What is the c cookie?
Used to synchronize user identifiers and exchange user data between various advertising platforms within the real-time bidding ecosystem.
Table of Contents
About the c cookie
| Vendor | BidSwitch |
|---|---|
| Cookie Domain | admedo.com,b2c.com,bidswitch.net,creative-serving.com,creativecdn.com,impact-ad.jp,liftdsp.com,mfadsrvr.com,peer-39.com,pointmediatracker.com,pubmine.com,sportradarserving.com |
| Category Category The functional category of the technology, such as Web Analytics or Social Media. Learn more | Advertising & Paid Media |
| Consent Category Consent Category The consent category this cookie most commonly falls under across sites we scan, normalized into four standard categories. Learn more | Targeting & Advertising |
| Prevalence | Very Common |
| Popularity Popularity Popularity is calculated from our dataset of 4.5B+ cookies analyzed across hundreds of millions of web pages. Learn more | Found on 16.1% of scanned pages |
| Expiration Type | Timestamp |
| Expiration Duration | 29 days |
| Party Type Party Type Whether the cookie is first-party or third-party. Learn more | 3rd-Party |
| Risk Level Risk Level Rates how sensitive the data stored by this cookie is (High, Medium, or Low) based on data classification and distribution. Learn more | Medium |
| Vendor Privacy Policy | https://www.bidswitch.com/privacy-policy/ |
| Vendor Website | https://www.bidswitch.com/ |
What is the purpose of the c cookie?
The c cookie is a third-party cookie observed across many independent programmatic ad-tech domains, with the dominant setter being BidSwitch (owned by IPONWEB, which was acquired by Criteo in 2022) and additional observed vendors including RTB House (creativecdn.com), Market One (impact-ad.jp), mfadsrvr, liftdsp, and sportradarserving. Because c is a generic single-character name reused by several vendors, its precise role depends on the setting domain, but in this ad-tech context it functions as part of cookie syncing (also known as user matching): in the real-time bidding (RTB) environment, demand and supply platforms need to recognize the same user across different domains to serve relevant advertisements, and this cookie stores a machine-generated value (frequently a numeric timestamp, and in some cases a UUID) that supports that process.
By matching identifiers, exchanges such as BidSwitch act as a central hub, allowing buyers and sellers in the programmatic ecosystem to align user IDs, target audiences, and optimize ad delivery. In some deployments the cookie value is simply a timestamp used to track cookie lifetime or to flag that the browser accepts cookies for tracking purposes, rather than a direct unique identifier.
What are the Privacy Risks of the c cookie?
Risk Level: Medium
The c cookie poses a medium privacy risk because it is used for cross-site tracking and profiling within the programmatic advertising ecosystem. By supporting the syncing of identifiers across multiple third-party ad networks and exchanges, it enables the aggregation of user behavior across different websites to build advertising profiles.
While the cookie value itself is typically a pseudonymous timestamp or opaque string rather than direct personal information, the resulting matched profiles are heavily used for targeted advertising and audience segmentation, which is why it is not classified as low risk. It is not rated high because the cookie value alone does not carry directly identifying personal data.
How to Remove the c cookie from a Website
To remove this cookie, website administrators must identify and remove the third-party ad tags, pixels, or exchange integrations (such as those from BidSwitch, RTB House, or Market One) that initiate it. Because this cookie is frequently loaded indirectly through a programmatic bidding cascade (for example, a primary ad tag or SSP calling out to various exchanges for user matching), completely eliminating it may require disabling programmatic ad serving entirely or restricting the primary ad vendor from conducting cookie syncing, since it is set by the vendor's service rather than the site itself.
This definition was last reviewed on August 19, 2026. ObservePoint regularly reviews these definitions to ensure our customers have the most up to date information about the cookies on their websites.