Data & identity
ID bridging
ID bridging is attaching an identifier observed in one context, such as a hashed email from a logged-in session, to impressions in other browsers or devices where it was not directly observed.
Authenticated identifiers are valuable because buyers pay more for impressions they can recognise. ID bridging extends that value: a vendor that saw a user log in on one device or site uses probabilistic matching, IP address or other signals to attach the same identifier to impressions elsewhere, for example in Safari where cookies are limited.
The controversy, which became prominent in 2024, is about disclosure. If an SSP presents a bridged ID in a bid request as though it were directly observed, buyers may pay a premium for "deterministic" identity that is really an inference, and they may unknowingly bid on the same user twice through different paths. Some buyers also consider undisclosed bridging a form of misrepresentation.
In response, industry groups including IAB Tech Lab and Prebid pushed for transparency, such as fields in OpenRTB's extended IDs that indicate how an ID was matched. Buyers can ask sellers about bridging practices and compare ID presence against expected login rates. Bridging is not fraud by itself, but it deserves disclosure.
Think of it like this
ID bridging is like a waiter assuming the person at table four is the same regular who sat there yesterday because they ordered the same coffee, and billing the "regular's" loyalty rate.
An example
A buyer sees 70% of Safari bid requests from one SSP carrying a hashed-email ID, even though publisher login rates are below 10%; the SSP confirms many IDs were bridged via IP matching.
Related terms
Hashed email
A hashed email is an email address transformed by a one-way cryptographic function, usually SHA-256, into a fixed string used to match users across systems without sharing the plain address.
Probabilistic matching
Probabilistic matching links devices or identifiers to the same person or household using statistical inference from signals like shared IP address, location, timing and device traits, with some uncertainty.
Deterministic matching
Deterministic matching links identifiers to the same person using definite, shared information, such as the same login email or phone number, rather than statistical inference.
Identity graph
An identity graph is a database that links the many identifiers belonging to the same person or household, such as emails, device IDs, cookies and IP addresses, into one profile.
Bid request
A bid request is the message an SSP or exchange sends to potential buyers describing an available ad impression, inviting them to bid on it in a real-time auction.
Sources: IAB Tech Lab OpenRTB