What the research claims
The security outfit Buchodi published an analysis on Sunday of the measurement pixel OpenAI gives advertisers who buy ads inside ChatGPT. Its conclusion is that the pixel lets OpenAI connect a person’s ChatGPT identity to what they do on websites that have nothing to do with ChatGPT.
The team says it observed the pixel across twelve commercial websites using thirteen distinct pixel identifiers, naming Chewy, Wayfair, HelloFresh, Coursera and SeatGeek among them. Of 932 tokens it decoded, 736 carried a subject tied to a ChatGPT account and 196 carried an anonymous subject. The anonymous identifier, Buchodi says, was just as stable as the account-linked one, persisting on a device for at least 27 days.
These are the researchers’ figures, drawn from their own instrumentation. OpenAI has not published numbers of its own.
How the cookie is said to work
According to the analysis, ChatGPT’s backend issues a signed token binding an identifier to either a ChatGPT account or an anonymous session. That identifier is stored in a cookie named __obi, set on OpenAI’s domain with a one-year lifetime and the SameSite=None and Secure attributes — the combination that allows a cookie to be sent from one site to another. When a page carrying the advertiser pixel loads, the cookie goes back to OpenAI alongside data about the page.
That data is not limited to what the advertiser chooses to hand over. Buchodi reports that the pixel’s software development kit scrapes the page it is on, collecting hashed email, phone and name values along with unhashed country, region, city and postal code, plus visited URLs, form fields and page content. Scraped identity outnumbered advertiser-supplied identity by 685 events to 255 in the traffic the team watched, and 638 of 881 pixels it saw had automatic data matching switched on.
Analytics or marketing
The sharpest point in the report is a classification question rather than a technical one. OpenAI’s cookie policy treats __obi as an analytics cookie rather than a marketing cookie. Under that classification, a user who declines marketing consent still receives it — which is how a cross-site identifier can survive a consent choice that most people would expect to stop exactly this.
The pixel also stores a click identifier in a first-party cookie on the advertiser’s own domain, which is the part of the design that looks most like conventional conversion tracking.
What OpenAI said
Buchodi says it put its questions to OpenAI on 14 September. The company acknowledged the inquiry, the report says, but did not answer how __obi is classified or how consent for it is handled. Aivio News has seen no public statement from OpenAI on the findings.
Logged-in users can switch ad tracking off under Settings, Data Controls, Marketing Privacy, according to the research — a control that sits inside the account rather than in the consent banner an advertiser’s site shows.
What to watch
The classification is the thing regulators tend to test. European data protection authorities have repeatedly held that a cookie’s purpose, not its label, decides whether consent is required for it, and OpenAI now runs an advertising business inside a product with several hundred million users. Whether a supervisory authority takes an interest in how __obi is described is the next concrete step to look for.
