What is the SRM_B cookie?
SRM_B is an HTTP cookie set by Microsoft, tied to the company's online advertising infrastructure (Microsoft Advertising / Bing Ads). It is placed on the .c.bing.com domain, a subdomain Microsoft uses for conversion-pixel and campaign-measurement services.
On a third-party website, this cookie typically appears when the site owner has integrated a Bing Ads conversion tag or pixel (UET – Universal Event Tracking) to measure the results of advertising campaigns run through Microsoft Advertising.
What data it stores
The cookie stores an identifier generated by Microsoft, used to recognize the visitor's browser across multiple visits and sites that use Microsoft Advertising scripts. The exact value format is not publicly documented by Microsoft.
The collected data is sent to Microsoft's servers, where it is linked to browsing activity to build interest profiles and attribute conversions from advertising campaigns.
What it is used for
- Lets Microsoft measure the effectiveness of Bing Ads/Microsoft Advertising campaigns (conversion attribution).
- Supports retargeting — showing relevant ads to visitors who already interacted with the site.
- Helps build audience segments for targeted advertising within the Microsoft network.
Does it require consent?
CookieFix classifies SRM_B as a marketing cookie (advertising/tracking). Under GDPR and the ePrivacy Directive (transposed in Romania through Law 506/2004), this type of cookie may only be set after the visitor's explicit consent, before the script runs.
Site owners must declare the cookie in their cookie policy, list it correctly under the marketing category in the consent banner, and ensure the Bing Ads UET script does not run before the visitor gives consent.
How to block or delete SRM_B
A visitor can delete or block SRM_B from their browser settings (Chrome, Firefox, Edge, Safari — cookies and site data section), or use Microsoft's ad personalization opt-out options available through their Microsoft account/privacy settings.
For site owners, the correct approach is to automatically block the Bing Ads conversion script until consent is given, using a CMP such as CookieFix, which blocks marketing scripts by default and only activates them after the visitor accepts.
Frequently asked questions
It is not malicious, but it is a tracking cookie used for advertising, which is why it requires visitor consent under GDPR.
It is set by Microsoft, through the Bing Ads/Microsoft Advertising infrastructure, on the .c.bing.com domain.
The typical duration observed is about 1 year from when it is set, after which it expires automatically.
Yes, as a marketing cookie it must be blocked until the visitor gives explicit consent, in line with GDPR and the ePrivacy Directive.