Ads.txt is a simple flexible and secure method for publishers and distributors to declare who is authorized to sell their inventory improving transparency for programmatic buyers. 

The ads.txt project aims to prevent various types of counterfeit inventory across the ecosystem by improving transparency in the digital programmatic supply chain. 

When a brand advertiser buys media programmatically they rely on the fact that the URLs they purchase were legitimately sold by those publishers. The problem is there is currently no way for a buyer to confirm who is responsible for selling those impressions across exchanges and there are many different scenarios when the URL passed may not be an accurate representation of what the impression actually is or who is selling it. While every impression already includes publisher information from the OpenRTB protocol including the page URL and Publisher.ID there is no record or information confirming who owns each Publisher.ID nor any way to confirm the validity of the information sent in the RTB bid request leaving the door open to counterfeit inventory.