Included is a screenshot taken of one of the multiple ads I found on the site, which linked back to Internet dating site True.com. A Neopets.com spokesperson said the ads could not have possibly have been served through its site, and that the ads must have been displayed by malicious software.
"This appears to be a 'malicious' software program and we are aggressively investigating its origin," the company said in an e-mailed statement. "We would never accept this type of ad on any of our company's sites as it doesn't meet any of Neopet's standards."
Neopets could not specify any particular adware or software in existence today that exhibits this type of ad-swapping behavior, but offered to put me in touch with an expert who could talk about how it would be theoretically possible for such malware to exist. Scans with several anti-spyware and anti-virus products returned a clean bill of health on my source's PC. [...]
Brian Krebs on Computer Security. The Washington Post Company.