Header bidding helps publishers raise CPMs because it allows them to make simultaneous calls to multiple SSPs, which drives up the demand for their inventory. One downside to this influx of bids is that it becomes more difficult for buyers to determine the legitimacy of inventory being sold through ad exchanges.
In response to advertisers' demands for more transparency, last year the Interactive Advertising Bureau (IAB) Tech Lab launched ads.txt, a text file on publishers’ sites that lists all the vendors that are authorized to sell their inventory. Because domain spoofing and arbitrage have plagued programmatic advertising, ads.txt was created so that ad buyers could have a tool to check whether a vendor’s claim to a piece of inventory was legitimate.