Understanding the Publisher Common ID Module in Prebid: What Publishers Need to Know
Maintaining user identity across advertising partners is getting harder as privacy technologies evolve. Browsers like Safari and Firefox limit the effectiveness of traditional third-party cookies, which directly impacts publishers’ ability to deliver targeted ads and maximize programmatic revenue.
The Publisher Common ID (pubcid) module in Prebid offers a straightforward, privacy-friendly way to assign unique user IDs on your own domain. Understanding and implementing this module can help publishers improve user matching while remaining compliant with modern privacy expectations.
What is the Publisher Common ID Module?
The Publisher Common ID module is a lightweight utility within Prebid.js designed to store a unique identifier for each site visitor on the publisher’s first-party domain. This ID is accessible to all participating Prebid adapters, enabling more reliable user recognition across different auction events and devices.
This approach addresses challenges presented by browser tracking restrictions. Unlike third-party cookies, first-party storage is more durable and compatible with privacy features like Apple’s Intelligent Tracking Prevention (ITP). The module can be a critical building block for publishers striving to maintain effective user-level targeting and identity resolution as third-party cookies phase out.
How the ID is Stored and Used
The Publisher Common ID is a UUID v4 value saved as both a first-party cookie and a localStorage item named ‘_pubcid’. When a new visitor lands on your site, the module checks for this ID and creates it if needed. It then appends this identifier to outgoing bid requests, so any ad tech partner supporting pubcid can use the same user reference.
For example, when a page loads, if ‘_pubcid’ is missing, the module generates one. On subsequent bid requests, this consistent value helps match users even if they visit your site on different occasions or via different devices.
Configuring and Integrating the Module
The pubcid module is simple to add to your Prebid.js build. Once included, it automatically handles ID creation, storage, and transfer to adapters that support it. Publishers can adjust several settings to fit their needs, such as the ID’s expiration interval and whether to store the ID in cookies, localStorage, or both.
Example Integration Flow
Suppose a publisher wants the ID to refresh annually for all users. You might add a configuration block like this to your Prebid setup:
pbjs.setConfig({
pubcid: { expInterval: 525600 }
});
This sets the ID’s lifespan to one year (in minutes). From there, as long as Prebid’s pubcid module is part of your build, all compatible adapters can read and use this ID in bid requests automatically.
Building and Deploying Prebid.js with pubcid
When preparing your Prebid.js custom build, ensure you include ‘pubCommonId’ in your –modules flag with the desired adapters. For example:
gulp build –modules=pubCommonId,appnexus,rubicon
After testing, deploy the bundled Prebid script to your CDN as you would with any site assets.
Maintaining User Privacy and Compliance
User privacy is a critical concern for any identity-related feature. The pubcid module builds in practical compliance controls, giving users the choice to opt out of ID storage and targeted advertising easily.
Opt-Out Mechanism
To honor user privacy preferences, publishers are required to offer opt-out functionality. This is typically provided via a privacy policy link or a dedicated site control. Opting out sets a ‘_pubcid_optout’ flag (via cookie or localStorage), which instructs the module not to create, read, or transmit the ID or to erase any existing value.
Operationally, you need to ensure your privacy policy explains this process and genuinely deletes the ID for users who opt out.
What this means for publishers
Implementing the Publisher Common ID module bridges the user identity gap left by third-party cookie restrictions. Publishers who add this module can expect higher user match rates with demand partners that support pubcid, leading to better programmatic fill and CPMs. However, you must also adapt your privacy practices, enabling a transparent opt-out and keeping documentation up-to-date for auditors or privacy assessments.
On the operational side, setup and ongoing management are straightforward. Mistakes like missing module inclusion, incorrect config, or opt-out non-compliance can quietly undermine your monetization, so regular testing and documentation reviews are essential.
Practical takeaway
For publishers looking to future-proof header bidding revenue, enabling the Publisher Common ID in Prebid is a best practice. Start by including the module in your Prebid custom build, and configure expiration and storage settings to fit your data policies.
Just as importantly, update your privacy documentation and clearly provide users with an opt-out function that works as described. Regularly test your implementation (including in incognito/private modes and across browsers) to verify IDs are being set and honored as expected.
Taking these steps will give your site a durable, privacy-respectful way to maintain user-level demand without relying on third-party cookies. This is key both for near-term earnings and longer-term compliance and competitiveness.
