Glossary
What is Universal ID?
Also called: identity solution, shared ID, UID2, ID5 and SharedID
A universal ID is one identifier that many buyers can read, used to recognise readers where third-party cookies are blocked or partitioned.
हिंदी में पढ़ेंChrome still allows third-party cookies by default, but Safari blocks them and Firefox isolates them per site, so on much of the web a buyer cannot recognise the reader. A universal ID fills that gap. IDs travel in the bid request’s extended IDs list, user.ext.eids, each tagged by its source.
Three common examples work differently. SharedID is a free Prebid ID stored in a first-party cookie on your domain; it needs no login but only works on your site. ID5 needs a partner number and returns an encrypted ID, built from signals your page shares such as a hashed email, that works across sites. UID2, open source from The Trade Desk, starts from a logged-in email or phone number, hashed and salted, and becomes a rotating encrypted token; readers can opt out at any time.
In Prebid, the User ID module loads these IDs. Without TCF Purpose 1 consent it makes no calls and stores nothing. Typing pbjs.getUserIdsAsEids() in the browser console shows which IDs actually loaded.
Example
Say a site has 100,000 daily readers: 30% on Safari and 8% logged in. SharedID can cover the 30,000 Safari readers, but only on this site. UID2 needs a login, so it can cover at most the 8,000 logged-in readers.
Why it matters to publishers
Recognised readers attract more bids, and universal IDs extend recognition beyond Chrome. Knowing which IDs your stack actually sends is the first step.