What is the sess cookie?
„sess” is an HTTP cookie placed on the .adnxs.com domain, belonging to AppNexus, an ad-exchange platform acquired by AT&T and later by Microsoft, and rebranded as Xandr. It isn't set by the visited website itself but by advertising scripts or pixels from partners (agencies, publishers, ad networks) using the Xandr/AppNexus infrastructure for programmatic bidding.
The cookie typically appears when a page loads an ad served through the Xandr/AppNexus chain, or when a tracking tag sends a request to adnxs.com, even if no ad is visibly displayed.
What data it stores
The cookie stores an identifier tied to the current browsing session, used to correlate successive ad requests made to AppNexus/Xandr servers during the same visit. The exact value format isn't publicly documented and may vary.
Data is sent to Xandr's servers (the .adnxs.com domain) and may be shared with advertising partners involved in the bidding chain for that page.
What it is used for
Its main purpose is to support real-time bidding (RTB) processes and ad performance measurement during a browsing session: preventing repeated display of the same ad, correlating multiple requests, and optimizing ad delivery.
For the site owner, this cookie brings no direct functional benefit — it's part of the advertising ecosystem of partners monetizing the site's ad space.
Does it require consent?
As a marketing/advertising cookie set by a third party for tracking and ad bidding, it falls under Article 5(3) of the ePrivacy Directive (implemented in Romania via Law 506/2004) and requires the visitor's prior explicit consent under GDPR.
- Must be listed in the cookie policy under the marketing/advertising category
- Cannot be set before the visitor gives consent
- The site owner should verify that any advertising partner triggering adnxs.com scripts is disclosed as a third party or subprocessor
How to block or delete sess
Visitors can manually delete or block the cookie via browser settings (Chrome, Firefox, Edge, Safari – cookie management for adnxs.com) or use ad-tracking blocking extensions.
At the site level, this cookie should not be allowed to load before consent is given. A CMP like CookieFix can automatically block the scripts and pixels that generate this cookie until the visitor accepts the marketing category, preventing unauthorized setting.
Frequently asked questions
It's a session cookie used by the AppNexus advertising platform (now Xandr, part of Microsoft) to correlate ad requests during a site visit.
It doesn't contain sensitive data in a technical sense, but it's part of ad-tracking infrastructure, which is why it's classified as a marketing cookie.
Yes, since it's used for advertising and tracking by a third party, it requires the visitor's prior consent under ePrivacy and GDPR.
The typical duration observed is 1 day, though this can vary depending on the advertising partner's configuration.