ads.txt and sellers.json: How Transparency Protects Publishers and Advertisers

Programmatic advertising moves billions of impressions through automated auctions every day — but that speed and scale once created a serious problem: how does an advertiser know the impression they're buying is really coming from the website it claims to be? Two industry standards, ads.txt and sellers.json, were built to answer that question. In this post you'll learn what they are, how they work together, and why they matter to both publishers and advertisers on any RTB platform.

The Problem: Domain Spoofing and Unauthorized Reselling

Before these standards existed, fraudsters exploited a simple weakness in the supply chain. In domain spoofing, a bad actor sells low-quality or bot traffic while labeling it as inventory from a premium, well-known site. An advertiser thinks they're buying an impression on a major news site; in reality, their budget is going to a worthless page dressed up in a fake domain.

A related problem is unauthorized reselling — intermediaries passing inventory along a long, murky chain of resellers, each taking a cut, with no way for the buyer to confirm who is actually authorized to sell that inventory. The result was wasted ad spend for advertisers and lost revenue for the legitimate publishers whose brands were being counterfeited.

The Interactive Advertising Bureau (IAB) Tech Lab introduced ads.txt and sellers.json to close these gaps by making the supply chain verifiable.

What Is ads.txt?

ads.txt stands for "Authorized Digital Sellers." It's a simple text file that a publisher places in the root directory of their website — for example, example.com/ads.txt. The file publicly lists every company authorized to sell that publisher's ad inventory.

Each line in the file names an advertising system and follows a straightforward format:

`` Domain of the ad system, Publisher's account ID, Relationship type, Certification ID ``

• Relationship type is either DIRECT (the publisher has a direct deal with that ad system) or RESELLER (the publisher has authorized that system to resell the inventory).

Because the file is public, any advertiser, DSP, or ad exchange can crawl it and confirm: "Is this seller actually allowed to offer this site's inventory?" If a bid comes in claiming to represent a domain, but the selling account isn't listed in that domain's ads.txt file, buyers can ignore it. That single check eliminates a huge share of domain spoofing.

For app-based inventory, the same concept exists as app-ads.txt, hosted on the developer's website and tied to the app's store listing.

What Is sellers.json?

If ads.txt lets publishers declare who is allowed to sell for them, sellers.json lets ad systems declare who they are selling on behalf of. Each advertising exchange or SSP publishes a sellers.json file that lists the sellers and intermediaries operating in its marketplace, along with their seller IDs and whether each is a publisher, an intermediary, or both.

Together with the SupplyChain Object (schain) — a record passed inside each bid request that maps every hop the impression traveled — sellers.json lets a buyer trace an impression from the original publisher all the way to the buyer, confirming each participant along the route. Think of ads.txt as the publisher's guest list, sellers.json as each venue's staff directory, and the SupplyChain Object as the itinerary showing the exact path taken.

Why This Matters for Publishers

For publishers, keeping an accurate ads.txt file is one of the easiest and highest-impact things you can do to protect and grow revenue:

• Stop counterfeiters from stealing your demand. When your authorized sellers are clearly listed, spoofers can't credibly sell fake versions of your inventory.
• Win more advertiser trust. Many advertisers and DSPs simply won't buy from domains with missing or misconfigured ads.txt files. A clean file keeps you eligible for more demand.
• Protect your eCPM. By reducing fraud in the ecosystem, you help ensure advertiser budgets flow to real inventory — including yours.

When you monetize with Squren, make sure to add the exact ads.txt line we provide so our demand can bid on your inventory without friction. If you're new to earning from your site, our guide on how publishers can monetize their traffic with Squren walks through the setup.

Why This Matters for Advertisers

For advertisers and media buyers, ads.txt and sellers.json are core tools for brand safety and budget efficiency:

• Buy authorized inventory only. Filtering to verified, ads.txt-authorized sellers keeps your spend on legitimate impressions instead of spoofed domains.
• Shorten the supply chain. By reading the SupplyChain Object, you can favor direct or short paths — a practice closely tied to supply path optimization — so more of your budget reaches the publisher instead of being eaten by resellers.
• Reduce fraud exposure. Combined with fraud filtering, these standards dramatically cut the risk of paying for invalid traffic.

These transparency standards work hand in hand with the fraud protections we build into the platform. To see how we defend campaigns further, read how Squren protects your campaigns from fraudulent traffic.

Best Practices Checklist

• Publishers: Host your ads.txt at your root domain, include only sellers you actually authorize, remove old or inactive lines, and re-check the file after any monetization change.
• Advertisers: Prefer authorized, DIRECT paths where possible, inspect the SupplyChain Object, and treat unauthorized sellers as a red flag.
• Everyone: Revisit these files regularly — supply chains change, and stale entries create both risk and lost opportunity.

Conclusion

ads.txt and sellers.json turned programmatic advertising from a black box into a supply chain you can actually verify. For publishers, they protect revenue and reputation. For advertisers, they safeguard budgets and brand safety. Transparency isn't just a compliance box to check — it's what makes trustworthy, high-performing RTB possible.

Ready to buy or sell inventory on a platform built for transparency and fraud-free traffic? Sign up as an advertiser or publisher at Squren.com today, or contact our 24/7 support team to learn how our RTB marketplace keeps your campaigns and your inventory protected.