An IAB Tech Lab standard that lets app publishers declare which ad networks are authorized to sell their inventory.

Where do I put the app-ads.txt file?

At the root of the developer website listed in your App Store / Google Play page, e.g. https://studio.com/app-ads.txt.

What is a Certification Authority ID?

An optional TAG-ID of the ad system, the 4th field of an app-ads.txt record.

538.nl is missing out on higher CPMs: its app-ads.txt has 6 invalid lines. Buyers' crawlers skip invalid lines, so those sellers can't be verified.

Checked 19 Oct 2026. 171 lines, 132 unique records, 2 duplicates, 6 invalid lines, 6 ad networks detected.

Xandr (Microsoft) — accounts 7463, 9302, 9994 and 4 more: 7 lines in the file
Google AdMob — accounts pub-5443109342713637, pub-5854238349292660, pub-1088068438510765: 3 lines in the file
Index Exchange — accounts 184476, 188249: 2 lines in the file
Magnite (Rubicon) — account 12108: 1 line in the file
PubMatic — accounts 162544, 160813, 161079 and 1 more: 4 lines in the file
Criteo — accounts B-060835, B-059589: 2 lines in the file

Yes — https://538.nl/app-ads.txt has 171 lines (checked 19 Oct 2026).

Its app-ads.txt lists Xandr (Microsoft), Google AdMob, Index Exchange, Magnite (Rubicon), PubMatic, Criteo.

538.nl is missing out on higher CPMs: its app-ads.txt has 6 invalid lines. Buyers' crawlers skip invalid lines, so those sellers can't be verified.