Online advertising is a major source of revenues in the Internet. In this paper, we identify a number of vulnerabilities of current ad serving systems. We describe how an adversary can exploit these vulnerabilities to divert part of the ad revenue stream for its own benefit. We propose a collaborative secure scheme to fix this problem. The solution relies on the fact that most of online advertising networks own digital authentication certificates and can become a source of trust. We also explain why the deployment of this solution would benefit the Web browsing security in general.