Hostname leakage across properties
Your measurement ID is visible in page source, so anyone can paste it on another site and have that traffic report into your property. Staging copies, scraped clones, and proxies do this too. The leaked hits inflate and pollute your data with another domain's traffic. This page explains hostname/property leakage and the valid-hostname filtering that contains it.
How traffic leaks across properties
A measurement or tracking ID is not a secret — it ships in the page source of every visitor. Anyone can copy it onto another domain, and those hits will report into your property. It also happens innocently: a cloned staging site, a scraped or mirrored copy of your pages, or a proxy that serves your content all carry the same ID and feed your reports.
The leaked traffic is real visits to a different domain, so it inflates totals and distorts segments with audiences that are not yours.
- Measurement IDs are visible in page source
- Clones, mirrors, and proxies carry the same ID
- Foreign hits inflate and pollute your property
Containing it with hostname filtering
Add a valid-hostname filter so only hits reporting your own domain(s) are counted; everything else is dropped. Review the hostname dimension periodically to spot domains you do not recognise. This is also the core defence against ghost spam, which relies on sending hits that do not match your hostname. Validate by confirming foreign hostnames stop appearing after the filter is live.
How it appears in analytics and logs
Hostnames you do not own appearing in a hostname report mean another domain is sending hits with your measurement ID.
Diagnostic use case
Detect and exclude traffic from domains that are not yours but report into your property, so your data reflects only your own site.
What WebmasterID can help detect
WebmasterID ties measurement to your own first-party domain, so foreign domains cannot trivially fold their traffic into your human analytics.
Common mistakes
- Assuming a measurement ID is private and cannot be reused.
- Never reviewing the hostname dimension for foreign domains.
- Skipping a valid-hostname filter that also blocks ghost spam.
Privacy and accuracy notes
Hostname filtering matches the reporting domain of a hit, not visitor identity. No personal data is required to contain leakage.
Related pages
- Referral spam and ghost traffic
Referral spam and ghost traffic are fake hits crafted to appear in your reports. Crawler spam loads pages to leave a referrer in your logs; ghost spam sends hits straight to a measurement endpoint without ever visiting your site. Both add phantom sessions with no engagement. This page explains the mechanics and the filtering that removes them.
- An analytics data-validation checklist
Before you act on a report, validate the data that produced it. This checklist walks the recurring failure points — duplicate tags, unfiltered bots, internal traffic, wrong time zone, broken events, sampling — and gives a concrete check for each. Run it after any tracking change and periodically, so a metric you trust is a metric you have verified.
- Developer and QA traffic in reports
Development, staging, preview, and automated test traffic can all reach a production analytics property if the same measurement ID is reused or environments are not separated. The hits look like engaged users but represent your own pipeline. This page explains how dev and QA traffic leaks into reports and the configuration that keeps environments cleanly apart.
- Multi-site analytics
Keep each site's traffic in its own property.
Sources and verification notes
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.