How will Google prevent malware from third-party ads?

Google is actively working with trusted advertisers and partners to reduce the risk of malware. We specifically forbid fourth-party calls or sub-syndication to advertisers or vendors we haven't certified.

Also, all third-party ads are checked for malware when they're initially entered into our system. Google also employs an automated malware checker that continuously scans all third-party creatives running through the network. Any ad with malware will be automatically pulled from the network to protect our partner websites and their users.