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ads.txt and app-ads.txt

October 22, 2026

ads.txt (Authorized Digital Sellers) is a plain text file a website places in its root directory, for example example.com/ads.txt, to publicly list the companies allowed to sell its ad space. app-ads.txt is the extension of ads.txt for mobile apps and connected TV (CTV) apps.

What is ads.txt and why does it exist?

Programmatic advertising sells ad space through automated auctions, and each bid request declares the website’s own domain. Fraudsters exploit this by labeling ad space on a junk site with the name of a major newspaper and selling it at the newspaper’s price. That scheme is called domain spoofing.

IAB Tech Lab drafted the ads.txt standard in spring 2017 and published version 1.0 on June 27, 2017. Like robots.txt, the file lives on the website’s own server, so only the site owner can post it. Version 1.1 was finalized in July 2022 and added the OWNERDOMAIN and MANAGERDOMAIN variables (IAB Tech Lab, ads.txt 1.1, July 2022).

In the 3ve case, 80.3% of the fraud operation’s bid requests in early 2018 came from sellers not listed in the ads.txt file of the domain they claimed. If every party refused unauthorized inventory, more than 80% of 3ve’s inventory would be blocked (Google and White Ops, The Hunt for 3ve, November 2018).

What does an ads.txt line contain?

Each line declares one account that may sell. Here is the sample line from Google AdSense (Google AdSense Help, Ads.txt guide, accessed October 2026):

google.com, pub-0000000000000000, DIRECT, f08c47fec0942fa0

Field Meaning In the example
1 Domain of the SSP or ad exchange google.com
2 Publisher account ID in that system, which must match the ID in the bid request pub-0000000000000000
3 DIRECT: the site owner controls the account. RESELLER: another party is authorized to resell DIRECT
4 Certification authority ID, optional f08c47fec0942fa0

How is app-ads.txt different from ads.txt?

Apps have no domain of their own, so app-ads.txt relies on the developer website listed on the app store page. If an app’s Google Play page lists myappsite.com, the file must sit at myappsite.com/app-ads.txt. The ad exchange passes the store URL (storeurl) in the bid request so buyers can find the file (IAB Tech Lab, app-ads.txt 1.0, accessed October 2026).

How do buyers use ads.txt?

Display & Video 360 (DV360), Google’s DSP, excludes unauthorized sellers for web, app and TV inventory. It checks every party in the selling chain, intermediaries included, against the SupplyChain object (schain). Sites without an ads.txt file can only be bought if the advertiser changes the setting and accepts the risk (Display & Video 360 Help, Authorized Digital Sellers, accessed October 2026).

sellers.json, an IAB Tech Lab standard finalized on July 31, 2019, fills the gap ads.txt leaves. ads.txt says which account IDs may sell. sellers.json says which company each ID belongs to (IAB Tech Lab, sellers.json, updated October 2026).

Hypothetical example: a news site lists 2 SSPs in its ads.txt. A DSP receives another 1 million bid requests carrying the site’s domain from a third SSP with an unknown account ID. At a CPM of 40,000 VND, that inventory is worth 40 million VND. With authorized seller filtering on, that money is never spent.

What can’t ads.txt stop?

IAB Tech Lab’s app-ads.txt document states the limit plainly: the standard cannot stop a dishonest platform from swapping an unauthorized publisher ID for an authorized one before offering the impression. Authorized inventory still has to be screened for invalid traffic. A real website can still pump in bots or stack ads. ads.txt is one layer in fighting ad fraud, not a complete filter.

Frequently asked questions

Does an online store need an ads.txt file?

Only if the site sells ad space, for example through Google AdSense (using the sample line above). AdSense says the file is not mandatory but highly recommended. A site that only buys ads for itself does not need one.

How do I check a website’s ads.txt?

Type the domain followed by /ads.txt into a browser, for example https://example.com/ads.txt. If the file appears, AdSense can most likely read it. Whether the site runs on WordPress or another platform, the file goes in the root directory. AdSense may take a few days, sometimes up to a month, to pick up changes.

If you buy display ads through a DSP and want a second look at your inventory settings, you can talk to Adtek through our contact page.

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