What is cookie syncing?
Cookie syncing is the process by which two ad tech companies match their separate cookie IDs for the same browser, so they can share data about that user. When a page loads, one platform redirects to another with its ID attached, and both store the mapping. It enables targeting and bidding across platforms, but raises privacy concerns because it spreads user IDs widely.
Because each company's third-party cookie is readable only on its own domain, a DSP and an SSP need cookie syncing to know they are looking at the same browser in a bid request. Sync pixels can fire dozens of times on a single page, adding latency.
Safari and Firefox block third-party cookies by default, making syncing ineffective there. Alternatives include shared IDs such as Unified ID 2.0, seller-defined audiences and server-side matching.