A Longitudinal Analysis of the ads.txt Standard

Programmatic advertising provides digital ad buyers with the convenience of purchasing ad impressions through Real Time Bidding (RTB) auctions. However, programmatic advertising has also given rise to a novel form of ad fraud known as domain spoofing, in which attackers sell counterfeit impressions that claim to be from high-value publishers. To mitigate domain spoofing, the Interactive Advertising Bureau (IAB) Tech Lab introduced the ads.txt standard in May 2017 to help ad buyers verify authorized digital ad sellers, as well as to promote overall transparency in programmatic advertising. In this work, we present a 15-month longitudinal, observational study of the ads.txt standard. We do this to understand (1) if it is helping ad buyers to combat domain spoofing and (2) whether the transparency offered by the standard can provide useful data to researchers and privacy advocates. With respect to halting domain spoofing, we observe that over 60% of Alexa Top-100K publishers that run RTB ads have adopted ads.txt, and that ad exchanges and advertisers appear to be honoring the standard. With respect to transparency, the widespread adoption of ads.txt allows us to explicitly identify over 1,000 domains belonging to ad exchanges, without having to rely on crowdsourcing or heuristic methods. However, we also find that ads.txt is still a long way from reaching its full potential. Many publishers have yet to adopt the standard, and we observe major ad exchanges purchasing unauthorized impressions that violate the standard. This opens the door to domain spoofing attacks. Further, ads.txt data often include errors that must be cleaned and mitigated before the data is practically useful.