https://safebrowsing.google.com/safe...ite=google.com
As you can see even google has an issue with this.. every ad network in the world has malware issues.
The solution for them is not an easy one... You have advertisers that will do everything they can to get their malware ads into any system they can. It's also not an easy thing to find.
All an ad network can do is try to find and deal with it as fast as possible.
example.
Malware advertiser places an ad in a system..
They leave the ad clean for X amount of time.
Then they target the malware to say load on only 1% of the traffic 1 time that they target to only display in 1 GEO, 1 OS, 1 Browser, and only new IP's that have not seen the ads.
Making it very hard to find even when doing manual or automated quality checks of ads.
An ad network could view the ad from the exact target that the malware advertiser has opted to target the malware to and never see the malware due to their IP already being in the advertisers system and flagged to not show the malware to them.
It is not possible for any ad network to say they this will never happen again.
Even the best systems I have seen that can flag 0day malware etc. Still have the issue of being able to trigger the malware to detect it. By that time it is possible some users have already triggered it before they have a chance to see it.