PageFair, an Irish ad analytics company, said Monday a small percentage of users were at risk after attackers compromised its systems over the weekend.
Ninety percent of publishers have less than ten million page views per month, and 60 percent have less than one million page views per month, he wrote.
PageFair has calculated that about 2.3 percent of the visitors to those sites would have been at risk of being infected.
The attackers gained access to a key email account at PageFair and then reset the password for a PageFair account at a content distribution network (CDN).
The attack was noticed within five minutes, but it took PageFair 83 minutes to completely stop the attacks.
Blanchfield wrote that an analysis of the malicious code delivered indicated that the majority of antivirus programs wouldn't have detected it at the time of the attacks.
But for the attacks to be successful, users would have had to approve the download of the executable file delivered, which Windows would have warned about, he wrote.
F-Secure wrote on its blog on Monday that the malware delivered was a remote access tool called NanoCore.