What is the _sharedid cookie?
_sharedid is an HTTP cookie set by Prebid.js, an open-source library used by websites and publishers to run real-time advertising auctions (header bidding) across multiple ad platforms (SSPs and DSPs).
The cookie belongs to the SharedID module, a "universal" identifier created by the Prebid community to partly replace traditional third-party cookies. It usually appears on the first page load, when the site's ad script initializes the programmatic auction.
What data it stores
_sharedid stores a unique, randomly generated pseudonymous identifier tied to the visitor's browser. The exact value format can vary depending on each site's Prebid configuration.
This identifier is shared with advertising partners (SSPs, DSPs, ad agencies) integrated into the site's Prebid auction, in order to link ad requests to a returning visitor.
What it is used for
Its main purpose is identifying and tracking visitors for advertising purposes, allowing ad partners to recognize the same user across multiple visits or pages and take part in more relevant ad auctions.
For the site publisher, this identifier helps increase programmatic advertising revenue by giving ad buyers a visitor-continuity signal, similar to classic third-party cookies.
Does it require consent?
_sharedid is classified as a marketing/advertising cookie, since it is used to track visitors for ad targeting purposes. Under GDPR and the ePrivacy Directive (transposed in Romania via Law 506/2004), this type of cookie requires prior, explicit user consent, obtained before the Prebid script runs.
- The site owner must disclose the cookie in the cookie policy, including provider, purpose and duration.
- Consent must be collected through a compliant banner (genuine opt-in, not just information), never assumed from continued browsing.
- Declining consent must effectively prevent the cookie from being set, not just hide the banner.
How to block or delete _sharedid
A visitor can manually delete or block _sharedid from browser settings (Chrome, Firefox, Edge, Safari – cookies and site data section), or use tracking/ad-blocking extensions.
For site owners, proper blocking before consent can't be achieved by deleting the cookie afterward — the Prebid script itself must be prevented from running until consent is given. A CMP platform like CookieFix handles this by automatically blocking marketing-category scripts, including Prebid, until the visitor gives consent.
Frequently asked questions
It doesn't hold sensitive data, but it counts as indirect personal data (a pseudonymous identifier) since it allows tracking a visitor's online behavior.
The site keeps working normally, but advertising partners will no longer recognize the visitor across sessions, which can reduce the relevance of ads shown.
Yes, as a marketing/advertising cookie, it requires prior consent under GDPR and Romania's ePrivacy-based legislation.
It's set by the Prebid.js script embedded on the site, as part of the SharedID module, but legal responsibility for consent lies with the site owner.