Hunting for the _uetsid cookie
A while ago we noticed a spike in Google search results for
_uetsid quite a lot, but so far we haven't recorded its purpose or originating company, which probably left most of the people visiting disappointed.
In fact, if you search for the cookie by its name it's indeed quite a mysterious one — it's only found in people's cookie policies where it's described only as "unclassified". There's no mention of this cookie in any of the popular advertisers documentation or policies, as it's the case with many well-known cookies (such as Google Analytics or CloudFlare). Solving this riddle required a bit of manual investigation...
We used one of the websites (
https://www.dimanoinmano.co.uk/) we knew was setting this cookie and rendered it using a custom PhantomJS script (see at the bottom) that records each resource loaded and scans its HTTP
document.cookie interface which is a bit more tricky to debug — the cookie name may not even appear in the script code as it may be generated programmatically. The next step was to see if
https://bat.bing.com/bat.js appeared always just before the cookie was created in PhantomJS. And we were lucky here, this is excerpt from the file contents:
function UET(o) ... this.sessionCookieName="_uetsid"
Now, knowing that it's Microsoft's Bing and armed with the "UET" acronym we were able to quickly find out that it's a Microsoft Bing Ads Universal Event Tracking (UET) and link appropriate Microsoft's policy document on the _uetsid cookie page, hopefully helping people to write more accurate policies!
And the PhantomJS script: