Adstxt

ads.txt, short for Authorized Digital Sellers, is a simple text file published by a website publisher to publicly declare which companies are authorized to sell that publisher’s digital advertising inventory. The goal is to increase transparency in the programmatic advertising ecosystem and reduce ad fraud, such as domain spoofing and unauthorized resellers. The term adstxt is sometimes used to refer to the contents of this file or the practice, though the standard file itself is typically named ads.txt.

The file is hosted on the publisher’s domain, usually at https://example.com/ads.txt. It is read by demand-side platforms, exchanges, and other buyers during programmatic auctions. Each line in the file represents an authorized seller and consists of four comma-separated fields: domain (the domain of the advertising system selling the publisher’s inventory), seller_account_id (the publisher’s account identifier with that seller), relationship (DIRECT or RESELLER), and an optional certification_id that identifies an external verifier for the relationship. Lines beginning with a hash (#) are comments and ignored.

Overview and governance: The ads.txt specification is maintained by the IAB Tech Lab. The practice began in 2017 and has since been widely adopted by publishers, ad tech vendors, and buyers as part of transparency and brand-safety best practices for open web advertising.

Adoption and limitations: While widely deployed in open web programmatic marketplaces, ads.txt is not a complete fraud solution. Some publishers may not publish an ads.txt, and lines can become outdated or misrepresented. It should be used alongside other verification and monitoring tools to improve accuracy and trust in the supply chain.