What is Ads.txt?

Ads.txt (Authorized Digital Sellers) is a plain text file publishers host at the root of their domain. Each line names an advertising system allowed to sell their inventory, the publisher’s account ID there, and whether the link is DIRECT or RESELLER. Buyers use it to confirm sellers are authorized before they bid.

The file is public, human-readable, and machine-parsed, which made adoption faster than heavier certification schemes alone.

How ads.txt is used in bidding

Demand-side platforms and related tools crawl example.com/ads.txt and cache the results. During an auction, the buyer compares the seller declared on the bid request with the publisher’s file. If the seller is not listed, many buyers skip the bid to reduce domain spoofing and unauthorized resale.

Syntax matters: typos or wrong publisher IDs can block legitimate revenue. Publishers update the file when they add or remove partners. Optional fields can include certification IDs where partners support them. Schedule reviews after every new SSP or exchange onboarding so sales teams are not surprised by sudden bid drops.

Why ads.txt matters for ad fraud

Spoofing is listing fake inventory as if it came from a premium site. Ads.txt shrinks the window by requiring a public match between domain and seller. It does not stop every abuse: bot traffic, invalid clicks, and creative fraud can still occur on authorized domains, so treat ads.txt as one layer in a broader ad fraud strategy.

For advertisers, enforcing ads.txt checks cuts spend on misrepresented web supply and complements click fraud work on other channels. Publishers gain trust and fewer blocked bids when the file is accurate. Layer policy with invalid impression hygiene and detection fundamentals you already trust.

Related transparency pieces

• Sellers.json on the sell side discloses who monetizes for each seller ID
• Supply-path reviews to remove redundant resellers
• Monitoring for stale or oversized files that signal poor governance
• Cross-checks with analytics when spoofed domains still send referral traffic flagged as suspicious activity downstream

Abisola

Meet Abisola! As the content manager at ClickPatrol, she’s the go-to expert on all things fake traffic. From bot clicks to ad fraud, Abisola knows how to spot, stop, and educate others about the sneaky tactics that inflate numbers but don’t bring real results.

View all posts by Abisola (Abisola)