What is the CMID cookie?
CMID is an HTTP cookie placed by the .casalemedia.com domain, belonging to Casale Media, the company that operates Index Exchange, one of the largest global supply-side platforms (SSPs) used for real-time bidding of ad inventory.
The cookie typically appears when a website displays ads through an advertising partner connected to Index Exchange, either directly or via other ad exchanges/DSPs. It is not set directly by the site owner but by third-party advertising scripts loaded on the page.
What data it stores
CMID stores a randomly generated unique identifier assigned to the visitor's browser. This identifier acts as a persistent pseudonym used to link repeat visits from the same user to an advertising profile.
The exact value format may vary and is not documented in detail by Casale Media publicly. Data associated with the identifier (ad interactions, sites visited within the ad network) is sent to Casale Media / Index Exchange servers, typically under domains such as casalemedia.com.
What it is used for
The main purpose of CMID is programmatic advertising: identifying the browser allows Index Exchange to participate in real-time bidding (RTB) auctions to serve relevant ads, avoid showing the same ad repeatedly, and measure campaign performance.
For the website owner, this cookie is usually a side effect of integrating an ad network or SSP/DSP that works with Index Exchange, rather than a deliberate, direct choice to set this specific cookie.
Does it require consent?
CMID is classified as a marketing cookie (advertising/tracking), since it builds a profile used for ad targeting and cross-site tracking. Under the ePrivacy Directive (as transposed into Romanian law via Law 506/2004) and GDPR, this type of cookie requires the user's prior consent, obtained explicitly before the script is loaded on the page.
Site owners must disclose this cookie in their cookie policy, correctly categorize it as marketing in the consent banner, and ensure the advertising scripts that generate it do not fire before the visitor grants consent, in line with data protection authority guidance.
How to block or delete CMID
A visitor can manually delete or block this cookie from browser settings (Chrome, Firefox, Edge, Safari), under the per-site cookie management section, by searching for casalemedia.com. Anti-tracking browser extensions or third-party cookie blocking modes also reduce its occurrence.
For site owners, proper blocking means the advertising scripts that set CMID must not execute at all before consent is given. A CMP such as CookieFix can automatically block these scripts (through scanning and classification) until the visitor grants consent for the marketing category, preventing the cookie from being set prematurely.
Frequently asked questions
It's a marketing cookie set by Casale Media, the company behind the Index Exchange ad platform, used to identify a browser for programmatic advertising purposes.
No, it's a standard advertising tracking cookie widely used across the ad-tech industry, not malicious software.
Its typical lifespan is around 1 year, though this can vary depending on the advertising partner's configuration.
Yes, as a marketing/tracking cookie, it requires the visitor's prior explicit consent under the ePrivacy rules and GDPR.