What is the _uetmsclkid cookie?
_uetmsclkid belongs to the UET (Universal Event Tracking) cookie family used by Microsoft Advertising, the platform behind Bing Ads campaigns. It is set by the UET tag (the "bat.js" script) installed on a website to measure conversions coming from Bing ads.
The cookie is typically set as soon as a visitor lands on a site through a Bing Ads advertisement whose destination URL contains a click identifier parameter (msclkid). The UET script captures this identifier and stores it locally in the cookie so it can later be matched to a conversion event (a completed form, a purchase, etc.).
What data it stores
The cookie mainly stores the click identifier (msclkid) taken from the ad's destination URL, along with a timestamp associated with the visit. The exact value format can vary and is not documented in detail by Microsoft.
Data collected through this cookie is sent to Microsoft Advertising's servers, where it is used to connect the ad click to the user's subsequent actions on the site (conversions).
What it is used for
The main purpose is conversion attribution for Bing Ads campaigns: it lets the advertiser see how many ad clicks actually turned into meaningful actions (purchases, sign-ups, quote requests).
- Measuring the effectiveness of Microsoft Advertising (Bing Ads) campaigns
- Linking a specific ad click to a later conversion on the site
- Optimizing bidding and ad spend based on results
Does it require consent?
From CookieFix's classification, _uetmsclkid falls under the marketing category (advertising/tracking). Under the GDPR and the ePrivacy Directive (transposed in Romania through Law 506/2004), this type of cookie is not strictly necessary for the website to function, so it requires the visitor's prior, explicit consent before being set.
Website operators must list this cookie (or the Microsoft Advertising tracking category it belongs to) in their cookie policy and make sure the UET script does not load until the visitor has ticked consent for the marketing category in the consent banner.
How to block or delete _uetmsclkid
A visitor can delete or block this cookie anytime from browser settings (Chrome, Firefox, Edge, Safari — the per-site cookie management section), or use dedicated ad-tracking blocker extensions.
For website operators, the correct approach isn't manual deletion but preventing the cookie from being set in the first place: a CMP such as CookieFix automatically blocks the Microsoft Ads UET script until the user gives explicit consent for the marketing category, avoiding setting the cookie without a legal basis.
Frequently asked questions
It doesn't contain sensitive data like passwords or payment details, but it is used to track online behavior for advertising purposes, which is why it requires consent under GDPR.
Advertisers won't be able to accurately measure conversions from Bing Ads for that visit, but browsing the site itself isn't affected.
Its typical duration is 90 days, after which it automatically expires from the browser.
Yes, as a marketing/tracking cookie, it must be listed in the cookie policy along with its purpose and duration, in line with GDPR and ePrivacy requirements.