Ads.txt (Authorized Digital Sellers)

Ads.txt, short for Authorized Digital Sellers, is a simple text file that a publisher hosts in a standard location on its own domain to publicly declare which companies are authorized to sell that publisher's advertising inventory. Each line names an authorized seller and the publisher's account identifier with that seller, so anyone, especially advertisers and their buying platforms, can read the file and confirm whether a given seller is genuinely permitted to offer the site's inventory. Introduced by the IAB Tech Lab (the technical standards arm of the Interactive Advertising Bureau), ads.txt was designed to combat a specific fraud: domain spoofing, where fraudsters sell fake inventory falsely claiming to be a reputable site. If a seller is not listed in a publisher's ads.txt file, buyers can treat any inventory it offers under that domain as unauthorized.

The mechanism is transparency by declaration. Because the file lives on the publisher's own domain and only the publisher controls it, its list of authorized sellers is trustworthy in a way that a claim inside a bid request is not. Buyers configure their platforms to check ads.txt and to buy only from sellers the publisher has authorized, which cuts off the market for spoofed and counterfeit inventory, because fraudsters who cannot appear in the file cannot easily masquerade as the site. Ads.txt does not verify the quality of an impression or stop every kind of fraud, and it only helps for the sites that publish and maintain it, but it closed a large, once-easy avenue for misrepresenting inventory. Its adoption became widespread precisely because it is cheap to implement and directly attacks a costly problem.

Ads.txt is one of three related transparency standards, and knowing which does what prevents confusion. Ads.txt is hosted by the publisher on its website and answers, from the publisher's side, which companies are authorized to sell its inventory. App-ads.txt is the same standard adapted for mobile and connected-TV apps, where there is no traditional website to host the file, so it lives at a location tied to the app developer's domain. Sellers.json is the mirror standard hosted by the intermediary, the exchange or reseller, and answers who those sellers are and what their real identities are. So ads.txt and app-ads.txt authorize sellers from the publisher's end, while sellers.json identifies them from the exchange's end.

Together with the SupplyChain object, which travels inside each bid request and lists the intermediaries an impression actually passed through, these standards form a cross-checking system. A buyer can confirm that a seller in a bid is authorized in the publisher's ads.txt or app-ads.txt, that the seller's identity is disclosed in the relevant sellers.json, and that the live SupplyChain path matches both. The key distinctions to hold onto are simple: the publisher hosts ads.txt, and app-ads.txt for apps, the intermediary hosts sellers.json, and the bid stream carries SupplyChain. Each closes a gap the others cannot, authorization, identity, and the actual path, which is why the industry treats them as a set rather than substitutes. Confusing ads.txt with sellers.json, a common slip, sends teams looking for the wrong file in the wrong place.

For a publisher, using ads.txt well means keeping the file accurate and current, listing every authorized seller with the correct account identifiers, marking each relationship's type honestly, and removing sellers that are no longer authorized. A stale or incomplete ads.txt is worse than mildly unhelpful: it can block legitimate demand if an authorized seller is missing, or leave a gap that fraud can exploit. For a buyer, using ads.txt well means actually enforcing it, configuring buying platforms to verify sellers against the file and to refuse inventory from sellers a publisher has not authorized. The standard only works when publishers publish carefully and buyers check, so both sides have to participate for the transparency to be real.

The failures are familiar. Publishers let the file drift out of date, so authorized partners get blocked or unauthorized ones slip through the gap. Teams confuse ads.txt with sellers.json and look to the wrong party for the wrong information. Buyers treat the file as a complete fraud defense, forgetting that it verifies authorization but not the quality or viewability of an impression. And some ignore the standard entirely, forgoing an easy, well-established protection against spoofing. The discipline is to maintain an accurate ads.txt as a publisher, enforce it as a buyer, use it together with app-ads.txt, sellers.json, and SupplyChain rather than in isolation, and remember that it closes the domain-spoofing gap specifically, a real and valuable job, but not the whole of supply-chain safety.

Ads.txt, for Authorized Digital Sellers, was introduced by the IAB Tech Lab in 2017 to let publishers declare authorized sellers and curb domain-spoofing fraud.

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