ads.txt
ads.txt is a public file publishers host to declare exactly which companies are authorized to sell their ad inventory.
ads.txt (Authorized Digital Sellers) is a public text file that a publisher hosts at the root of its domain (for example, example.com/ads.txt) to declare which companies are authorized to sell its ad inventory. Introduced by the IAB Tech Lab, it is a simple, machine-readable allowlist designed to stop fraud in programmatic advertising.
How it works#
Each line names an authorized seller: the ad system's domain, the publisher's seller account ID, the relationship type (DIRECT or RESELLER), and an optional certification ID. Before bidding on inventory, buyers and Demand-Side Platforms crawl the publisher's ads.txt and check that the seller offering the impression is on the list. If it isn't, the inventory is treated as unauthorized and can be rejected.
Why it matters#
ads.txt directly attacks domain spoofing, where fraudsters sell fake impressions claiming to be a premium site, and unauthorized reselling. By publishing an authoritative list, publishers make it far harder for counterfeit inventory to pass as theirs. It is the publisher-side counterpart to sellers.json, which exposes the same relationships from the seller's side, and together with the SupplyChain Object (schain) it gives buyers a verifiable view of the ad supply chain. For app inventory, the equivalent file is app-ads.txt.
Related terms: sellers.json, SupplyChain Object (schain), and Ad Supply Chain.
