Cookie sync

Cookie sync is the process in which two ad tech companies map their cookie IDs for the same browser to each other through redirects, so one can recognise the other's user in later bid requests.

Each company sets cookies on its own domain and cannot read anyone else's. A sync connects them one browser at a time. Prebid Server shows the mechanics clearly. Prebid.js calls the /cookie_sync endpoint with the list of server-side bidders and any consent strings. Prebid Server reads its uids cookie on the host domain and returns sync URLs for bidders that are missing or stale (Prebid.org, accessed 9 November 2026).

The page then drops a pixel or iframe for each bidder. The bidder reads its own cookie and redirects the browser to Prebid Server's /setuid endpoint with its ID in the URL. If privacy rules allow, Prebid Server writes that ID into the uids cookie. A "cooperative syncing" mode can sync enabled bidders even when they are not on the current page.

Syncs depend on third-party cookies, so they fail where browsers block them. Prebid.js does not fire registered syncs in Safari, and since version 11.9 it skips them in Firefox and Chrome on iOS too (Prebid.org, bidder adapter guide).

Example: A news site runs four server-side bidders through Prebid Server. On a reader's first visit, the /cookie_sync response marks all four with "no_cookie": true, and the page fires four sync pixels. By the second page view, the uids cookie holds four bidder IDs that Prebid Server can place in each bidder's requests.

Related terms: user matching, Prebid Server, DMP, third-party data, UID2, CHIPS.

Authoritative source: Prebid.org, "Prebid Server User ID Sync" (accessed 9 November 2026).

Read the full guide: Prebid Server cookie sync and user matching explained.