Cookie syncing, also known as cookie matching, is a process used by advertising technology platforms to share user identifiers across different domains. Since cookies are domain-specific, an ad exchange cannot directly read a cookie set by a demand-side platform.
Cookie syncing solves this by having one platform redirect the user’s browser through another platform’s domain, allowing both platforms to link their respective cookie IDs for that user. This creates a shared identifier that enables cross-platform targeting, frequency capping, and attribution. Cookie syncing typically happens through pixel fires and redirect chains that are invisible to the user.
Cookie syncing raises significant privacy concerns because it enables extensive cross-site tracking without explicit user awareness. Under the GDPR and ePrivacy Directive, cookie syncing requires informed consent because it involves setting third-party cookies and sharing personal data between multiple parties.
As browsers block third-party cookies, traditional cookie syncing is becoming technically impossible in many environments. The Privacy Sandbox and similar initiatives are developing privacy-preserving alternatives that achieve some of the same advertising objectives without cross-site cookie matching.
Seers’ CMP detects cookie syncing activities during its automated website scan, identifying the tracking pixels and redirect chains involved. These are categorised under advertising cookies and blocked until the user provides explicit consent.
Seers’ integration with Google Consent Mode v2 and server-side tagging offers alternative approaches that reduce dependence on cookie syncing while maintaining advertising effectiveness.
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