What is the MUID cookie?
MUID is an HTTP cookie acting as a unique identifier, set by Microsoft domains including bing.com. It typically appears when a site loads Microsoft resources, most commonly the Bing Ads conversion tracking script (UET tag), a Microsoft widget or button, or any integration that contacts Microsoft's servers.
It isn't limited to bing.com – many websites running Microsoft Advertising for conversion tracking set it on their own domain as a third-party cookie tied to Microsoft's infrastructure.
What data it stores
MUID stores a unique identifier associated with the visitor's browser, generated by Microsoft. The value is an opaque alphanumeric string with no meaning readable by the site owner.
The data is sent to Microsoft/Bing servers, where it is linked to browsing activity across sites using Microsoft services (Bing Ads, Bing.com, some Microsoft products), building a cross-site behavioral profile.
What it is used for
For Microsoft, MUID identifies the same user across multiple visits and sites, enabling conversion measurement for Microsoft/Bing Ads campaigns, ad targeting, and behavioral analysis for marketing purposes.
For the site owner, this cookie is only useful if they run paid Bing Ads campaigns and want to measure the conversions they generate; otherwise it provides no functional benefit to the site itself.
Does it require consent?
MUID is classified as a marketing (advertising/tracking) cookie and is not strictly necessary for the site to function. Under GDPR and the ePrivacy Directive (transposed in Romania via Law 506/2004), setting and reading this cookie requires prior, explicit visitor consent given through an affirmative action (checkbox/click), not by mere browsing.
Site owners must disclose MUID in their cookie policy, listing its provider, purpose, and duration, and ensure the script that sets it (typically the Bing Ads tag) does not load before consent is obtained.
How to block or delete MUID
- Visitors can delete or block MUID via browser cookie settings, use private browsing, or install tracker-blocking extensions; this does not affect site functionality, only Microsoft's ad tracking.
- Site owners must prevent the Bing Ads/UET script from loading before consent is given.
- A CMP like CookieFix can automatically block this script until the visitor accepts the marketing category, preventing MUID from being set prematurely.
Frequently asked questions
It's a unique identifier set by Microsoft, mainly through Bing Ads, used to recognize visitors across multiple sites and measure advertising conversions.
It doesn't contain directly readable information like a name, but it's considered personal data under GDPR because it enables tracking an individual across the internet.
Its typical duration is around 1 year from being set or last updated, though this can vary.
Yes, since it's a marketing/tracking cookie, it requires prior visitor consent under GDPR and ePrivacy, and the script that sets it must be blocked until consent is given.