What it does
MUID identifies a browser to Microsoft and is used across Microsoft sites and services, including advertising. Because the same identifier is recognised on other properties, it supports cross-site recognition in a way a site-scoped analytics cookie does not.
It commonly appears alongside `_uetsid` and `_uetvid`, the Universal Event Tracking cookies, when a Microsoft Advertising tag is present.
What to write in a cookie declaration
List Microsoft as the provider and be explicit that the identifier is shared across Microsoft properties. That is the fact a visitor would want to know.
| Field | Value |
|---|---|
| Name | MUID |
| Provider | Microsoft |
| Purpose category | advertising |
| Consent category | Marketing |
| Expiry | About a year (vendor default) |
| Storage | Third-party |
Treat the expiry as indicative
The value above is the vendor’s documented default. Vendors change configuration and several browsers cap script-written lifetimes. Record what repeated scans observe on your own site, with the browser, region, path and interaction state; one run is not authoritative for every visitor.
Can a visitor refuse it?
Yes, in marketing. It appears because a Microsoft advertising or Clarity script is running; not loading that script is the control.
A verification scan supplies runtime evidence for the pages and states it exercises. Test a fresh profile with no choice made, then after refusal, and reconcile observed storage with response headers, server-side integrations and paths the scan did not visit.
Common questions
Why does MUID appear when I only installed Clarity?
Clarity is a Microsoft product and shares parts of Microsoft’s identity infrastructure. A scan reporting MUID on a site with no ads tag is worth investigating rather than dismissing.
Sources and verification
Verified on . Product-behaviour statements were checked against the current implementation and tests. The links below are the verification basis recorded for this article. They support the stated facts, not a legal conclusion for every site or configuration; recheck changing vendor behaviour before relying on it in production.
- Microsoft Advertising: Universal Event Tracking
Vendor documentation
Checked
See what your own site is loading
A browser scan reports the requests and storage it observed during the sampled journey. Use configured workspace scans to compare the states and pages that matter to your implementation.