CNAME cloaking
CNAME cloaking points a subdomain of your own site (via a DNS CNAME record) at a third-party tracking provider, so the tracker appears first-party to browsers and ad blockers. This page explains the mechanism, the security and privacy risks it introduces, and how browser anti-tracking features have started to counter it.
How the mechanism works
Normally a third-party tracker is loaded from its own domain, which browsers and blockers can recognise. With CNAME cloaking, the site creates a subdomain (for example metrics.example.com) and uses a DNS CNAME to point it at the tracking provider's host. Requests now appear to go to the site's own domain, so they are treated as first-party — including for cookie scoping — even though the data reaches a third party.
- A site subdomain CNAMEs to the tracker's host
- Browsers see the request as first-party
- First-party cookies can be exposed to the third party
Risks and browser response
Researchers documented that CNAME cloaking can leak unrelated first-party cookies (including session cookies) to the tracking endpoint, creating security and privacy exposure. Browsers have responded: Safari's ITP caps the lifetime of cookies set via CNAME-cloaked responses, and uBlock Origin and Firefox added CNAME-uncloaking to detect the underlying third party. The technique is therefore both risky and increasingly countered.
How it appears in analytics and logs
A subdomain whose CNAME resolves to a third-party tracker means requests to it are functionally third-party — cookies and data flow to that provider despite the first-party appearance.
Diagnostic use case
Recognise CNAME-cloaked setups in your own stack so you can assess the cookie-leakage and security risks rather than treating the subdomain as genuinely first-party.
What WebmasterID can help detect
WebmasterID uses genuine first-party measurement rather than CNAME-cloaked third parties, so there is no hidden third-party data flow behind a disguised subdomain.
Common mistakes
- Treating a CNAME-cloaked subdomain as truly first-party.
- Overlooking session-cookie leakage to the tracking endpoint.
- Assuming cloaking evades all browser anti-tracking measures.
Privacy and accuracy notes
This page is educational and not an endorsement. CNAME cloaking can undermine user expectations and is treated by some browsers as tracking; it does not change a vendor's role under privacy law.
Related pages
- Safari ITP and analytics privacy
Intelligent Tracking Prevention (ITP) is WebKit's privacy feature that partitions and limits storage to stop cross-site tracking in Safari. It blocks third-party cookies, caps script-set first-party cookie lifetimes, and constrains other client-side storage. This page summarises ITP's documented behaviours and what they mean for measuring audiences.
- First-party cookie lifespan and caps
Even first-party cookies no longer live as long as their stated expiry. Safari's Intelligent Tracking Prevention caps script-set first-party cookies to seven days (or 24 hours in some cases), and other browsers apply their own storage limits. This page explains how those caps work and why they fragment returning-visitor and retention metrics.
- Server-side tagging and privacy
Server-side tagging runs tag logic in a server container you control instead of the visitor's browser. It can reduce data exposed to third-party scripts, give you a place to strip or anonymise fields before forwarding, and improve load on the client. But it does not by itself reduce what you collect, and routing data through your server can shift, not remove, responsibilities. This is educational, not legal advice.
- Privacy-first analytics
Genuine first-party measurement, no cloaked third parties.
Sources and verification notes
- WebKit — Tracking prevention and CNAME cloakingSafari/WebKit defence against CNAME cloaking.
- MDN — CNAME recordHow a CNAME DNS record aliases a hostname.
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.