Understanding Prebid’s Storage Control Module: Ensuring Transparent Device Storage Use

Device storage access—such as setting cookies or using localStorage—is a core part of how many header bidding modules operate. Yet, with privacy regulations tightening and transparency expectations increasing, it’s never been more important for publishers to understand and control which scripts are storing data on users’ devices.

Prebid’s Storage Control module aims to bring order and accountability to this process. By enforcing disclosure requirements for storage use, it helps publishers avoid accidental privacy violations and gain clearer visibility into third-party activity on their domains. For publishers and ad ops teams, this is a concrete step toward maintaining compliance and protecting user trust.

Why Storage Use Disclosure Matters in Header Bidding

Modern header bidding relies heavily on device storage for user identification, frequency capping, and syncing. However, regulations like GDPR and increasing demand for privacy have placed a spotlight on undisclosed storage access.

If vendors set cookies or other identifiers on your users’ devices without appropriate disclosure, you risk legal exposure and diminished trust with your audience. The IAB’s TCF framework makes disclosure mandatory, but manual oversight is nearly impossible at the scale of typical ad stacks.

Real-World Example: Undisclosed Identifiers

Imagine a new analytics module added to Prebid.js that quietly sets an additional cookie for cross-domain tracking. Without disclosure, neither your legal team nor your consent management platform would be aware. Such surprises are exactly what Storage Control helps prevent.

How the Storage Control Module Works in Prebid.js

The Storage Control module audits the storage access patterns of installed Prebid modules. It compares every attempt to set or read a cookie (or use localStorage) against a list of disclosed identifiers, which are drawn from published JSON files per the TCF’s requirements.

When a module tries to use an undisclosed storage key, Storage Control will either log a warning or block access outright, depending on its configuration. This applies not just to IAB-registered vendors, but also any adapters or modules that properly publish their own disclosure metadata.

Configuring Enforcement Levels

Storage Control supports several enforcement modes:

– ‘off’: Only logs warnings for undisclosed storage use (default).
– ‘strict’: Actively blocks undisclosed storage access, offering maximum compliance.
– ‘allowAliases’: Blocks undisclosed storage, except in cases where aliases are linked to compliant modules—useful for multi-branded bidders.

Switching between these modes is a single configuration call, giving publishers flexibility as they tighten or relax enforcement over time.

Integrating and Using Storage Control as a Publisher

To benefit from Storage Control, publishers must include it as part of their Prebid build. For those using npm, it’s essential to also import storage disclosure metadata alongside each module.

Once installed, publishers can query all active storage disclosures using Prebid’s API. This returns a clear, structured list of every identifier declared by loaded modules—offering immediate visibility into what’s happening on the page.

Practical Example: Querying Disclosures for Audits

Suppose your privacy officer asks for a list of all cookies being set by header bidding code. By invoking `pbjs.getStorageUseDisclosures()`, you receive a ready-made breakdown showing each identifier, its purpose, and which module is responsible—dramatically simplifying internal audits or regulatory reporting.

Common Pitfalls and Best Practices for Storage Disclosure

While Storage Control reduces risk, it is only as accurate as the metadata it checks. Publishers that add new modules or update adapters must ensure disclosure files are present and up to date. Skipping metadata imports or integrating custom modules without disclosures reintroduces gaps in your compliance posture.

Best Practice Checklist

– Always include the Storage Control module in all Prebid.js builds.
– When adding or updating modules, verify that disclosure files are present and accurate.
– Use the enforcement mode that matches your regulatory environment and risk profile.
– Regularly audit your disclosures via the Storage Control API.

What this means for publishers

Storage Control empowers publishers to proactively manage privacy risks by preventing the use of undisclosed device storage. It provides visibility and enforcement against vendor modules that might otherwise operate outside your compliance boundaries, while also simplifying audits and transparency reporting. Integrating and maintaining this module reduces the likelihood of regulatory issues and boosts confidence with both users and partners.

Practical takeaway

Integrate the Storage Control module as a standard part of your Prebid.js deployment. Choose an enforcement mode (‘strict’ for high-compliance environments or ‘off’ for initial monitoring) to match your requirements. Always ensure that newly added or updated bidder modules supply complete disclosure files—this keeps your site transparent and defensible during audits.

Make routine use of the Storage Control API’s reporting capabilities to periodically verify which storage identifiers are in use on your property. By setting up these controls, publishers put themselves in a strong position for privacy compliance and build trust with their users and commercial partners.