-grade security software, and by a free online scanning tool it makes available on its Web site, Panda found 111,379 PCs infected with the worm out of a pool of 2 million machines.

"I'm pretty confident in this number," said Ryan Sherstobitoff , chief corporate evangelist at Panda Security, as he cautioned it was just a snapshot. "Conficker is still infecting high volumes of machines and is a fast-propagating worm."

Conficker is an alternate name for the Downadup worm.

In fact, Panda's estimate is probably very conservative, Sherstobitoff said, since the bulk of the infected computers were scanned when their owners took the time to steer their browsers to the company's online scanner .

"The 6% was of people coming to our site, and opting in for the scans. That's somewhat scary," said Sherstobitoff. "If we were actually to look at the [general] population, all the people who don't have antivirus or if they do, who haven't updated definitions, the infection rate might be in the range of 20% to 30%."

While there has been some disagreement among security researchers about Downadup's infection volume -- last week, for instance, some disputed F-Secure Corp. 's estimate of 8.9 million infected PCs -- there has been little argument about the relative size of the worm attack. Nearly every researcher has pegged it as the biggest in years.

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