Criminals flooded several online ad networks with malicious advertisements over the weekend, causing popular Web sites such as the Drudge Report, Horoscope.com and Lyrics.com to inadvertently attack their readers, a security company said Wednesday.The trouble started on Saturday, when the criminals somehow placed the malicious ads on networks managed by Google's DoubleClick, as well as two others: YieldManager and ValueClick's Fastclick network, according to Mary Landesman, a senior security researcher with ScanSafe.The attack comes just a week after the New York Times Web site was tricked into displaying a deceptive 'scareware' advertisement for fake antivirus software from scammers pretending to be ad buyers with Vonage, an Internet telephony company.Instead of trying to trick Web surfers into buying bogus software, these ads attacked.They would pop up a nearly invisible window in the victim's browser that contained a maliciously encoded pdf document, which included attack code that placed a variant of the Win32\/Alureon Trojan horse program on the victim's computer. Sometimes, the ads would also try to exploit a previously patched flaw in Microsoft's DirectShow software, Landesman said."The user would have seen a very brief opening of a blank pdf window and it would be at the bottom portion of their screen," she said. The Alureon Trojan is known to download additional malware and often hijack victims' search results, she said.The pdf attacks apparently only affected victims with out-of-date versions of Adobe's Reader or Acrobat software, she added.Between Saturday and Monday, the ads accounted for 11 percent of all Web pages blocked by ScanSafe's Web filtering software, a sign that many people were being presented with the malicious ads. And because the pdf pages were modified slightly every time they were displayed, most antivirus products didn't detect them.In tests, ScanSafe found that only 3 out of 41 antivirus vendors detected the malware."To be honest, they were pretty clever in the way they carried this out," Landesman said. "They managed to infiltrate sites that enjoy very good traffic and they were able to use a mechanism for creating this pdf that caused it to be nearly completely undetected."This is not the first time Google's DoubleClick has been associated with this type of malicious advertising. Earlier this year criminals placed similar ads on the home page of technology trade magazine eWeek, whose ads were managed by DoubleClick.