What is cookie sync?
Cookie sync, also called user sync or ID sync, is the process by which two ad tech companies learn each other's cookie IDs for the same browser, so one can send IDs the other understands. Most often an exchange keeps a match table of its IDs against each DSP's IDs and places the DSP's ID in User.buyeruid on later bid requests.
Also called: cookie syncing, user sync, ID sync, cookie matching, user matching.
How it works
Cookies belong to one domain, so an exchange cannot read a DSP's cookie and a DSP cannot read the exchange's. Syncing works around that with redirects. The exchange loads the DSP's sync pixel in the browser, the DSP reads its own cookie and redirects to an exchange URL with its user ID in a query parameter, and the exchange reads its own cookie on that call and stores the pair. Appendix C of OpenRTB 2.6 walks through this flow for match tables hosted by either side.
Sync URLs usually carry privacy parameters: the GDPR flag and TCF string, the GPP string and section IDs, and the US Privacy string. Each party should check them before writing a match. From OpenRTB 2.6-202409, an ID passed in User.eids can record that it came from a cookie sync with EID.mm = 2.
Cookie sync works only where third-party cookies do. Safari and Firefox block or partition them by default, and apps and CTV devices have no shared cookie at all, which is why identity has shifted toward User.eids and device advertising IDs. Pitfalls: stale buyeruid values that point to expired cookies, sync pixels that slow the page, and syncing with partners that never bid.
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Related terms
Every term is in the glossary.