What is ads.txt?
ads.txt (Authorized Digital Sellers) is a public text file publishers host on their website listing every company allowed to sell their ad inventory, and whether each is a direct seller or reseller. Buyers check it to avoid buying from unauthorized sellers, which makes domain spoofing harder. The IAB Tech Lab introduced it in 2017.
Each line in ads.txt names an ad system, the publisher's account ID in that system, the relationship (DIRECT or RESELLER) and optionally a certification authority ID. Anyone can view it at example.com/ads.txt. app-ads.txt extends the idea to mobile and CTV apps, with the file hosted on the developer's website listed in the app store.
ads.txt only says who may sell; it does not prove where an individual impression came from. It works best together with sellers.json, which identifies the sellers behind account IDs, and the supplyChain object, which records each hop in a transaction. Publishers should keep their files current, as stale entries give fraudsters room.