ID5 ID

The ID5 ID is a shared identifier from the company ID5 that publishers and ad platforms use to recognise users without third-party cookies, built from publisher signals, hashed emails and probabilistic links.

Prebid's documentation describes the ID5 ID as a shared, neutral identifier that is free to use after registering with ID5 and getting a partner number (Prebid.org, accessed 9 November 2026). Publishers add the ID5 module to Prebid.js or call ID5's service, and the ID goes into bid requests in the user.eids array.

Unlike UID2 or EUID, the ID5 ID does not need a login. It combines signals the publisher passes, a hashed email where one exists, and probabilistic matching across devices. That gives it reach on pages where few users sign in. It also means its accuracy and its privacy standing depend on how each publisher collects consent.

PPC Land reports that the ID is encrypted with a rotating key, so only permissioned partners can read it. According to the same report, publishers should refresh it every 7,200 seconds or less, and storage typically expires after 90 days (PPC Land, 22 August 2026). ID5 Technology Limited was incorporated in London in April 2017, the report adds.

Example: On 19 June 2023 The Trade Desk added ID5's graph to its Identity Alliance and let publishers that use ID5 generate EUID values for logged-in traffic (PPC Land, 22 August 2026). A publisher can therefore pass both an ID5 ID and an EUID in the same bid request.

Related terms: UID2, EUID, RampID, household graph, user matching.

Authoritative source: Prebid.org, "ID5 ID for Header Bidding" user ID module documentation (accessed 9 November 2026).