But ads.txt is not an ad fraud panacea. Though it can help advertisers and publishers fight domain spoofing, it does not block bot traffic.
Publishers also make mistakes in their ads.txt files, which makes it difficult for demand-side platforms (DSPs) to properly filter unauthorized inventory. FirstImpression.io estimates that among the top 1,000 publishers according to Alexa ranks, 27% have errors in their ads.txt files.
Per Q4 2018 Pixalate estimates, websites that had adopted ads.txt had display ad fraud rates that were 3.5 percentage points lower than websites that hadn't implemented ads.txt. While the initiative can help lower fraud rates by keeping ads away from spoofed domains and low-quality arbitraged inventory, it’s possible the difference in these fraud rates is partly driven by self-selection bias. Websites with ads.txt adoption might have taken ad fraud seriously from the beginning and have other guards in place that drive their fraud rates down.