ChatGPT ad cookie links browsing on other sites to your account

A researcher has documented that OpenAI's ad collector at bzr.openai.com sets a cookie named __obi, scoped to .openai.com and tied to a ChatGPT account, which is then sent to OpenAI from ordinary websites the user visits.

The flow: the chatgpt.com client generates 16 random bytes and calls /backend-api/bazaar/obi/sync-token, and the backend returns an RS256 JWT that expires in 60 seconds. The client posts that token to bzr.openai.com/v1/obi/sync, which writes a one-year cookie configured with SameSite=None; Secure. On advertiser-page requests, every other OpenAI cookie was blocked by the browser; __obi was the only identifier that travelled.

What the traffic showed

The findings cover several months of observed traffic across 936 distinct advertiser pixels on 1,029 hostnames. On a single device, one __obi value reached OpenAI from 12 commercial sites under 13 pixel IDs, including Chewy, Wayfair, ThriftBooks, Eventbrite, HelloFresh, Coursera and SeatGeek, with every request accepted with a 202 response. Of 30 distinct __obi values in the wider sample, 12 appeared under more than one advertiser.

The pixel SDK also gathers identity from the advertiser's page. Scraped identity, taken from form fields, rendered page text and the tag-manager bus, outnumbered advertiser-supplied identity 685 events to 255. Email, phone and names are SHA-256 hashed before transmission, while country, region, city and postal code are sent in the clear. URLs arrive without query strings, but paths survive.

OpenAI's cookie policy lists __obi under analytics cookies. All 932 decoded sync tokens carried consent_decision: analytics_allowed, meaning a user who permits analytics but refuses marketing still receives it. The researcher emailed OpenAI on 14 September with two questions; OpenAI Support said the observations would be reviewed internally and answered neither.

The mechanism was observed on Chrome for Android and does not operate on iOS browsers. Full technical detail is in the original writeup.