What is Cookie sync?

Each ad tech company sets its own cookie on its own domain, so an SSP and a DSP each hold a different ID for the same browser. A cookie sync links the two, usually through a small pixel or iframe call that passes one company’s ID to the other. Once they are matched, a bid request carrying the SSP’s ID can be read by the DSP as one of its own users.

Syncing matters most for server-side bidding. In Prebid Server the bidders are called from a server, not the page, so they cannot read their own cookies directly. Prebid Server relies on cookie syncing to pass their reader IDs. Without a match, a bidder still sees the request but may not recognise the reader.

Cookie syncs depend on third-party cookies. Chrome still allows them by default, but Safari blocks cookies for cross-site resources by default and Firefox gives each site its own cookie jar, so syncs fail on much of that traffic. That is one reason publishers add first-party and universal IDs.